Allicin Bioavailability and Bioequivalence from Garlic Supplements and Garlic Foods

Abstract

Allicin is considered responsible for most of the pharmacological activity of crushed raw garlic cloves. However, when garlic supplements and garlic foods are consumed, allicin bioavailability or bioequivalence (ABB) has been unknown and in question because allicin formation from alliin and garlic alliinase usually occurs after consumption, under enzyme-inhibiting gastrointestinal conditions. The ABB from 13 garlic supplements and 9 garlic foods was determined by bioassay for 13 subjects by comparing the area under the 32-h concentration curve of breath allyl methyl sulfide (AMS), the main breath metabolite of allicin, to the area found after consuming a control (100% ABB) of known allicin content: homogenized raw garlic. For enteric tablets, ABB varied from 36⁻104%, but it was reduced to 22⁻57% when consumed with a high-protein meal, due to slower gastric emptying. Independent of meal type, non-enteric tablets gave high ABB (80⁻111%), while garlic powder capsules gave 26⁻109%. Kwai garlic powder tablets, which have been used in a large number of clinical trials, gave 80% ABB, validating it as representing raw garlic in those trials. ABB did not vary with alliinase activity, indicating that only a minimum level of activity is required. Enteric tablets (high-protein meal) disintegrated slower in women than men. The ABB of supplements was compared to that predicted in vitro by the dissolution test in the United States Pharmacopeia (USP); only partial agreement was found. Cooked or acidified garlic foods, which have no alliinase activity, gave higher ABB than expected: boiled (16%), roasted (30%), pickled (19%), and acid-minced (66%). Black garlic gave 5%. The mechanism for the higher than expected ABB for alliinase-inhibited garlic was explored; the results for an alliin-free/allicin-free extract indicate a partial role for the enhanced metabolism of γ-glutamyl S-allylcysteine and S-allylcysteine to AMS. In conclusion, these largely unexpected results (lower ABB for enteric tablets and higher ABB for all other products) provide guidelines for the qualities of garlic products to be used in future clinical trials and new standards for manufacturers of garlic powder supplements. They also give the consumer an awareness of how garlic foods might compare to the garlic powder supplements used to establish any allicin-related health benefit of garlic.

Keywords: S-allylcysteine; aged garlic extract; allicin bioavailability; allicin metabolism; alliin; allyl methyl sulfide; black garlic; cooked garlic; garlic supplements; pickled garlic.

Figure 1

Structures of the known significant S-allyl compounds derived from garlic, including transformation reactions. The compounds above the horizontal line comprise alliin and alliin-derived dithioallyl compounds (AADD).The first set of values for the allyl thiosulfinates represent the average, minimum, and maximum mol % of total allyl thiosulfinates found for 26 samples from six countries; the second set is the S-allyl mol % or percent of the alliin transformed to each thiosulfinates—allicin increased because it is has two S-allyl groups [19,20]. The values for the allyl polysulfides represent the average mol % and S-allyl mol % for five samples of steam-distilled garlic oil capsules; small amounts of penta- and hexasulfides (3%) are also present in these oils [21,22] (p. 100). Spontaneous formation of the allyl polysulfides from the thiosulfinates (rapid in hot water, slow in ambient water) results in loss of about 25% of the total S-allyl, due to the formation of SO₂, propene, and allyl alcohol [23,24].

Figure 1

Structures of the known significant S-allyl compounds derived from garlic, including transformation reactions. The compounds above the horizontal line comprise alliin and alliin-derived dithioallyl compounds (AADD).The first set of values for the allyl thiosulfinates represent the average, minimum, and maximum mol % of total allyl thiosulfinates found for 26 samples from six countries; the second set is the S-allyl mol % or percent of the alliin transformed to each thiosulfinates—allicin increased because it is has two S-allyl groups [19,20]. The values for the allyl polysulfides represent the average mol % and S-allyl mol % for five samples of steam-distilled garlic oil capsules; small amounts of penta- and hexasulfides (3%) are also present in these oils [21,22] (p. 100). Spontaneous formation of the allyl polysulfides from the thiosulfinates (rapid in hot water, slow in ambient water) results in loss of about 25% of the total S-allyl, due to the formation of SO₂, propene, and allyl alcohol [23,24].

Figure 2

Metabolism of allyl thiosulfinates and allyl polysulfides. R represents allyl, methyl, or trans-1-propenyl. When R is allyl (allicin or diallyl sulfides), two moles of allyl mercaptan and allyl methyl sulfide (AMS) are formed.

Figure 3

Examples of standard curves for AMS standards, 13–106 ng/L, over the course of 14 months.

Figure 4

Typical concentration curves for breath AMS, after consumption of various garlic products. (A–D), 1.4 g of raw garlic homogenate (0.88 g raw garlic) consumed in capsules with low-protein meal (the standard control). (E–H), enteric-coated tablets, brand E2 (2 tablets), consumed with low or high-protein meals by a female or male. (I–L), normal tablets, brand N1 (3 tablets), consumed with low-protein meal (not the same persons as A–D). (M–P), garlic foods consumed with low-protein meals (four different persons): M = 12 g pickled garlic; N = 7 g acid-minced garlic; O = 6.1 g roasted garlic (160 °C for 30 min); P = 5.5 g boiled garlic (45 min). (Q–T), garlic extracts and pure compounds: Q = protein-free high alliin extract (PFHA); R = alliin-free high GSAC extract (AFHG); S = pure l(+)-alliin; T = pure S-allylcysteine. S and T were consumed by the same person. Q and R were consumed by another person, but very similar curves were found when Q and R were consumed by the person who consumed S and T.

Figure 5

Dose response of the breath AMS concentration curve (AUC) after consumption of A, the control (raw garlic homogenate) or B, N1 tablets, with high- or low-protein meals. Values are means ± standard deviation (SD) (n = 13).

Figure 6

Possible metabolic pathways for the partial transformation of alliin and SAC (S-allylcysteine) to AMS (allyl methyl sulfide) upon consuming alliinase-inhibited garlic. For structures, see Figure 1 and Figure 2. Of the three proposals for alliin, only pathway C is likely to be relevant.