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Residual Methanol in Botanical Dietary Ingredients: Perspectives of a Manufacturer

Organic solvents such as methanol, ethanol, ethyl acetate, and acetone are employed during the extraction and downstream processing of medicinal herbs to manufacture standardized herbal extracts (SHEs). The SHEs from medicinal plants increasingly are used as ingredients in herbal dietary supplements (also known as food supplements in some countries), complementary medicines, licensed nonprescription drug preparations, functional foods, and natural cosmetics all over the world. It is almost impossible to remove the residual solvents completely from liquid and dried herbal extracts. Since many organic solvents can be toxic to humans — i.e., depending on the level of exposure — maximum residue limits (MRLs) are necessary for SHEs. The intention of this brief article is to provide an overview of various issues associated with methanol as a residual solvent in SHEs used in botanical dietary ingredients and supplements.

Manufacturers of SHEs often favor the use of methanol for extraction of medicinal plants due to its lower boiling point, higher volatility, and higher extraction efficiency compared to ethanol (depending on the desired secondary metabolite composition).1-3 Methanol also is used as a co-solvent to enhance extraction efficiency in supercritical fluid extractions.4 As a means to minimize the abuse of ethanol, most government-sanctioned licensing systems normally require producers, distributors, and sellers of ethanol to obtain licenses, the availability of which may be restricted, particularly in the retail sector. Overall, licensing requirements (in which any license is required) exist in 142 countries worldwide.5

In the United States, dietary supplements are governed under food laws and are frequently available in solid dosage forms, e.g., tablets or capsules. Doses of botanical dietary supplements typically range from 250 mg up to 2.5 g per person per day.6 Based on our experience, properly dried SHEs processed with methanol tend to contain 50 ppm to 1,000 ppm (parts per million) of methanol, which is well within the guidelines established by the International Conference on Harmonization (ICH) for residual solvents for pharmaceutical products. (These guidelines have been adopted as legally binding for SHEs in several countries).7 The MRLs of residual methanol, as per the regulations of different selected countries, are shown in Table 1.

Although the toxicity of methanol at high doses is well established, less is known about potential adverse effects from lower levels of exposure over a long period of time,17 which often is the case with methanol-containing SHEs. To our knowledge, no studies seem to have been performed, so far, specifically to evaluate the toxicity of SHEs due to the presence of residual methanol. Under these circumstances, regulatory bodies typically apply the same limits for residual methanol that are applicable to conventional foods, though the intake of dietary supplements is far less than conventional food.

Sources of Exposure to Methanol

Methanol is normally present in the human body as a naturally occurring byproduct of protein formation.17 According to the International Programme on Chemical Safety Poisons Information Monograph, the normal blood methanol concentration in humans is approximately 15 mg/L (range 2-30 mg/L).18§

Naturally occurring methanol in food and beverages

Methanol has been found in food, particularly fresh fruit and vegetables, which is absorbed during digestion.19 It occurs as free methanol or can be converted to methanol in the gastrointestinal tract after hydrolysis of methyl esters of fatty acids or methoxy groups on polysaccharides (e.g., pectin).17 Concentrations of methanol in fresh orange (Citrus sinensis, Rutaceae) and grapefruit (Citrus x paradisi, Rutaceae) juices are in the range of 11-80 mg/L and 12-60 mg/L, respectively. In human volunteers, consumption of 10-15 g isolated pectin or of one kg apples (Malus spp., Rosaceae) containing approximately 10 g natural pectin induced a significant increase in methanol in the breath and, by inference, in the blood. Consumption of one kg apples was estimated to release 500 mg methanol. It has been estimated that humans may be exposed to approximately 1,000 mg methanol per day from fruits and vegetables. Ripe fruit was found to release more methanol than unripe fruit.17,20,21

Methanol also occurs at low concentrations in alcoholic drinks. Concentrations of 6-27 mg/L have been measured in beer, 96-321 mg/L in wine, and 10-220 mg/L in distilled spirits.22 The European Union regulatory limit on methanol in vodka is set at 10 g per hectoliter of 100% vol. alcohol (i.e., 100 mg methanol per liter of alcohol, equivalent to 37 mg/L if the vodka contains 37% alcohol).23

Naturally occurring methanol in plants and SHEs

The methanol content of plant leaves and the potential methanol released from leaves into the atmosphere have been investigated by several researchers. Free methanol has been found in common bean (Phaseolus vulgaris, Fabaceae) leaves at levels ranging from 10-27 µg/g fresh weight. Pectin demethylation, mediated by pectin methylesterase, in the cell walls is considered the likely source of methanol in leaves,24,25 as well as in fruits like tomato (Lycopersicon esculentum, Solanaceae).26 Methanol is known to be produced in plants during the early stages of leaf expansion.27 As such, it can be inferred that methanol is naturally present in plants, and its level may differ depending on the growth stage during which the particular plant part was collected. Low levels of methanol also were detected in tobacco (Nicotiana tabacum, Solanaceae) leaves (27.8 ppm).28

Reports on methanol contents of medicinal plants are somewhat scarce. A methanol content of approximately 50 ppm was found in dried licorice (Glycyrrhiza spp., Fabaceae) roots.29 Recently, methanol was detected in leaves and root tissues of the traditional Indian medicinal plant ashwagandha (Withania somnifera, Solanaceae) using high-resolution magic angle spinning nuclear magnetic resonance spectroscopy.30

There are very few publications on the analysis of solvent residues in SHEs and dietary supplements. According to one research paper,31 up to 740 ppm methanol was found in some health foods and dietary supplements sold in the natural products marketplace, including aloe extract (from Aloe spp., Xanthorrhoeaceae), Chlorella powder (from Chlorella spp., Chlorophyceae), Garcinia extract (from Garcinia spp., Clusiaceae), Ginkgo biloba (Ginkgoaceae) leaf extract, and mume plum extract (from Prunus mume, Rosaceae). The authors indicated that since this solvent can be present naturally in volatile components of plants, it was not clear as to whether it originated in the plants, was a residual solvent from the manufacturing process, or perhaps was present from other sources. The same research group29 had indicated earlier that there are many possible origins of methanol in plant material, such as methanol being naturally present as a volatile component of food, or methanol being formed, for example, by a fermentation process. Similar to ethanol, methanol also could be formed during the manufacture or storage of food additives after hydrolysis of compounds in raw materials, or during chemical analysis by hydrolysis of compounds at high temperatures. The formation of methanol by the latter process, according to the authors, would lead to an overestimation and should be minimized.31

Based on our in-house analyses of residual solvents (unpublished results), some crude powders of Indian medicinal plants and dried aqueous extracts show methanol residues in the range of 10-100 ppm when analyzed by headspace gas chromatography. These results need to be further investigated, confirmed, and published.

Observations and Recommendations

The ICH guidelines7 — which apply to the manufacture or purification of drug substances, their excipients, or drug products — list methanol as a class 2 solvent, which means that it should be limited in pharmaceutical products to a permitted daily exposure of 30 mg per day and a concentration limit of 3,000 ppm in finished consumer products. The SHEs intended as dietary ingredients in dietary supplements (typically in the form of tablets or capsules) in most cases have an average daily dose of less than 2.5 g per day.6 Assuming the maximum limit for methanol given in the ICH guidelines (3,000 ppm) is present in SHEs with a dose of, for example, 10 g per day (given a combination of five extracts at two g each), the maximum daily consumption of methanol would be about 30 mg per day. Compared to a potential exposure of 1,000 mg of methanol per day from fruits and vegetables,17 the maximum exposure of methanol (30 mg per day) from dietary supplements appears relatively low and is highly unlikely to pose a toxicity concern. It may also be noted that US Environmental Protection Agency’s Integrated Risk Information System revised the Reference Dose for Chronic Oral Exposure of methanol to two mg/kg per day (120 mg per day for a person weighing 60 kg), which is likely to be without an appreciable risk of deleterious effects during an average lifetime. The revised RfD of methanol is in addition to the background levels of methanol derived from a diet that includes fruits and vegetables.32

In this context, we believe that the limit of 3,000 ppm in the ICH guidelines adequately addresses the safety concerns that could arise from methanol residues in SHEs. We propose that the ICH guidelines should be adopted by all regulatory agencies across the world for residual methanol concentrations in botanical extracts that are meant to be used as ingredients in dietary supplements.

Deepak Mundkinajeddu, PhD, is head of research and development and Amit Agarwal, PhD, is director of Natural Remedies Pvt. Ltd. (Bangalore, India), a manufacturer of botanical extracts used in foods, dietary supplements, drugs, and cosmetics.

§A panel of the European Food Safety Authority (EFSA) recently released a scientific opinion in which it concluded that methanol released in the human body by the metabolism of the artificial sweetener aspartame is not expected to pose a safety risk. The panel noted that aspartame-derived methanol contributed to less than 10% of the total mean anticipated exposure to methanol from all sources.19


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