Black Ginger (Kaempferia parviflora) for Nutraceuticals: From Methoxyflavone Marker Standardisation to Pharmacokinetics and Human Evidence

1. Introduction

Running Title: Black Ginger: Markers, Pharmacokinetics and Human Evidence

Kaempferia parviflora Wall. ex Baker, known as black ginger, Thai ginseng or krachai dam, is a member of the Zingiberaceae whose dark purple rhizome has been used in Thai and neighbouring folk medicine for vitality, fatigue, sexual function and gastrointestinal complaints [1,2]. Its extracts are now ingredients of capsules, functional drinks and powders sold across Asia, and they have been tested in human trials of physical fitness, body fat, energy expenditure and male sexual health [3,4]. The characteristic constituents are methoxyflavones, especially 5,7-dimethoxyflavone, 5,7,4’-trimethoxyflavone and 3,5,7,3’,4’-pentamethoxyflavone, which are used as analytical markers and are considered the main active compounds [1,2].

Three types of review exist. Chen and colleagues summarised the chemistry and pharmacology of the plant and its methoxyflavones [2]. Saokaew and colleagues systematically reviewed seven clinical studies and judged the evidence promising but inconclusive because of small trials [3]. Na Takuathung and colleagues meta-analysed animal and human studies and reported benefits for fasting glucose, sexual function and physical performance [4]. None of these asks a prior question: are the extracts used in the trials chemically comparable, and how much of the presumed active constituents reached the participants?

This question is the central concern of herbal quality control. Fernandes and colleagues recently reviewed flavonoids as markers in herbal medicine quality control and emphasised that marker selection should move from single-compound assays towards fingerprints and quality markers linked to effectiveness and pharmacokinetics [5]. Two black ginger studies illustrate why this matters: a pharmacokinetic study of an ethanol extract in healthy volunteers [1] and a randomised trial of a functional drink made with a water extract [6]. Both report doses of 90 and 180 milligrams of extract, but, as shown below, these doses are not chemically equivalent.

The aim of this review is to connect the three levels of evidence, namely marker chemistry and standardisation, pharmacokinetics and formulation, and human outcomes, and to propose how black ginger products and trials should be specified so that results can be compared and reproduced. The perspective is that of a manufacturer of botanical extracts.

2. Methods of Literature Selection

This is a narrative review. The three source publications provided by the author [1,5,6] were read in full. Further literature on Kaempferia parviflora chemistry, analysis, pharmacokinetics, formulation, safety and human trials was identified through searches of the Europe PubMed Central database using the plant name combined with terms for each topic, and from the reference lists of existing reviews [2,3,4]. Bibliographic details were verified against the Crossref registry. Marker doses delivered in the two fully examined trials were calculated from the compositions reported by their authors. No quantitative synthesis was attempted.

3. Methoxyflavones as Markers

3.1. Composition and Natural Variation

The rhizome contains more than a dozen methoxylated flavones and flavanones. A validated gas chromatographic method quantified 11 of them in rhizomes from 12 origins and found total flavonoid content ranging from 23.9 to 61.0 milligrams per gram, with 5,7-dimethoxyflavone (up to 21.7 milligrams per gram) and 5,7,4’-trimethoxyflavone (up to 9.9 milligrams per gram) as the major constituents; two distinct chemical patterns were distinguished by the ratio of these two compounds [7]. Raw material therefore varies more than twofold in total marker content and differs in marker profile, before any extraction step.

3.2. Variation Introduced by Extraction

The extraction solvent amplifies this variation, because the methoxyflavones are lipophilic and poorly soluble in water [8]. The two source trials provide a direct comparison (Table 1). The pharmacokinetic study used an ethanol extract obtained at 5.7 per cent yield and reported that each capsule containing 90 milligrams of extract delivered 13.4 milligrams of pentamethoxyflavone and 10.4 milligrams of trimethoxyflavone [1], which corresponds to about 15 and 12 per cent of the extract mass. The functional drink trial used a water extract containing 2.63 per cent dimethoxyflavone, 0.86 per cent trimethoxyflavone and 0.48 per cent pentamethoxyflavone [6]. Ninety milligrams of this extract therefore supplied about 0.43 milligram of pentamethoxyflavone and 0.78 milligram of trimethoxyflavone, roughly one-thirtieth and one-thirteenth of the amounts in the ethanol extract capsule. A trial using purified polymethoxyflavone, by contrast, specified a daily dose of 12 milligrams of polymethoxyflavone [9]. Expressing doses as milligrams of extract, as is common, conceals differences of more than an order of magnitude.

A second issue concerns the finished product. The drink trial reported methoxyflavone concentrations in the finished drinks of 0.054 and 0.066 micrograms per millilitre of dimethoxyflavone for the 90- and 180-milligram drinks [6]. From the stated extract composition, 90 milligrams of extract in 80 millilitres would be expected to give about 30 micrograms per millilitre, and the 180-milligram drink would be expected to contain twice as much as the 90-milligram drink rather than about 20 per cent more. Whether the difference reflects a reporting error, poor dispersion of the lipophilic markers in a water-based drink, or losses during heating and carbonation cannot be determined from the publication. It illustrates the value of assaying markers in the finished product and reporting them per serving.

3.3. Selecting Quality Markers

Fernandes and colleagues describe quality markers as constituents that are traceable from raw material to product, measurable, related to effectiveness, specific to the plant, and appropriate to the formulation, and they note that marker content is sensitive to extraction, storage and matrix [5]. For black ginger, the three main methoxyflavones meet these criteria reasonably well (Table 2). They are characteristic of the species, measurable by liquid or gas chromatography [1,7], and linked to activity: dimethoxyflavone and related 7-methoxyflavones inhibit phosphodiesterase type 5 [10], and purified polymethoxyflavones reduced visceral fat in a randomised trial [9]. Pentamethoxyflavone and trimethoxyflavone are also the compounds detected in human blood [1], so they connect product content with systemic exposure, an element of the content–pharmacokinetics–pharmacology approach to quality-marker selection [5].

A single marker is not sufficient. Because raw materials show distinct marker ratios [7], a specification should include the three main markers individually, their ratio, and a chromatographic fingerprint that confirms identity and detects substitution with other dark-rhizome Zingiberaceae. High-performance thin-layer chromatography is suitable for identity, and liquid chromatography with diode-array detection for assay and fingerprint, as recommended generally for flavonoid markers [5].

4. Pharmacokinetics

4.1. Animal Data

After oral administration of an ethanol extract to rats, dimethoxyflavone, trimethoxyflavone and pentamethoxyflavone reached peak concentrations of 0.55 to 0.88 micrograms per millilitre within one to two hours, with half-lives of three to six hours and oral bioavailability of only 1 to 4 per cent [12]. They distributed mainly to liver and kidney and were also found in lung, testes and brain, and they were excreted in urine and faeces largely as demethylated, sulfated and glucuronidated metabolites, with less than 3.1 per cent of each dose recovered unchanged in urine [12].

4.2. Human Data

The only human pharmacokinetic study gave single doses of 90 or 180 milligrams of ethanol extract to healthy volunteers [1]. Pentamethoxyflavone reached peak concentrations of 26.0 and 71.2 nanograms per millilitre at about four hours, with exposure increasing linearly with dose, and was undetectable by ten hours. Trimethoxyflavone was quantifiable only at the higher dose, with a peak of 63.0 nanograms per millilitre at about six hours. Half-lives were two to three hours. Dimethoxyflavone, the most abundant marker in many raw materials [7], was not detected, which the authors attributed to the limited sensitivity of their liquid chromatography with ultraviolet detection. Complete concentration profiles were obtained in only nine of 14 volunteers for pentamethoxyflavone at the higher dose and in four for trimethoxyflavone [1].

These data have three implications. First, systemic exposure to parent methoxyflavones is low and transient, so once-daily dosing produces a short daily peak rather than sustained levels. Second, the absence of detectable dimethoxyflavone and the incomplete profiles mean that exposure is uncertain rather than necessarily absent; mass spectrometric methods with lower limits of quantitation are needed. Third, metabolites, which dominate excretion in rats [12], were not measured in humans and may contribute to activity. Pharmacokinetic data are lacking for water extracts, for drinks and for repeated dosing.

4.3. Formulation to Improve Absorption

The low aqueous solubility of methoxyflavones suggests formulation as a lever. In rats, a self-microemulsifying system raised the oral bioavailability of pentamethoxyflavone, trimethoxyflavone and dimethoxyflavone 25-, 42- and 26-fold, and a hydroxypropyl-beta-cyclodextrin complex 22-, 34- and 23-fold, compared with unformulated extract [8]. Self-nanoemulsifying systems and nanosuspensions have also improved dissolution and intestinal absorption [13,14]. None of these has yet been tested in humans. If they translate, formulation would further widen the gap between extract mass and delivered dose, and marker-based dosing would become even more important.

5. Human Evidence

Table 3 summarises representative human studies. In physical performance, 8 weeks of 25 or 90 milligrams of extract improved chair-stand and six-minute walk performance in 45 older adults [15]; 12 weeks of 180 milligrams increased hand-grip strength but not five other fitness measures in 60 soccer players [16]; and 12 weeks of a functional drink with 180 milligrams of water extract improved estimated maximal oxygen uptake, timed shuttle run and five-minute run distance in healthy adults [6]. In body composition, 150 milligrams of extract for 12 weeks reduced visceral, subcutaneous and total abdominal fat measured by computed tomography in 76 overweight Japanese adults [17], 12 milligrams of purified polymethoxyflavone did the same in 80 adults [9], and a commercial extract reduced body fat measured by dual-energy X-ray absorptiometry in overweight Korean adults [18]. A single dose increased whole-body energy expenditure, particularly in men with active brown adipose tissue [19]. For sexual health, an open-label pilot study of 100 milligrams of ethanol extract for 30 days in 14 men reported improved erectile function scores [20]. A 28-day study found no effect on glucose tolerance in healthy volunteers [1], whereas a meta-analysis reported a modest reduction in fasting glucose across human studies [4].

5.1. Appraisal

The body-composition trials are the strongest: they used objective imaging outcomes, were randomised and placebo-controlled with 76 to 108 participants, and one used a purified, marker-defined preparation [9,17,18]. The physical-performance trials are smaller and rely on field tests. In the functional drink trial, maximal oxygen uptake was not measured directly but estimated from age-predicted maximal heart rate and resting heart rate, so a change in this estimate largely reflects a change in resting heart rate; and although the title refers to physical flexibility, no flexibility test is described in the methods or results [6]. Positive results were confined to the higher dose and to some of several outcomes, and multiplicity was not addressed. The sexual-health evidence rests on one small uncontrolled study [20] and on mechanistic plausibility from phosphodiesterase type 5 inhibition [10].

Across all trials, doses are expressed as extract mass (25 to 180 milligrams) or, in one case, purified polymethoxyflavone (12 milligrams). The two trials examined in detail show that equal extract masses can deliver marker doses differing more than tenfold (Table 1). Pooling such trials in meta-analysis [4] combines interventions that may not be pharmacologically equivalent, and dose–response relationships cannot be established without marker-based dosing.

6. Safety

Toxicity studies in rodents have shown no adverse effects at high oral doses, including up to 2000 milligrams per kilogram daily for six months in rats [1]. Human trials have reported few and mild adverse events, such as transient nausea in one participant [1], and no clinically relevant changes in laboratory parameters [9,17]. Two interaction signals deserve attention. Dimethoxyflavone suppressed hepatic cytochrome P450 3A expression in mice and increased the exposure to midazolam by 130 per cent [11], suggesting possible interactions with drugs metabolised by this enzyme. And because methoxyflavones inhibit phosphodiesterase type 5 [10], additive effects with prescription inhibitors of this enzyme or with nitrates are theoretically possible. Neither has been studied in humans; both justify caution statements and interaction studies, particularly for formulations that increase bioavailability.

7. Recommendations

7.1. Specification of Ingredients and Products

A black ginger ingredient specification should state the plant part and origin, the extraction solvent and drug-to-extract ratio, the individual contents of dimethoxyflavone, trimethoxyflavone and pentamethoxyflavone with acceptance ranges, their ratio, and a chromatographic fingerprint. Finished products should declare marker content per serving, verified by assay of the finished product rather than calculated from the extract.

7.2. Reporting Standard for Trials

The following minimum information is proposed for clinical and pharmacokinetic studies.

• Botanical identity with voucher specimen, plant part, origin and, where known, chemical pattern of the raw material.
• Extraction solvent, drug-to-extract ratio and any purification or formulation technology.
• Content of each main methoxyflavone in the extract and in the finished product, determined by a validated method, with the daily dose of each marker in milligrams.
• Stability of markers in the finished product over the trial period.
• For pharmacokinetics, a bioanalytical method sensitive enough to quantify all main markers and their principal metabolites, with repeated-dose data.
• Objective, directly measured primary outcomes, pre-registration, and correction for multiple outcomes.
• Monitoring of adverse events and, where relevant, concomitant medicines metabolised by cytochrome P450 3A.

7.3. Research Priorities

Priorities are a human pharmacokinetic study using mass spectrometry that compares ethanol, water and formulated extracts at equal marker doses; dose-finding trials expressed in marker units; replication of the physical-performance findings with directly measured outcomes; a controlled trial of sexual-function outcomes; and a clinical interaction study with a probe substrate of cytochrome P450 3A.

8. Conclusions

Black ginger has a growing evidence base in humans, strongest for reduction of abdominal fat and more preliminary for physical performance and sexual function. Its methoxyflavones are good quality markers: characteristic, measurable, linked to activity and, for two of them, detectable in human blood. Yet trials describe doses as milligrams of extract, and the two trials examined here show that the same extract mass can deliver markers in amounts differing by more than an order of magnitude depending on the extraction solvent. Combined with low and variable oral bioavailability, this makes the current literature difficult to compare and pool. Specifying and reporting doses in methoxyflavone units, using fingerprints to control raw materials, and measuring exposure with sensitive methods would turn black ginger from a promising traditional ingredient into a reproducible, evidence-based one.