Fermented Bread Consumption: Effects on Glycaemia in Healthy Populations

1. Introduction

Fermented foods (FFs) constitute a diverse and ancient category of food products that are produced via the controlled activity of microorganisms (bacteria, yeasts or molds), acting on raw food materials of both plant and animal origins (including cereals, legumes, vegetables, roots and tubers, milk, meat or fish). Defined as “foods made through desired microbial growth and enzymatic conversions of food components” [1], FFs have been used in human cultures worldwide for millennia. The fermentation process extends shelf-life, influences flavours, aromas, textures and modifies the nutritional and functional characteristics of foods. Microbial metabolism transformations involve biochemical changes that contribute to food safety by inhibiting spoilage and pathogenic organisms while also improving nutrient bioavailability, reducing anti-nutritional factors and generating compounds with potential health benefits. Therefore, FFs hold a unique position as both traditional dietary components and objects of renewed scientific interest and investigation, being increasingly recognised for their roles in modulating the gut microbiota, supporting metabolic health and contributing to the prevention of chronic diseases, including type 2 diabetes mellitus (T2D) [2].

The increasing prevalence of T2D represents one of the most critical public health challenges of the 21st century, placing a considerable burden on healthcare systems worldwide [3,4]. T2D is characterised by chronic hyperglycaemia resulting from impaired insulin action and secretion. The condition is driven by a complex interplay of genetic, metabolic and lifestyle factors, with dietary factors playing an important role in the disease pathogenesis and development. According to the World Health Organization, the diagnosis of T2D is established by elevated fasting plasma glucose or abnormal glucose tolerance [5]. As rates of T2D continue to rise globally, a lot of research has been invested to identify dietary strategies for prevention or delay of the disease onset, among healthy individuals and particularly populations at increased risk of T2D, such as those with prediabetes or obesity. Carbohydrate quality and consumption, are important dietary components implicated in risk [6,7] and management of T2D [8,9]. Of the different dietary sources of carbohydrates, bread is widely understood to induce rapid rises in glycemia and indeed white bread is often used as the reference (in place of glucose) for assessing glycaemic index (GI), a measure indicating the rise of blood glucose in the 2 hours following consumption of the food [10]. In this context, understanding the glycaemic impact of different breads particularly in relation to processing and fermentation methods, is essential for dietary management. The practice of fermenting grains to produce bread has historical roots, with both baker’s yeast and sourdough breads consumed across cultures for millennia. While some breads are processed without a fermentation step (e.g., soda bread), fermentation of bread is common, improving sensory properties, shelf-life, nutrient bioavailability of bread, and generating bioactive compounds that alter carbohydrate structure in ways that may positively affect postprandial metabolism [11]. A number of systematic reviews have already considered the growing body of evidence investigating different types of fermented breads on glycaemic control and insulin sensitivity [12,13,14]. However, these previous reviews often pooled broad study populations and diverse bread types, comprising different cereal or fibre contents. Such an approach prevents solid conclusions from being made for healthy people and makes the role of fermentation itself in the observed effects difficult to elucidate.

The present review attempts to address the challenges in this research topic by first characterising bread, its constituents and modes of production, and how these might explain purported effects on glucose homeostasis, then using a systematic approach to search for relevant clinical trials investigating the effects of fermented bread on glycemia, and finally reporting on possible mechanisms of action underlying the observed effects. Using key outcomes of glucose homeostasis such as fasting blood glucose, insulin sensitivity, postprandial glucose and insulin responses, we sought to determine whether regular consumption of fermented breads can support metabolic health in healthy adults, including obesity and pre-diabetes to reflect populations at risk of T2D. Within this overarching aim we examined three specific research questions: does the type of fermentation influence the effects of bread on glycaemia (i.e., mainly sourdough versus yeast-based fermentation)? Does fermentation impact the glycaemic effects (i.e., fermented bread versus non-fermented bread or other kind of cereal-based food)? Does the consumption of specific breads affect glycaemic parameters considering both differences captured by low versus high intakes and changes between pre- and post-intervention? Through this comprehensive and systematic synthesis, we intend to clarify whether fermented breads offer measurable benefits for glucose regulation in healthy adults and to provide evidence that may inform both clinical nutrition practice and public dietary recommendations.

2. Methods

This review was conducted following a generic search strategy, developed within the COST Action CA20218 Promoting Innovation of ferMENTed fOods PIMENTO initiative detailed in a corresponding position paper [15], and in agreement with the EFSA guidances [16,17]. This review is structured in three main parts, (i) a systematic review of human studies in accordance with published guidelines [18] as well as (ii) complementary parts on food characterization and (iii) mechanism of action. The review protocol, including specific search terms and strategy, was registered in the Open Science Framework (OSF protocol, DOI: 10.17605/osf.io/qrza3).

2.1. Systematic Review of Human Studies

2.1.1. Literature Search

A literature search was conducted using Medline, Cochrane, and Scopus. The search was limited to English-language studies and used predefined strings (see Supplementary Material 1). The studies included in this review were identified through (i) selection of articles that met the predefined eligibility criteria aligned with the P/I/C/O (Population, Intervention, Control, Outcome) framework (no exclusion criteria on study design), (ii) screening of relevant systematic reviews for possibly missed studies meeting the same criteria, as shown in the flow diagram (Figure 1). From 7,382 initial records (post-deduplication), 1,660 met PIO (Population/ Intervention/ Outcome) criteria based on title and abstract. After full-text assessment applying refined inclusion criteria, 314 entries were retained.

The selection was further narrowed to 65 articles focused on bread as the fermented product of interest. Of these, eighteen included an appropriate control but two used exactly the same data from one trial, so one of these was excluded and seventeen were retained for this review.

An update of the search (from September 2023 to January 2025), focusing on fermented cereals only, retrieved 329 new entries (317 after deduplication), resulting in fourteen full-text eligible studies and one meeting full PICO (Population/ Intervention/ Control/ Outcome) criteria.

Review articles were retained at the screening stage if they addressed related topics, allowing for the identification of missed pertinent primary studies: Systematic reviews identified during the search contributed fifteen entries meeting the PIO framework; nine met the full PICO criteria and were included.

2.1.2. Selection Criteria

Eligibility according to the PIO criteria was assessed using CADIMA, an open-source platform designed to support systematic literature reviews and evidence mapping [19]. Two reviewers screened independently the studies and resolved discrepancies through discussion, involving a third reviewer in case of conflict. Inclusion was limited to original studies involving, at least in part, healthy or prediabetic adults, but no athletes or pregnant women. Additionally, only participants without regular use of medications were eligible, with exceptions for contraceptive pills, postmenopausal hormone treatment and well-adjusted thyroid hormone therapy. Studies with blood measures of glucose or insulin homeostasis were selected.

The intervention or exposure was initially defined as the oral intake or consumption of any type of fermented food (FF), or instances where FF consumption could be derived from dietary questionnaires. Interventions were excluded if FFs were mixed with other components that prevented the effect of the FF to be isolated/assessed independently. Also, studies testing only probiotics (without a FF matrix, or without a direct participation in the fermentation of the food matrix) or only an extract from the FF were excluded, as we were interested in the effect of the whole food. Non-oral applications were not considered. Outcomes of interest were defined as measures related to glucose or insulin homeostasis, or, at a minimum, the collection of relevant blood samples indicative of metabolic responses. In a second step, we focused our search on studies assessing bread. Eligible controls for fermented breads were defined as non-fermented cereal products (e.g., pasta, porridge, unfermented bread), providing that they used the same cereal and flour type. Comparisons of leavening method such as yeast-fermented vs. sourdough breads, were also eligible where breads were produced using similar flour/cereal. ‘Long-term’ intervention studies offering a before/after intervention comparison were accepted in the absence of a proper control. Studies that only compared breads to glucose drinks were excluded. Observational studies employing dietary questionnaires comparing high vs. low intake of bread were retained unless it was impossible to discern fermented food consumption (i.e., described only within broader categories such as cereals). Primary outcomes of interest were measures related to glucose homeostasis or insulin, including both postprandial responses for short term studies and fasting values for long-term studies, such as HbA1c, or different measures of insulin sensitivity. Therefore, studies without collection of relevant blood samples indicative of metabolic response analyses were excluded, at the level of abstract screening. Incidence of T2D alone was, however, not a valid outcome for inclusion at full text level screening. Additionally, studies addressing diabetes incidence or risk were included provisionally at abstract level, pending full-text evaluation for relevant glucose or insulin data. Studies assessing glucose changes solely during exercise were excluded. At full text level, studies assessing the GI of breads when a valid control was present were also taken into consideration.

Data on adverse effects for assessment of security safety and effects on gut parameters for possible mechanisms of action were noted when available.

2.1.3. Data Extraction

A standardized data extraction form was used. Each study was independently reviewed by two researchers. Discrepancies were resolved by discussion. Extracted data included general information on the study such as study design, participant characteristics, and location of the study. Additionally, details about the fermented bread and the control used were extracted, the outcome measures and the effect detected. Depending on the type of control identified, studies were clustered for each of the three specific research questions. Finally adverse events, and mechanistic hypotheses described in the studies were collected.

2.1.4. Risk of Bias (RoB) and Quality Assessment

For each included study the risk of bias using the “Revised Cochrane risk-of-bias tool for randomized trials (RoB 2) [20] was evaluated. The outcomes were visualized using Robvis [21].

Potential sources of bias across multiple domains were considered, including the randomization process, period and carryover effects, effect of adhering to interventions, missing outcome data, outcome measurement, and the selection of reported results. Each domain was evaluated independently by two researchers, and an overall risk of bias judgment was subsequently assigned to each study, resolving discrepancies by discussion.

2.1.5. Data Synthesis

A descriptive synthesis was conducted. Due to heterogeneity in study designs and outcomes, meta-analysis was not performed. The strength of the evidence was assessed from the identified studies with respect to each of our questions, pooling all considered glycaemia/insulinaemia outcomes together to identify any effect on glucose metabolism in general. Criteria that were applied consisted in the number of studies identified, their quality (considering the RoB assessment) and the consistency of the effects observed, in a modified form of the GRADE approach [22]. Four levels of evidence were considered: (i) a high level (strong evidence), if more than 10 randomized control trials (RCT) were found, with the majority displaying a low-risk of bias evaluation and none a high risk and the vast majority reporting a consistent effect; (ii) a moderate level if at least 10 human studies (RCT or cohort studies) were available, with a maximum of one study with a high-risk of bias and the majority reporting a consistent effect; (iii) a low level of evidence if at least 10 human studies (RCT or cohort studies) were available, a minority displaying a high RoB and a majority reporting a consistent effect; (iii) a very low level of evidence if either only few human studies were found, and/or most identified studies showed a high RoB and/or no consistency in the reported effects were found.

2.2. Non-Systematic Parts of the Review

These complementary sections of the review address important parts of an EFSA dossier.

2.2.1. Characteristics of the Fermented Foods

Narrative synthesis was used to describe product characteristics using data from included studies and complementary sources (e.g., Codex Alimentarius, AACC standards). Attributes such as fermentation type, microbial strains, processing parameters, physical quality characteristics, proximate composition and bioactive components possibly affecting glycaemic responses were also included.

2.2.2. Supportive Evidence: Mechanism of Action and Bioavailability

Similarly to the product characterization, we screened selected studies for information on possible mechanisms of action and bioavailability of bioactive compounds, and complemented them with additional literature, particularly adding animal and in vitro studies which offered a deeper understanding of the involved mechanisms, including the potential role of the gut microbiota in mediating the effect of bread on glycemia, as well as bioaccessibility and absorption of bioactive compounds from bread.

2.2.3. Safety of Fermented Foods

We screened all included studies for adverse effects reported and complemented this information with additional relevant literature.

2.3. Summary of the Systematic and Non-Systematic Parts of the Review

We evaluated the current collective evidence using systematic and non-systematic parts of this review, taking into account the results for the human study search, the coherence/plausibility of the observed effects, and the characteristics of fermented breads. Therefore, three levels of overall evidence were defined: (i) a high level of evidence (“convincing and sufficient”) if strong evidence was found from the all human studies, the mechanism of action pathways are plausible and bioactive compounds are well described; (ii) a low level of evidence (“neither convincing nor sufficient”) if there was either a strong evidence from human studies but lack of knowledge upon pathways and/or bioactives or a low/very low evidence from human studies but well characterised bioactives and pathways; (iii) a very low level of evidence (“no or very limited”) if both the evidence from human studies was very low and the characterisation of bioactives and pathways was limited.

3. Results and Discussion

To understand how bread consumption could affect glycaemic regulation, we first summarize the characteristic of bread as a food type, then analyse the systematically selected human studies and report on possible mechanisms explaining the observed effects.

3.1. Characterisation of the Food/Constituent

3.1.1. Food Description

Bread is a staple food consumed daily by billions of people worldwide. In many societies, bread is the primary source of carbohydrates as it is easily accessible and inexpensive. Bread is a bakery product; the dough of bread is prepared primarily from flour of cereals and water, with the addition of one or more leavening agents (biological or chemical), and salt. Although Codex Alimentarius does not provide an explicit definition of bread, it classifies bread and similar bakery products under the category “bread and ordinary bakery wares” (Category 07.1) within the General Standard for Food Additives (GSFA). This classification includes specific allowances and restrictions regarding food additives, reflecting bread’s role as a fundamental dietary staple. EU Food Categorization (Annex II of Reg. 1333/2008) classifies bread under Bakery wares – Bread and Rolls as products prepared mainly with cereal flour or cereals that have undergone a treatment, such as baking, steaming or extrusion.

Considering breads that use biological leavening agents, two subtypes of bread can be defined: bread fermented by baker’s yeast and bread fermented using sourdough. Baker’s yeast fermented bread is typically produced from refined wheat flour, water, Saccharomyces cerevisiae, and salt. It is leavened using baker’s yeast and undergoes a relatively short fermentation time. The use of S. cerevisiae as the primary leavening agent in industrial bread production is due to the prioritisation of consistency, speed and uniform texture and flavour.

Sourdough bread is leavened by fermentation with lactic acid bacteria (LAB) and yeasts, either spontaneously or using a starter culture. It has a longer fermentation period than bread produced using baker’s yeast. This typically results in a sour flavour and alters the nutritional profile by producing a range of bioactive compounds that are absent in breads fermented using only baker’s yeast [23]. The microbiota involved in sourdough fermentation varies depending on the ingredients and fermentation conditions, but typically includes species such as Fructilactobacillus sanfranciscensis, Limosilactobacillus fermentum, Lactiplantibacillus plantarum and Levilactobacillus brevis along with yeasts including S. cerevisiae, Candida milleri [24,25]. Recently, a starter culture containing the LAB Companilactobacillus crustorum LMG 23699 and the yeast Wickerhamomyces anomalus IMDO 010110 has been developed for sourdough fermentation [26].

Bread is primarily made from wheat flour (refined or whole; [27,28,29,30,31,32,33]. Additionally, various grains such as rye, barley, oats, millet, spelt, and rice are also incorporated in bread formula [25,29] 34-41]. To enhance quality characteristics and nutritional profile of bread, additional ingredients may be introduced such as malt, ascorbic acid, gluten, flour from pulses, milk powder, flaxseed, semolina, β-glucan, starch, fat, olive oil and sugar [31,35,42,43,44]. The yeast leavened bread may also be acidified by lactic and acetic acid to replicate sensory properties of sourdough [29].

3.1.2. Production Methods

Both S. cerevisiae and sourdough fermented bread involve similar processing steps including mixing, kneading, proofing and baking. However, sourdough bread production includes a longer fermentation period, because sourdough is a co-ferment and the sourdough microbiota requires a relatively long time to adapt [11].

In the industrial process of baker’s yeast-leavened bread, all ingredients like flour, water, S. cerevisiae and salt are mixed to obtain dough prior to proofing. The dough is then kneaded for approximately 5-7 min, followed by a bulk proofing lasting 20-45 min at 28-38 °C and 80-85% relative humidity. After bulk fermentation, the dough is divided, shaped, and subjected to a final proofing stage for 30–60 min at 34–38 °C, maintaining similar humidity levels [29,38,43,45]. Baking is typically carried out at temperatures between 175–250 °C for 20–45 min, depending on the type and size of the bread [38,43,45,46]. To produce a glossy and well-developed crust, a short burst of steam is often applied at the start of baking. Some specialized methods use a single high-temperature holding step (e.g., at 70 °C for about 1 h), followed by baking at lower temperatures such as 165 °C [33]. Achieving consistent and standardized quality is generally more straightforward in bread leavened with baker’s yeast due to its uniform microbial composition.

In sourdough bread production, the basic processing steps are similar to those used in bread fermented by baker’s yeast. However, there is no standard formula or procedure for sourdough fermentation, which leads to significant microbial variation in the sourdough culture. Factors such as the type of flour, water quality, and fermentation temperature contribute to variability in microbial growth, leavening performance, and formation of bioactive compounds, all which in turn influence the final product characteristics and therefore its potential health impact, sensory properties and shelf life.

There are two broad types of sourdough ferment. Type 1 sourdough is the spontaneous fermentation of a mixture comprising flour and water at ambient temperature. Within this type of sourdough ferment, the fermentation process necessitates daily ‘feedings’ at a specific ratio of flour suspension, and it is continued until the pH level reaches approximately 4.0 ± 0.2. The production of type 1 sourdough typically requires a period of 7–15 days [41,47]. Type 2 sourdough ferment aims to expedite the fermentation period by specific LAB and yeast culture. In type 2 fermentation, the desired pH level is achieved within 24-48 h without daily feeding [24,26,48].

Following sourdough ferment preparation, sourdough is incorporated into the final dough mixture along with the remaining bread ingredients and is kneaded. Afterwards, the dough is subjected to a two-step fermentation process. First, bulk fermentation is carried out for approximately 60-120 min at a temperature of 28-29 °C and 66-80% relative humidity, allowing for the initial development of gas retention and flavour compounds [31,42]. After this stage, the dough is divided and shaped, followed by a second proofing phase lasting 30-180 min at 30-38 °C and similar humidity levels [31,41,42]. The proofing period can be reduced to 60 min by using Baker’s yeast in addition to sourdough [24]. Finally, the fully proofed dough is baked at temperature varying between 165 and 250 °C for 30-80 min, depending on loaf size, dough composition, and the desired crust and crumb characteristics [28,29,30,31,33,38,40,42,43].

3.1.3. Analytical Methods

Processing and type of ferment used can alter crumb structure, texture, composition and bioavailability of different compounds in bread, which might affect the glycaemic responses to bread consumption[41].

The main quality characteristics of bread fermented with both sourdough and baker’s yeast include a high specific volume, a brown and glossy crisp crust and a soft and chewy crumb, with a homogeneous structure with small and homogenous pores. Specific volume is measured by calculating the ratio of bread volume (mL) to weight (g) using the rapeseed displacement method AACC 10.05.01 [49]. Porosity and internal microstructure can be examined via X-ray imaging, assessing air cell distribution, integrity of the protein matrix, and the extent of starch gelatinization [43]. Another physical quality measurement technique is texture profile analysis, which uses a texture analyser to evaluate hardness, springiness, cohesiveness, chewiness and resilience of bread crumbs [50].

The physical and microstructural properties of bread play important roles in determining starch digestibility and influencing glycaemic response. Zamaratskaia et al. [41] reported that white bread exhibited greater starch gelatinization and amylose leakage compared to sourdough bread. Furthermore, in the same study rye crispbreads were characterised with a more heterogeneous structure, with bran fragments dispersed throughout the matrix. These fragments were notably larger in unfermented samples than fermented ones. In addition, lamellae in crumb of yeast-fermented wheat and unfermented rye crispbreads were thinner than those observed in sourdough-fermented rye crispbreads. These structural distinctions were proposed by Zamaratskaia et al. to contribute to a slower disintegration rate, increased viscosity, and the presence of larger particles in the digesta of unfermented rye crispbreads, which can ultimately result in delayed glucose absorption and a reduced insulin response. Moreover, increased bread crumb firmness was associated with limited starch digestibility, potentially due to enhanced agglomeration and crystallinity of starch fractions. The development of compact amylopectin–amylose networks may further limit the susceptibility of starch to amylolytic enzymes and hinder enzymatic penetration [51]. Additionally, specific microstructural features such as a dense matrix and an amylose layer surrounding starch granules are thought to play a key role in reducing starch accessibility and digestibility [41].

The chemical composition of bread is determined using standardised analytical methods. Total protein content is quantified by the Kjeldahl method (Method no: 46.12.01), total fat content is measured using ether extraction (Method no: 30.10.01), and total ash content is determined by incinerating the sample at 575-590 °C until a constant weight is obtained (Method no: 08.01.01) 01 [49]. Starch (Method no: 76.13.01), dietary fibre (Method no: 32.07.01), resistant starch and β-glucan content (Method no: 32.22.01) are highly correlated with glycaemic response of bread and they are assessed using the enzymatic hydrolysis methods 01 [49]. Furthermore, fermentation has been shown to increase the content of some bioactive compounds such as phenolic compounds which may contribute to lowering glycaemic and insulin responses [52].

The interactions between starch and different components such as lipids, organic acids, or polyphenols decrease the digestibility rate of starch since these compounds limit the accessibility of amylolytic enzymes to amylose/amylopectin or inhibit amylase activity [53], and their quantification in the breads is therefore important. In sourdough bread, fermentation leads to a decrease in pH and an increase in organic acids concentration, mainly lactic and acetic acid. Therefore, parameters such as pH, total titratable acidity, lactic acid and acetic acid are routinely monitored in sourdough bread. The pH value is measured by a pH meter, and the titratable acidity is determined by titrating a homogenised sample with 0.1N NaOH (Method no: 02.31.01) [49]. Organic acids are quantified by High-Performance Liquid Chromatography (HPLC) [25].

The acidity in sourdough bread contributes to the compositional changes that result in its low GI. The presence of organic acids in sourdough reduces the starch hydrolysis rate by promoting interactions between starch and gluten during starch gelatinization. Moreover, the presence of organic acids supports the debranching of amylopectin moieties during baking and debranched amylopectin may induce a high level of resistant starch. The resistant starch can in turn surround starch granules and form a physical barrier against enzymatic attack by α-amylase [46,54]. Moreover, the organic acids generated in sourdough fermentation could affect satiety by delaying gastric emptying [41,55].

A further characteristic of bread that should be analysed to understand its impact on glycaemia is the solubility of its constituents. Sourdough fermentation, besides increasing resistant starch content as mentioned above, also leads to increased solubility of dietary fibre and protein, and decrease molecular weight of β-glucans in bread [41,56,57]. Higher solubility and lower molecular weight of dietary fibres could increase their fermentability in the colon, which may contribute to increased satiety [41]. On the other hand, high molecular weight of soluble fibre contributes to increased digesta viscosity, which might reduce gastric emptying rate and the digestion and absorption rate of nutrients [28,41].

3.2. Identification of Pertinent Human Efficacy Studies

3.2.1. Overview of Identified Studies

To answer the three specific research questions about the effect of bread consumption on glycaemia, studies from the literature based on pre-defined PICO criteria were selected. Checking them first only for PIO criteria allowed us to identify 65 studies, from which only 27 were included. The main reasons for exclusion at this step were that the effect of bread was compared to a glucose solution. As we wanted to identify the effect of fermented bread as a whole and not the effect of fibre specifically, studies directly comparing two breads only differing in their fibre content were not included, except for long-term studies, in which a change in glycaemia for each bread separately between start and end of intervention was considered.

A criterion on the study design was not defined, as different types of studies could potentially help answer our questions. However, the studies that were included were exclusively intervention studies. The main reasons for the absence of observational studies, were that bread was often pooled in one category with other cereal products, mostly separated into refined and wholegrain, precluding the analysis of the effect of bread alone, and that the outcome assessed was typically the prevalence of T2D without a direct measurement of blood parameters of glycaemia and insulinaemia.

Population:

Following the EFSA guidelines, we considered studies with healthy participants. However, the population characteristics still varied between the studies as we defined ‘healthy’ as not taking medication (with specific exceptions) and the absence of a diagnosed disease. However, we included studies in participants with a high BMI or those with pre-diabetes. Among the 27 selected studies 23 comprised healthy participants, two studies involved participants with pre-diabetes/impaired glucose tolerance and two healthy and prediabetic (one separately, one as a mixed population).

We found 21 studies from Europe (eleven from Nordic countries), three from North America (Canada), one from Middle East, one from China and one from New Zealand, revealing an uneven distribution of data from different continents, not directly related to the importance of bread in the local nutrition.

The mean size of study population considered was rather small (n= 18.1), ranging from 6 participants to 64 participants. Both sexes were represented, either in mixed populations (24 studies) or in studies with only male participants [31,43] and a unique study focusing on postmenopausal women [58]. This led to a total number of 488 participants, among them at least 237 female and 190 male participants (the exact distribution of sex in several studies was not reported).

We considered only adult populations, and therefore all included studies had participants aged above 18. Still, the mean age and the range of age for participants varied between studies (from 22.1 ± 3.6 up to 60.1 ± 12.1 y.).

Outcomes:

Most studies (n=24) assessed both glucose and insulin or C-peptide, some calculated additional parameters of such as insulin resistance or sensitivity, and a few measured other glycaemia related parameters (such as fructosamine, glucose-dependent insulinotropic polypeptide (GIP) or glucagon-like peptide-1 (GLP-1)). Most studies reported postprandial effects on glycaemic parameters after either the intake of a piece of test bread only or a standardized meal containing the test bread (n=20), while one study reported on similar responses after a standard breakfast following an evening meal with the test bread [59]. Only seven studies reported on long-term effects of daily bread consumption, replacing usual bread and/or cereal products by the test bread within the usual diet.

Intervention:

All selected studies were intervention studies in which participants consumed different kinds of bread, during one meal (post-prandial focus) or over several days (from 1 to 12 weeks). Most studies used a cross-over design.

We could assess different aspects of the effect of fermented bread on glycaemia depending on the control used in the studies. We identified twelve studies that compared yeast fermented with sourdough fermented breads based on the same cereal/flour quality, which allowed the effect of the type of fermentation to be evaluated. Ten studies deliver insights on the general effect of fermentation, comparing fermented bread to unfermented cereal products such as unfermented bread, pasta or porridge. Finally, we identified six studies that used long-term interventions allowing a comparison of glycaemic parameters before and after consumption of the tested breads without control

3.2.2. Impact of the Type of Fermentation on the Glycaemic Effects of Bread

Of the twelve studies comparing a sourdough with a similar yeast bread (Table 1), only one intervention did not consider an acute, postprandial response, instead testing the effect of four weeks of daily consumption of the breads [25]. All other studies assessed the postprandial responses after consuming the test breads, either alone (with only a glass of water) or as a breakfast (including same toppings/drinks between groups) [46,60,61]. Test bread portions were mainly matched for carbohydrate content (50g), whereas in three cases test breads were matched on weight per portion (100g for Maioli et al. [61], 126g for Darzi et al.[60] and 150g for Chatonidi et al. [26]). Most studies considered breads made from wheat flour only (n=8), including whole grain flour, white flour or both. One study assessed the effect of gluten-free breads based on rice, corn and buckwheat [24], one study used mostly barley with 20% wheat [46] and another wheat semolina and 30% corn [61] to produce their breads. In one study the information on the type of flour (white or whole meal) was missing [25]. Considering the characterisation of the breads, all included studies gave some information on the chemical composition of their bread, mostly nutritional composition or, for six studies pH or organic acids [24,26,31,46,60,61]. Only Pagliai et al. reported on microbiological analyses of their sourdough [25]. Scazzina et al. [62] and Liljeberg et al. [46] both analysed the in vitro digestibility of starches from their breads, whereas Maioli et al. [61] reported on microstructural analyses using electron microscopy and Novotni et al. [24] on several physical parameters (specific volume, crumb firmness, viscosity).

All twelve studies measured a glycaemia parameter, three also calculated the GI of tested breads [24,45,47] and seven measured an insulinaemia parameter, either C-peptide [26] or insulin [28,31,42,46,60,61]. Pagliai et al. reported that the change in fasting blood glucose (FBG) after a 4 week-intervention was not significatively different between the two groups, even if FBG increased after consumption of the yeast bread while no significant changes on FBG were observed with the sourdough treatment [25]. Among the postprandial response studies that calculated the GI of breads, differences were observed depending on fermentation method in two studies. An incremental effect was observed by Novotni et al., who tested different quantities of sourdough added to a standard yeast-based recipe and, found a significantly lower GI with 15% and 22,5% of sourdough, when compared to a yeast-fermented bread with no sourdough [24]. Similarly, Borczak et al. found a lower GI for their sourdough bread than the yeast control [45]. In contrast, Fredensborg et al. did not identify significant differences between breads prepared using different leavening agent (backer yeast, desem, sourdough) [47].

Differences in other measures of the glucose response based on bread fermentation method were also reported with only Darzi et al. noting no significant difference between control yeast and sourdough bread. All of the other studies found either a significantly lower overall response for sourdough vs yeast bread [31,42,45,62], or at least at some specific time points [24,26,28] or for partial Area Under the Curve (AUC) [46,61].

From the seven studies reporting on insulinemia response, only Najjar et al. and Darzi et al. did not find any significant difference between breads using sourdough and yeast. The other five studies reported significantly lower insulinemia responses after sourdough than yeast breads, at least for some specific time points during the postprandial response [26,28,42,46,61].

Within this specific question of the effect of type of leavening, only Najjar et al. reported on incretin release, measuring both GIP and GLP-1, which stimulate insulin production. Interestingly, even if in their study no significant differences in insulin release or calculated insulin sensitivity were found, the overall response of GLP-1 was significantly lower for sourdough than for yeast and the second meal response of GIP was also reduced after sourdough compared to after yeast bread.

In total ten of the twelve studies, or five from seven, reported some positive effect on glycaemia, or insulin responses respectively, for sourdough compared to yeast bread consumption.

3.2.3. General Impact of Fermentation on the Glycaemic Effects of Bread

From the ten studies comparing fermented bread to a non-fermented control, four used a non-fermented flat bread [27,41,43,63], four used pasta [35,43,59,64], two porridge [36,39] and one boiled kernel [38] as control products (Table 2). The fermented breads were all produced using yeast, except for one study in which a sourdough bread was compared with an unfermented bread [41). Cereals used for the food item preparation differed between studies, wheat [27,35,39,43,59,64] and rye [38,39,41,63] being the most common, with only one study using barley [36]. Similarly, the content of fibre in the test products differed between studies, some comparing food items with white flour without fibre [27,35] or with addition of dietary fibre [43,59] or whole grain flour [36,38,41,63] or both [39,64]. We only considered comparisons between food items from similar composition (cereal type and fibre content). All studies reported on the nutritional composition of the breads, with more or less details on fibre and starches, and for Zamaratskaia et al., on organic acids [41]. Only Rosen et al. performed an in vitro starch hydrolysis in both their studies [38,39]. Only Eelderink et al. analysed the microstructure of their breads (density, porosity) [43].

All studies assessed the postprandial response after a breakfast containing the test products, and, in the case of Liljeberg et al. [35], also after a standard second meal, with the exception of Nilsson et al., who assessed the response after a standard breakfast following an evening meal made of the test products [59]. Test breads were consumed as part of a breakfast, mainly with cheese, in six studies [35,36,41,43,63,64]; in the four other studies they were only accompanied by a glass of water. Most studies adjusted the quantities of tested food to a defined quantity of available starch (from 30 to 50 g) [27,35,36,38,39,43,59,64].

All ten studies measured a postprandial glucose response, five calculated the GI of tested breads [27,35,36,38,39] and eight also measured the insulin response.

The four studies comparing fermented bread to unfermented bread showed variable results. While two studies did not report any significant difference for the glucose response despite a lower insulin response (AUC) after consumption of the unfermented crisp breads compared to either yeast [63] or sourdough fermented breads [41], Eelderink et al. found a lower insulin response (AUC, different time points, peak value) and a lower glucose peak value after unfermented bread consumption [43]. In contrast, fermented arabic bread was found to have a lower GI than unfermented regag bread [27].

Of the four studies comparing fermented bread to pasta, three reported significantly higher postprandial responses after bread consumption: either in glucose peak value [43], total AUC or time-point values for both refined and whole grains [64], or GI and second-meal responses [35]. Similarly, two of these studies, which also assessed insulin showed significantly lower insulin responses after pasta than after bread consumption either as whole AUC [43] or as insulin index at different time points after the second standard meal [35]. In contrast Nilsson et al. did not find a significant difference, neither in glucose nor in insulin response to a standard breakfast following an evening meal containing the test foods [59]. The time between the evening meal and the test breakfast of the study by Nilsson et al. [59] was however longer than the time between the test breakfast and the standard midday meal of the study by Liljeberg et al. [35], which might explain different results observed. Generally, it seems that pasta is rather more favourable than fermented breads on acute glycaemic responses, which is probably due to their high content of resistant starches.

We found two studies meeting our criteria that compared fermented breads to porridges. Both showed a significantly lower insulin response for the breads in the early phase (AUC 0-30 min.), compared to the porridges, with a lower peak value for endosperm rye bread [39] and a lower insulin index for high-fibre barley bread [36]. Whereas the insulin changes were not accompanied by a significant difference in glucose response for Liljeberg et al. [36], Rosén et al. found lower early response (AUC 0-30 min.) of glucose for their rye breads compared to the rye porridges [39].

Finally, Rosén et al. compared a fermented bread to boiled kernels (both prepared from whole grain rye), and did not detect any significant difference in the glucose response (GI, peak glucose and AUC) but found a significantly higher late insulin response (AUC 120-170 min.) for the kernels [38].

To summarize, the effect of fermented bread compared to unfermented cereal products on glycaemia is not generalizable. Few studies compared fermented with unfermented breads limiting the direct assessment of a generic fermentation effect. Where suitable comparators were described, the effects on glycaemia were conflicting and inconsistent perhaps due to specific characteristics of the test products selected. Whereas, most unfermented breads and pasta produced lower glucose responses, particularly on insulin responses, porridges and boiled kernels could be less favourable than fermented breads. Still for each case, the number of studies is too low to draw a clear conclusion.

3.2.4. The Effect of Fermented Bread Consumption on Glycaemia

Finally, we analysed the overall effect of fermented bread consumption, instead of focusing solely on comparisons with unfermented cereal foods or between different fermentation types.

As no observational study comparing low versus high consumption of bread was identified, applying our other selection criteria, we considered studies for which data was available for before and after a longer intervention were identified (Table 3). Even if effects observed in such before-after comparison without a control group eating no bread or a different quantity are to be interpreted carefully, we chose to comprehensively review all relevant evidence. From the six longer intervention studies identified, most tested at least one wheat-based bread [25,33,44,58,65], two considered high fibre/whole grain rye bread [58,65] and one a foxtail millet bread [37]. Nutritional composition of the bread was given only in three studies [44,58,65]. Organic acids and microbial composition were analysed only in Pagliai et al. [25], whereas Todesco et al. reported upon in vitro starch digestion analyses of their breads [33] and Ren et al. referred to previous work for similar analyses [37].

The period for which the participants had to consume the tested bread daily ranged from one week [33] to twelve weeks [37]. The daily dose of bread was either the same for each participant (from 90 g in Ren et al. [37], to an amount of bread corresponding to 150 g carbohydrate in Todesco et al. [33]), or was adapted to local nutrition guideline [44], or defined as a minimum number of portions [58]. For all studies, the participants were advised to maintain their habitual diet and lifestyle, replacing usual breads and cereals products with the test breads. All studies compared at least two different breads for their effect on fasting blood glucose, some also considered fasting insulin [37,44,58,65] and few also analysed the acute response to an oral glucose tolerance test (OGTT) [37,44], a test meal [65] or a frequently sampled intravenous-glucose-tolerance test (FSIGTT) [58] before and after the intervention.

In most cases, the fermented bread interventions did not lead to a significant change on glycaemic outcomes. However, in one case a negative effect was observed: Pagliai et al. detected an increase in FBG after 4 weeks of the control wheat bread, whereas their sourdough bread did not induce a significant change [25]. In few cases a positive effect was found on some outcomes: Lappi et al. observed a decrease in the late insulin response (at 120 min, after a test meal) and an increase in the calculated disposition index after four weeks of wholegrain sourdough rye bread [65]; Todesco et al. detected a lower glucose response after a standard meal after one week of a propionate-enriched white wheat bread [33]; Ren at al. found, after 12 weeks of a foxtail millet bread, a high impact in subject with IGT with a decrease in FBG and 2h-glucose response to an OGTT as well as a decrease in calculated insulin resistance index (HOMA-IR) [37].

To summarize, except for a few specific breads and specific outcomes, bread consumption over several days or weeks was not associated with significant changes on glycaemia parameters in healthy people. However, it must be pointed out that for all these studies the effects observed were not controlled for as we considered only the change within one group. Moreover, participants did consume bread before joining the respective studies and were just advised to replace it with the test breads. None of the identified studies gave details on the consumption of bread before intervention, which minimises the possible interpretation of the effects observed. This is probably due to the fact that all studies except Ren et al. [37] compared different breads with each other. Observational studies comparing people eating variable quantities of bread would have supported better evidence on this research question but no suitable studies were identified, due to the strict criteria of healthy population and glycaemia outcomes as blood parameters.

3.2.5. Risk of Bias (RoB) Assessment

For the 26 studies using a randomised controlled trial design, we assessed the risk of bias using the Cochrane RoB2 tool. Only two (7.7%) were judged to have a low risk of bias across all domains [24,45]. In contrast, five studies (19.2%) were judged to have a high overall risk of bias. The reasons were mainly deviations from intended interventions [25,27,47], selective reporting of results [65], or both [60]. The majority of studies, 19 (73.1%), were classified as having “some concerns” (Figure 2, Figure 3 and Figure 4). For our first specific question on the effect of the type of fermentation, all studies showed a low risk of bias in measurement outcomes (Figure 2), whereas for the general effect of fermentation the strength of included studies concerned missing outcome data, as all were assessed with a low risk of bias in this domain (Figure 3).

Overall, the most frequent concern was related to selective reporting, due to the absence of a publicly available protocol or a pre-specified statistical analysis plan. Due to the crossover design, some issues were identified in domains related to the randomization process, reporting or control of carryover effects and the adequacy/appropriate choice of the control condition. In addition, some studies also raised concerns about missing outcome data [25,61] and the measurement of outcomes [33,35,36,59].

3.2.6. Evaluating the Certainty of the Evidence from Human Studies

Considering the number of identified relevant studies (12), their risk of bias (majority with some concerns) and the consistency of effects (10 out of 12 showing a protective effect on glycaemia, 5 out of 7 on insulinaemia) we estimate overall level of the evidence for the effect of the type of fermentation as ‘low’ for a beneficial effect of sourdough fermentation compared to yeast fermentation on postprandial glycaemia parameters.

For the general effect of fermentation on postprandial glycaemia, due to a low consistency of observed effects, even if ten human studies were found most of them with only some concerns of risk of bias, we estimate the evidence as ‘very low’. The number of studies for subgroups of control products (unfermented breads, pasta, porridge/kernels) were not sufficient to reach a higher level of evidence, even if the effects observed inside each subgroup were more consistent.

Finally, we found only a few studies that contributed evidence on the overall effect of bread consumption on long term glycaemia/insulinaemia parameters. Most had some concerns for risk of bias and highly inconsistent effects, therefore we conclude to ’very low’ evidence for a general effect on bread consumption on glycaemia/insulinaemia in over-weeks interventions. Still, for this specific question, identified studies did not use an appropriate design to respond to our research question, as discussed above

3.3. Biological Plausibility – Bioavailability and Mechanism of Action

The potential role of commonly consumed fermented cereal-based products like bread, in glycaemic regulation is of increasing relevance for public health, considering the pandemic of T2D. While clinical trials in humans have provided supportive evidence for reduced postprandial glucose levels after the consumption of sourdough or lactic-acid-fermented bread, in vitro and in vivo studies have helped to define/uncover the possible mechanisms that contribute to this process, which can be grouped in (i) changes in bread structure that reduce the accessibility of starch to digestive enzymes, slowing glucose release, (ii) the production of organic acids and other bioactive compounds during fermentation, which lower bread pH and may slow gastric emptying and starch digestion and (iii) the potential modulation of gut microbiota and its metabolites (Figure 5).

3.3.1. Bread Structure and Starch Accessibility

Fermentation chemically and structurally modifies the starch and protein content of bread, influencing digestion kinetics and postprandial glucose dynamics. Fermentation increases the formation of resistant starch fractions, which are less bioavailable for enzymatic hydrolysis, thereby decreasing the amount of rapidly digestible starch and contributing to a lower glycaemic potential [66,67,68]. Furthermore, while gelatinisation during baking typically increases starch digestibility, other processes like fermentation and subsequent cooling promote the formation of resistant starch type 3 (retrograded starch), that further resists enzymatic hydrolysis and thereby can contribute to lower glycaemic potential [69]. Moreover, bread fermented with LAB exhibits a denser crumb structure and altered fibre matrix, both of which are physical properties that reduce enzymatic access to starch substrates in vitro [68].

It has been reported that the type of fermentation and the baking conditions also affect the postprandial glycaemic and insulinemic responses by altering gastric emptying rates, starch digestibility, and nutrient bioavailability [25,29]. Specifically, sourdough fermentation has been associated with changes in the composition and functionality of dietary fibre, releasing of bound phenolics, an increase in organic acid content, and reductions in the amount of simple carbohydrates and starch digestibility. All of these outcomes may delay gastric emptying and affect glucose metabolism [25,31,41].

3.3.2. Organic Acids from Fermentation Reduce pH and Delay Starch Digestion

Additionally, several in vitro models simulating human digestion have shown that organic acids produced during fermentation, mainly lactic and acetic acid, slow starch hydrolysis by inhibiting amylase activity and reducing the rate of glucose release [62,70,71]. Besides those effects on carbohydrate digestibility, organic acids produced by fermentation, including several short-chain fatty acids (SCFA), can act on gastric emptying and affect appetite regulation. For example, acetate has been shown to cross the blood-brain barrier and activate hypothalamic pathways that regulate appetite and energy expenditure [72]. This leads to increased satiety hormone levels, reduced food intake, and slower gastric emptying, contributing to more gradual postprandial glucose rises. In an older rodent study, lactic acid was also found to modulate postprandial plasma glucose concentration through delayed gastric emptying due to increased secretin and cholecystokinin gut release [73,74].

Fermentation does not only change the carbohydrate fraction of the cereal. It increases protein digestibility through their degradation by cereal proteases, activated by the organic acids produced by LAB and yeast [23]. This can lead to the formation of bioactive peptides such as ACE-inhibitory peptides, whose primary function is to help lower blood pressure, but they may also influence glucose metabolism by improving insulin sensitivity and glucose uptake and reducing oxidative stress and inflammation, both of which are implicated in insulin resistance [75] and kokumi peptides, with anti-inflammatory and hypoglycaemic effects [76]. Other products of fermentation include free amino acids, some of which are precursors to bioactive compounds such as 4-hydroxyphenyllactic acid, 2-hydroxy-isocaproic acid, vanillic acid, and 3-phenyllactic acid or γ-aminobutyric acid, which might also impact diabetes risk through their antioxidant activities [77].

3.3.3. Modulation of Gut Microbiota and Its Metabolites

The final mechanism that has been proposed to participate in the effect of sourdough bread in glucose regulation is the gut microbiota and its metabolites that are affected by dietary exposure to sourdough bread. Both undigested carbohydrates from cereals but also exopolysaccharides formed by the LAB during fermentation [78] can be an energy source for the gut microbiota, which can metabolise them into SCFA. In vitro fermentation experiments using human faecal inocula have demonstrated that sourdough and LAB-fermented breads yield higher SCFA concentrations compared to conventional breads, with coarse wholemeal rye and wheat-rye breads showing particularly favourable SCFA production patterns [79,80]. These SCFAs, especially butyrate and propionate, have been shown in cell culture to activate AMP-activated protein kinase (AMPK) in skeletal muscle, enhancing GLUT4 translocation and glucose uptake. They also stimulate the secretion of incretin hormones such as GLP-1 and Peptide YY (PYY) from enteroendocrine cells [81] through activation of free fatty acid receptors (FFAR2/3). FFAR2/3, found also in vascular tissue, adipose tissue, pancreas and other organs are associated with improved glucose homeostasis, enhanced insulin sensitivity and modulation of blood pressure. This association has been largely established through gain of function (overexpression) or loss of function (knockout) in transgenic rodents or cell models, often with SCFA administration to activate the receptors, showing that FFAR2/3 play beneficial roles in glucose and insulin regulation [82,83]. Activation of these receptors has also been linked to elevated plasma leptin and reduced non-esterified fatty acid levels [60,61,74]. Moreover, improved endothelial function, partially mediated by SCFA and stimulating nitric oxide production, is closely linked to better vascular insulin sensitivity, which facilitates glucose uptake by tissues [84].

The impact of sourdough on the gut microbiota composition and metabolites have been investigated principally using in vitro systems to simulate food-microbiome interactions [85], with several animal studies, some investigating fermented bread [86,87] while others focus on the effects of isolated sourdough cultures [88] but there is little evidence from human studies [30]. Despite the potential mechanisms by which fermented bread could affect the gut microbiota to improve glucose regulation, in vivo data is inconsistent. Shifts in specific microbial groups were observed for two studies that supplemented mice [87] and rats [86] with fermented bread (sourdough and yeast-fermented) compared to the control group but the effects on glycemia both pre and post meal were minimal for all groups. Consistent changes in the microbiota were found for sourdough and yeast-fermented bread interventions compared to the control within these two studies, but the taxa differed between the studies. Moreover, significant differences were observed in both studies for Mucispirillum genus comparing the two types of fermented breads but the genus was higher for mice in the sourdough group than those in yeast-fermented bread group in the mice study [87], while reduced for rats in the sourdough group relative to the yeast-fermented bread group [86]. Rats fed with diets incorporating sourdough-fermented breads showed significantly improved postprandial glucose profiles compared to those consuming conventionally leavened bread [89]. The study reported reductions in glycemia and insulinemia, as well as lower levels of pro-inflammatory cytokines, suggesting broader metabolic benefit associated with the consumption of sourdough-fermented bread. These results highlight that sourdough fermentation may not only modulate glycemic response but also exert anti-inflammatory effects and improve overall metabolic health in animal models.

Similarly to the described animal studies, effects of sourdough bread on the gut microbiota have been observed in human studies but not in parallel to changes in glycemia. Notably, Korem et al. found significant increases after white bread consumption, relative to sourdough bread consumption on two butyrate producers, (Eubacterium ventriosum species and the Anaerostipes genus), in a short-term crossover intervention [30]. High intraindividual variability in glucose response to the different breads was observed in this study but surprisingly the direction of response (higher in sourdough or yeast-fermented bread) could reliably be predicted from features of the baseline microbiome data. These findings indicate that not only can the gut microbiome be altered by fermented bread to affect glucose homeostasis but the current gut microbiota may play a critical role in mediating the response to different bread interventions.

The role of SCFA in glucose regulation is widely investigated. For example, it has been reported that propionate suppresses gluconeogenesis in hepatocytes by inhibiting phosphoenolpyruvate carboxykinase (PEPCK), while butyrate enhances glycogen synthesis, both contributing to improved postprandial glucose regulation [90,91]. Moreover, in isolated pancreatic islet cells from rodents, butyrate showed the most pronounced effect on inducing insulin secretion in comparison to other SCFA tested [92].

3.3.4. Conclusions on Mechanisms of Action

Together, in vitro and in vivo studies provide mechanistic evidence that supports and complements clinical research on the glucose-lowering potential of fermented breads. By modulating the rate and extent of starch digestion and influencing host metabolic and inflammatory pathways, fermented breads, especially those produced using traditional sourdough or lactic acid, could be a promising dietary intervention for glycaemic control. However, the impact of fermented bread on glycaemia appears to both influence and be influenced by additional individual factors such as the gut microbiome composition itself that should be considered in interventions assessing the role of fermented bread on glycaemia [93]. Additionally, the role of the upper-intestinal tract in glycaemic response is still to be understood. Different food structures have been proven to elicit different postprandial gut hormones and satiety responses via GLP-1, PYY [94]. This suggests that dietary effects might be more complex and interconnected than initially suspected and that it would require integrating person-specific factors with food characteristics.

3.4. Safety of Fermented Breads

None of the studies included in this review reported major safety concerns related to fermented bread intake. Mild, non-serious adverse effects were occasionally observed, such as the flatulence noted by Eelderink et al. [43], which was not linked to any specific test meal and was deemed clinically irrelevant. Similarly, Rave et al. recorded 30 gastrointestinal events during their intervention (15 during wholegrain treatment, 13 with meal replacement, and two during the washout phase), though these effects were attributed to bread ingredients rather than the fermentation process [32]. Three participants experienced transient constipation early in the wholegrain treatment, which resolved without consequence, and no such effects were reported during the meal replacement phase. Available evidence does not indicate any safety issues associated with bread consumption.

From a technological perspective, sourdough fermentation lowers the dough pH and produces organic acids, creating an environment that inhibits the growth of spoilage and pathogenic microorganisms [95]. Moreover, fermentation can reduce levels of potentially harmful compounds such as mycotoxins and acrylamide precursors, further improving product safety [96]. Furthermore, sourdough fermentation can mitigate acrylamide formation by lowering dough pH, which inhibits the Maillard reaction, and by promoting the activity of LAB and yeast that degrade free asparagine, a key acrylamide precursor [97,98]. Studies have demonstrated that the use of specific sourdough starters or extended fermentation time can significantly reduce acrylamide concentrations in bread compared with yeast-leavened controls, with reductions of up to 70% reported in some formulations [99]. Nevertheless, as with other fermented foods, inadequate process control or inappropriate storage conditions may permit the accumulation of biogenic amines or survival of undesirable microbes, highlighting the importance of maintaining good manufacturing practices and controlled fermentation parameters [100].

Other safety issues should be considered for cereal-based foods, particularly wheat foods, such as their potential allergenicity and inflammatory potential. The extent to which fermentation, particularly sourdough, can influence these risks was not the focus of our study and is reviewed elsewhere [12].

3.5. Characterization of the Relationship Between Consumption of Fermented Bread and Functional Effects

Fermented breads, particularly those using sourdough starters with LAB and yeasts, have been studied widely for their potential to modulate postprandial glycaemic and insulinaemic responses. Human dietary intervention studies mostly show that sourdough bread tends to slightly reduce postprandial glucose and/or insulin levels compared to conventional yeast-leavened bread. According to the findings from these studies, sourdough bread is associated with delayed gastric emptying, reduced starch hydrolysis, and lower availability of quickly digestible sugars. These effects are primarily observed in short-term postprandial settings, while longer-term effects remain unclear due to study design limitations. A consistent tendency is observed toward lower glycaemic and/or insulinemic responses after consuming sourdough bread, as demonstrated in acute intervention studies [24,46,61,62]. The mechanisms proposed for these effects include acid-induced delay of gastric emptying and inhibition of amylolytic enzymes, as demonstrated by studies with 13C-octanoic acid labelling [26,101]. According to some studies, such as those conducted by Chatonidi et al. [26] and Liljeberg et al. [46], breads with similar acid contents, whether from fermentation or direct acid addition, produced comparable metabolic responses, highlighting the role of fermentation by-products. However, only part of the selected studies did analyse their test bread for acidity-related parameters, some only reporting pH, or one specific organic acid. This lack of standardized information on the bread characteristics impairs a detailed comparison of studies and reduce the strength of evidence on the relationship between these parameters and effects on glycaemia response.

Beyond the reductions in glucose and insulin peaks, some studies [26,65,101] reported lower C-peptide levels and altered insulin dynamics, suggesting potential improvements in insulin sensitivity underlying the glycaemia changes. However, not all studies have reported consistent outcomes. For example, Dall’Asta et al. found no significant differences in metabolic outcomes, potentially due to variations in flour type, fermentation duration, or microbial strains involved [42]. The lack of reporting on the microbial composition of sourdough used or in bread dough after fermentation in several studies however again impairs comparison between studies and a strong conclusion on its exact effect on observed outcomes.

Dose-response effects of bread consumption were not directly explored in identified studies. Novotni et al. demonstrated a non-linear dose-response, where moderate amounts of sourdough ferment used for bread preparation reduced GI more effectively than bread with higher doses of ferment, possibly due to changes in food matrix viscosity, showing the importance of the structure characterisation of tested breads [24]. Still structural characterisation was absent from most of the selected studies. Liljeberg et al. found that the presence of lactic and propionic acids at defined concentrations significantly lowered glucose and insulin responses, mimicking the effects of sourdough fermentation [46].

Despite the relatively consistent findings in acute interventions, only few studies were identified that assess long-term glycaemia/insulinaemia effects, making it difficult to extrapolate findings to habitual consumption patterns. Moreover, findings from these studies were not consistent, and poorly controlled for factors relevant to our research question. The effects observed in acute interventions were mostly of a small magnitude and thus may not translate to a significant impact on the fasting measures of glycaemia and insulinaemia in relatively short exposures used in the ‘long-term’ intervention studies that we identified. Moreover, while dynamic and variable responses are observed in postprandial glycaemia measures, fasting measures of glycaemia are tightly regulated in healthy individuals reflecting a physiological homeostatic control that should not be perturbed by short-term dietary intervention. Observational studies, considering changes over years of consumption, are better suited to assess the long-term impact of fermented breads. However, we could not identify any such study addressing our blood-measurement based outcomes of interest while also offering sufficient granularity on the dietary assessments to permit such investigation. Bread formulations also vary significantly across studies, where described, with different flour types, fermentation times, pH levels, and inclusion of additives (e.g., gluten, fat, salt), which makes direct comparisons and meta-analysis difficult.

4. Conclusion

Overall, considering the well-studied food characteristics and mechanisms of action there is some evidence for a functional benefit of sourdough fermented bread, in offering modest reduction of postprandial glycaemic and/or insulinemic responses. However, the available evidence remains neither convincing nor sufficient to fulfill the requirements for scientific substantiation according to EFSA standards for a claim about positive impact on glycaemia/insulinaemia maintenance, due to the absence of observational studies, lack of dose-response trials, and insufficient long-term data. For the effect of fermented bread intake in general (compared to non-fermented cereal product or over a longer intervention period), mainly due to inconsistency in observed effects, there is currently no or only very low evidence for a positive effect on glycaemia/insulinaemia homeostasis.

To advance the scientific substantiation of health benefits associated with fermented bread, future studies should focus on long-term (>= 4 weeks) randomized controlled trials with well-characterized (i.e., structural characteristics, for sourdough bread microbial composition of sourdough/dough as well as pH and organic acids content), well described bread formulations and preparations and an adequate number of participants to observe a change in the selected outcome. The inclusion of mechanistic endpoints (e.g., gastric motility, enzymatic hydrolysis, gut microbiota) in such controlled studies could help understand disparity between studies. Finally, data from observational, prospective cohort studies that assess real-world outcomes such as T2D incidence should be used to assess whether the observed short-term metabolic effects in healthy individuals translate into meaningful health benefits at the population level.