Bowel Preparation for Colonoscopy in Patients with Diabetes Mellitus – a Gap We Have to Bridge: A Review

1. Introduction

Colonoscopy remains the cornerstone diagnostic and therapeutic intervention in gastroenterology, serving as a pivotal tool for the detection and resection of precancerous lesions, including adenomas, as well as for the diagnosis and management of various colonic pathologies. The efficacy of colonoscopy is inextricably linked to the adequacy of bowel preparation, which is essential for optimal mucosal visualization. High-quality bowel cleansing has been unequivocally associated with improved adenoma detection rates, a reduced likelihood of missed lesions, and a diminished need for repeat procedures [1]. Conversely, inadequate bowel preparation compromises diagnostic accuracy, prolongs procedural duration, elevates the risk of procedural complications, and exacerbates patient discomfort, collectively imposing a substantial economic and logistical burden on healthcare systems [2]. Colonoscopy does not detect all colonic lesions, with a missed adenoma detection rate of 26% [3].

Despite continuous refinements in bowel preparation regimens, achieving optimal colonic cleansing remains a formidable challenge, particularly in specific high-risk patient populations, among which individuals with diabetes mellitus (DM) constitute a distinct subgroup. Diabetic patients exhibit a disproportionately high risk of suboptimal bowel preparation due to a constellation of pathophysiological mechanisms, including delayed gastric emptying (gastroparesis), autonomic neuropathy, and altered colonic motility, all of which contribute to prolonged intestinal transit and impaired bowel cleansing. Compared with non-diabetic patients, persons with diabetes have slower gastric emptying, colonic transit, and colon evacuation [4]. Moreover, systemic factors such as fluid and electrolyte imbalances, dietary restrictions, and the complexities of antihyperglycemic therapy modifications further compound the challenges associated with bowel preparation in this patient population.

According to the International Diabetes Federation (IDF), the IDF Diabetes Atlas (2021) reports that an estimated 537 million adults aged 20–79 years are currently living with diabetes, a figure projected to escalate to 643 million by 2030 and 783 million by 2045 [5]. Concurrently, colorectal cancer remains one of the most prevalent malignancies worldwide, ranking as the second most common cancer among women and the third most common among men [6].

Bearing in mind epidemiological data on the prevalence of diabetes and colorectal cancer in the world, and the importance of colonoscopy as a screening method, the goals of our article are as follows:

1. search the literature on the topic of bowel preparation for colonoscopy in diabetics;

2. to determine whether the currently valid guidelines of the most important Gastroenterology associations contain specific instructions on bowel preparation for colonoscopy;

3. to see if there are possibly insufficiently detailed recommendations of the current guidelines and to point out the importance and specifics of bowel preparation for colonoscopy in diabetics.

2. Materials and Methods

Regarding the fact that this article is a review article, we used the PubMed electronic database for the literature search.

To collect literature for our first research aim, we used the keywords "diabetes mellitus" AND "colonoscopy" AND "bowel preparation", and 57 articles were found. For literature selection, we have used the PubMed filter option and selected only the following types of studies, such as: Clinical Trial, Controlled Clinical Trial, Guideline, Meta-Analysis, Multicenter Study, Observational Study, Practice Guideline, Books and Documents, Randomized Controlled Trial, Review, Systematic Review.

Specific inclusion criteria were involving only human studies with adult and young study population. Language other than English was not a limitation. The exclusion criteria were as follows: all other types of literature and articles listed among the search PubMed options, in vitro or animal studies, and articles without full-text availability. The full texts of manuscripts that appeared potentially relevant for our article were obtained and evaluated by the both authors.

To collect the literature for our second study aim, we searched for current guidelines using the websites of important global Gastroenterology associations and/or organizations.

3. Results

For our first aim, the article search identified twenty two relevant full text articles from the PubMed electronic database. A twenty of twenty two studies that met the full inclusion criteria for this article were retrieved and fully reviewed.

Summarizing all articles, five review articles, two meta-analyses, seven randomized controlled trials, two multicenter studies, and four cohort studies were included in this review [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]. The first study on this topic was published in 2009 by Ozturk NA. et al. who concluded that optimal bowel cleansing is poorer in diabetics with autonomous neuropathy than in those without autonomous neuropathy and non-diabetics controls [26]. The same author published another clinically controlled study in 2010, where they investigated the safety and tolerability of sodium phosphate in diabetics, and concluded that colon preparation for colonoscopy is proportional to the duration of diabetes and the presence of late complications [25]. Hayes A. et al. in 2011 found that colon preparation is better achieved with specialized protocols than with standardized ones [24]. Rotondano G. et al. confirmed in 2015 that diabetes mellitus is independent predictors of inadequate bowel cleansing, both at the level of right and left colon, in the overall population [23]. Kim YH. et al. in 2017 published the results of randomized controlled trial and the conclusion was that diabetic patients had a worse preparation quality and longer cecal intubation and total procedure time compared with non-diabetic patients [22].

In subsequent years, from 2019 to 2025, published articles have shown that diabetes mellitus is an independent risk factor/predictor of inadequate bowel preparation [7,8,12,14,16,17,18]. Bearing in mind that the results of this are provided by randomized clinical research and meta-analysis with a large number of patients, the value of such evidence should be considered significant.

See the results in the Table 1.

For our second goal, we identified three articles/guidelines from: European Society of Gastrointestinal Endoscopy (ESGE) Guideline, US Multi-Society Task Force on Colorectal Cancer: American College of Gastroenterology, American Gastroenterological Association, and the American Society for Gastrointestinal Endoscopy, and Canadian Association of Gastroenterology.

For the results, please see Table 2.

4. Discussion

Globally, there is an alarming rise in the incidence of diabetes mellitus and colorectal cancer, marking both conditions as critical public health issues [5,6,30]. Diabetes mellitus, particularly type 2, is escalating in prevalence primarily due to modern lifestyle factors such as poor dietary habits, obesity, reduced physical activity, and increasing lifespan [31]. Concurrently, colorectal cancer remains one of the leading causes of cancer-related morbidity and mortality worldwide, exhibiting steadily rising incidence rates [6,30]. Recent studies consistently document an increased incidence of colorectal cancer among patients with diabetes compared to the non-diabetic population [32]. Several biological mechanisms have been suggested to explain this correlation, including hyperinsulinemia, insulin resistance, chronic inflammation, oxidative stress, and significant alterations in gut microbiota [33]. These metabolic disturbances create an environment conducive to carcinogenesis, amplifying CRC risk among diabetic patients. Colonoscopy represents a cornerstone of colorectal cancer screening and prevention. However, its efficacy is highly dependent on optimal bowel preparation. Poor bowel preparation significantly compromises the diagnostic accuracy of colonoscopies, prolongs procedure durations, increases healthcare costs, and necessitates repeat procedures [2]. Diabetic patients particularly struggle with adequate bowel preparation, primarily due to complications like gastrointestinal dysmotility, gastroparesis, and altered colonic transit times [4]. The adequacy of bowel preparation is often evaluated using the Boston Bowel Preparation Scale (BBPS). BBPS is a standardized, validated scoring system designed to assess bowel cleanliness in three colonic segments: the right colon, transverse colon, and left colon. Each segment is rated from 0 (inadequate, mucosa not visible) to 3 (excellent, mucosa clearly visible), resulting in a total score ranging from 0 to 9. Scores equal to or greater than 6, with at least 2 points per segment, indicate sufficient preparation quality [4]. Given the substantial diabetic population and their increased colorectal cancer risk, optimizing colonoscopy efficacy through improved bowel preparation is crucial. Surprisingly, current literature and guidelines specific to bowel preparation in diabetic patients are sparse. A comprehensive search of the medical databases yielded only 20 studies addressing the specific challenge of bowel preparation among diabetic patients undergoing colonoscopy [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]. This limited volume of research is particularly concerning given the high global prevalence of both diabetes and colorectal cancer, highlighting a significant gap in clinical evidence. In light of this evidence gap, we advocate strongly for further research dedicated to the specific needs and physiological challenges of diabetic patients in bowel preparation. Enhanced research outcomes could provide critical insights that would inform and necessitate a revision of existing clinical guidelines. Specifically, targeted guidelines for diabetic patients would support improved clinical outcomes, patient safety, and cost-effectiveness. Although Canadian researchers previously addressed this concern and published recommendations aiming to improve bowel preparation among diabetic patients, these recommendations have unfortunately not been integrated into daily clinical practice nor formally incorporated into broader clinical guidelines [34]. This underscores the need for an active implementation strategy, ensuring recommendations transition effectively from research findings into routine practice. Ultimately, the revision of current bowel preparation guidelines to explicitly accommodate diabetic patients is imperative. Implementation of evidence-based, diabetes-specific bowel preparation protocols is critical to enhancing the diagnostic accuracy and effectiveness of colonoscopies, thereby facilitating timely CRC detection, better clinical management, and improved outcomes for this high-risk patient group.The following text addresses the key issue to this topic.

4.1. Pathophysiological Mechanisms Affecting Bowel Preparation in Diabetic Patients

The mechanisms contributing to inadequate bowel preparation in diabetic patients are multifactorial. Gastroparesis, a frequent complication of long-standing diabetes, leads to delayed gastric emptying and altered intestinal motility, which in turn disrupts the efficacy of bowel cleansing solutions [35]. Autonomic neuropathy, another common consequence of diabetes, further impairs gastrointestinal motility, reducing colonic peristalsis and contributing to inefficient evacuation of bowel contents. Additionally, colonic dysbiosis associated with diabetes, characterized by alterations in gut microbiota composition, may further impact bowel preparation efficacy by influencing intestinal transit and fluid absorption [36].

Beyond motility disturbances, metabolic factors play a crucial role. Diabetic patients often experience dehydration due to polyuria associated with hyperglycemia, which can exacerbate fluid shifts induced by bowel cleansing agents. Electrolyte imbalances, particularly sodium and potassium disturbances, may further hinder colonic motility and increase the risk of adverse events associated with bowel preparation. Moreover, the need for dietary modifications prior to colonoscopy can be particularly challenging for diabetic patients who must carefully manage blood glucose levels. The requirement for a low-residue diet followed by a clear liquid diet may lead to fluctuations in glycemic control, increasing the risk of hypoglycemia or hyperglycemia.

4.2. Impact of Antihyperglycemic Medications on Bowel Preparation

Diabetes pharmacotherapy presents another layer of complexity in bowel preparation. Many antihyperglycemic agents have implications for fluid balance, gastrointestinal motility, and metabolic stability, necessitating careful medication adjustments in the peri-colonoscopy period. The Canadian Association of Gastroenterology provides specific recommendations regarding the management of these medications:

• Metformin should be discontinued upon initiation of a clear liquid diet due to its association with lactic acidosis risk [37].
• GLP-1 receptor agonists, which delay gastric emptying, should be withheld if a once-weekly dose is scheduled within two days before colonoscopy. This class of medications may exacerbate delayed bowel transit and contribute to inadequate cleansing [38].
• DPP-4 inhibitors should be omitted on the morning of the procedure, as they have minimal risk of hypoglycemia but may still interact with fasting metabolism [39].
• SGLT-2 inhibitors should be stopped three days before colonoscopy to reduce the risk of dehydration and euglycemic ketoacidosis [40].
• Insulin therapy should be carefully adjusted, with dose reductions or omissions as appropriate, to prevent hypoglycemia during fasting [34].

These recommendations underscore the necessity for individualized bowel preparation protocols in diabetic patients, as the interplay between medication use, glycemic control, and bowel cleansing agents poses a unique set of challenges.

4.3. Clinical Implications of Inadequate Bowel Preparation

Suboptimal bowel preparation in diabetic patients not only diminishes procedural efficacy but also increases the likelihood of requiring repeat colonoscopies, thereby exposing patients to additional procedural risks and healthcare costs. Studies have consistently demonstrated that diabetic patients are nearly twice as likely as their non-diabetic counterparts to exhibit inadequate bowel preparation, with reported prevalence rates of up to 25% [2].

A poorly prepared bowel significantly reduces the adenoma detection rate, a key quality metric in colonoscopy, potentially leading to missed precancerous lesions. Given the increased risk of colorectal neoplasia in individuals with type 2 diabetes, failure to achieve adequate bowel preparation may delay the detection of malignancies, ultimately impacting colorectal cancer outcomes [41]. Furthermore, prolonged procedural times associated with inadequate preparation increase patient discomfort, elevate sedation requirements, and contribute to higher complication rates, including perforation and post-procedural bleeding.

4.4. The Burden of Diabetes and Colorectal Cancer

The importance of optimizing bowel preparation among diabetic patients is underscored by current global epidemiological patterns. According to the International Diabetes Federation (IDF) Diabetes Atlas (2021), approximately 537 million adults aged 20–79 currently have diabetes. This number is projected to rise significantly to 643 million by 2030 and further to 783 million by 2045 [5]. At the same time, colorectal cancer (CRC) continues to be among the most widespread cancers globally, ranking second in incidence among women and third among men. CRC represents about 10% of all new cancer cases and cancer-related deaths each year [6].

Notably, type 2 diabetes has been identified as a risk factor for colorectal adenomas-the benign neoplastic precursors to CRC-predominantly in populations of White/European ancestry [41]. The intersection of diabetes and colorectal cancer risk highlights the critical need for ensuring high-quality colonoscopy preparation in diabetic patients, as inadequate visualization may result in missed lesions, thereby exacerbating cancer-related morbidity and mortality. Patients with colorectal cancer (CRC) and concurrent type 2 diabetes mellitus (T2DM) experience a higher economic burden compared to non-diabetic patients. Notably, individuals with both T2DM and Stage II CRC face a significantly greater economic burden, while surgical patients exhibit a substantially higher disease burden than those managed non-surgically. Efforts should focus on both primary and secondary prevention strategies to mitigate the financial burden associated with colorectal cancer [42].

4.5. The Need for Standardized and Evidence-Based Guidelines

A growing body of research highlights the inconsistencies in current bowel preparation guidelines for diabetic patients. Many of the available protocols are generalized and do not address the heterogeneity in diabetes management, comorbidities, and treatment strategies. The physiological challenges posed by diabetes necessitate a more structured and targeted approach to bowel cleansing. Furthermore, newer bowel preparation regimens, including split-dose and low-volume polyethylene glycol-based solutions, need further evaluation in diabetic populations to determine their efficacy and tolerability.

Future studies should also explore the role of patient education in improving adherence to bowel preparation instructions. Misinterpretation of dietary and medication guidelines remains a significant factor contributing to inadequate cleansing. Structured educational interventions, including pre-procedural counseling, written instructions, and digital resources, may improve compliance and preparation quality in diabetic patients.

5. Conclusions

A dedicated, prospective, multicenter clinical investigation is critically needed to systematically evaluate the comparative efficacy, tolerability, and safety profiles of diverse bowel preparation regimens specifically in diabetic patients undergoing colonoscopy. Given the complex pathophysiological mechanisms unique to diabetic populations, including gastroparesis, autonomic neuropathy, altered gastrointestinal motility, fluid-electrolyte imbalances, and medication-related challenges, research efforts must focus on refining existing bowel cleansing strategies and exploring innovative preparations that effectively address these specific issues. Future studies should rigorously examine the effectiveness of split-dose and low-volume polyethylene glycol based preparations, alongside novel osmotic and stimulant laxatives specifically adapted for diabetic patients with compromised gastrointestinal function. In parallel, comprehensive evaluations should be undertaken to assess the impact of structured patient education interventions and adherence enhancement strategies, including interactive multimedia educational tools, patient counseling, and personalized medication adjustment protocols. These measures have significant potential to improve patient compliance and, consequently, bowel cleansing outcomes. Moreover, investigations into emerging bowel preparation solutions designed explicitly for diabetic patients with impaired gastrointestinal motility should be prioritized. Innovations such as prokinetic adjunct therapies, novel formulations improving intestinal transit, and patient-specific dosing schedules should be explored and rigorously validated through randomized controlled trials. These tailored bowel preparation protocols could notably enhance the quality of mucosal visualization, adenoma detection rates, and overall diagnostic accuracy. The urgent development and formal implementation of comprehensive, evidence-based bowel preparation guidelines explicitly tailored for diabetic patients are imperative. Such guidelines should integrate evidence from robust clinical trials, meta-analyses, and systematic reviews, ensuring standardized, practical, and individualized protocols that are feasible for broad clinical adoption. This targeted approach would substantially enhance procedural efficiency, reduce healthcare-associated costs, and markedly improve patient safety, satisfaction, and clinical outcomes. The development of an evidence-based, standardized bowel preparation protocol specific to diabetic patients would not only enhance procedural safety and diagnostic yield but also improve long-term colorectal cancer prevention. As global diabetes prevalence continues to rise, optimizing colonoscopy effectiveness among diabetic patients becomes increasingly critical for effective colorectal cancer screening and prevention. Therefore, prioritizing focused clinical research and evidence-based guideline development aimed explicitly at addressing diabetic patients' unique bowel preparation challenges must remain a paramount objective within gastroenterological practice and healthcare policy.