Background
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.
Table 1. MRLs of Residual Methanol as per Guidelines from Different Regulatory Agencies
Country/Region
|
Resource*
|
Category
|
Limits (ppm)
|
Reference
|
Australia
|
ARTG
|
Complementary
medicines
|
3,000
|
8
|
Canada
|
Health
Canada
|
Natural
health products
|
3,000
|
9
|
Europe
|
EMA
|
Herbal
medicinal products
|
3,000
|
10
|
EFSA
|
Botanical
food supplements
|
10
|
11
|
Japan
|
JETRO
|
Foods
and food additives
|
50
|
12
|
Korea
|
MFDS
|
Food
additives
|
50
to 200†
|
13
|
Health
functional foods
|
‡
|
14
|
USA
|
USP
(Compendia and Codex)
|
Herbal
medicines and dietary supplements
|
3,000
|
15
|
Food
ingredients
|
50
to 200**
|
16
|
* ARTG:
Australian Register of Therapeutic Goods; EMA: European Medicines Agency; EFSA:
European Food Safety Authority; JETRO: Japanese External Trade Organization;
MFDS: Ministry of Food and Drug Safety; USP: United States Pharmacopeia † Values are specific to the ingredients listed in the respective food
additives regulation. In general, most food additives have been given the limit
in the range specified in Table 1 with the exception of pectin, which has a MRL
of 10,000 ppm. ‡ If the use of a solvent remains inevitable in the course of manufacturing,
the residue levels shall be kept at the minimum quantity achievable in the
manufacturing process.
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) and grapefruit (Citrus x paradisi) 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
1 kg apples (Malus spp.) containing
approximately 10 g natural pectin induced a significant increase in methanol in
the breath and, by inference, in the blood. Consumption of 1 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) 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).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) 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.) roots.29 Recently,
methanol was detected in leaves and root tissues of the traditional Indian
medicinal plant ashwagandha (Withania somnifera) using high-resolution magic
angle spinning (HR-MAS) nuclear
magnetic resonance (NMR) 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.), Chlorella
powder (from Chlorella spp.), Garcinia extract (from Garcinia spp.), Ginkgo biloba leaf extract, and ume plum extract (from Prunus mume). 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 (GC). 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
with a permitted daily exposure of 30 mg/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 2 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/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 (EPA) Integrated Risk Information System (IRIS) revised the
Reference Dose for Chronic Oral Exposure (RfD) of methanol to 2 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.
**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
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.
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