Allicin Defined
Allicin Chemistry
Allicin Stability
Commercial Garlic 
"
Allicin Potential"
Enteric Coatings
Allicin Absorption
Allicin Activity in Vivo
Other Compounds
References

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Allicin Defined:

Garlic (Allium sativum), like other plants, has an exquisite defense system composed of as many different components as the human immune system. In order to protect itself from insects and fungi, garlic enzymatically produces allicin when it is injured. Thus, allicin is mother nature's insecticide. Allicin was discovered in 1944 by Cavallito et al.1 who first noted its potent antimicrobial activity. Allicin received a patent for its antifungal activity in test tubes. However, no clinical trials have been performed with allicin and it was never developed into a drug or commercial product due to its instability, inability to be absorbed, and offensive odor. Allicin itself is considered to be of limited value inside the body and is presently regarded by the scientific community as just a transient compound which rapidly decomposes to other compounds.11

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Allicin Chemistry:

Allicin Chemical Structure

Chemically, allicin is known as 2-propene-1-sulfinothioc acid S-2-propenyl ester; thio-2-propene-1-sulfinic acid S-allyl ester.2  Allicin is produced by an enzymatic reaction when raw garlic is either crushed or somehow injured. The enzyme, alliinase, stored in a separate compartment in garlic, combines with a compound called alliin in raw garlic and produces allicin. 

Allicin + Alliin = Allicin

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Allicin Stability:

Because allicin is so unstable, once it is generated it readily changes into other compounds. Thus cooking, aging, crushing and otherwise processing garlic causes allicin to be decomposed into other compounds. According to two studies of garlic preparations, allicin decreased to non-detectable amounts within one3 to six4 days. 

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Allicin in Commercial Products:

Research conducted at the Chemistry Department of the University of California showed that commercial garlic products on the market all contain an undetectable amount (<1 ppm) of allicin.5

Allicin Chart of Commercial Products

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"Allicin Potential" of Questionable Value

To prevent the typical loss of allicin, some manufacturers have attempted to stabilize alliin and alliinase so that these compounds would not come together until after consumption in hopes of producing allicin inside of the body. Such "allicin potential" is measured by adding water to garlic products which contain both alliin and alliinase to determine how much allicin can be produced. However, the actual production of allicin inside the body is not the same as that produced in a test tube because intestinal conditions hinder the generation of allicin:

1) Stomach acid destroys alliinase, preventing allicin production. 
2) Intestinal fluids further diminish the amount of allicin that can be produced.

99% loss of allicinSimulated stomach fluids and simulated intestinal fluids have commonly been used to determine the effects of typical digestion on nutrients or chemicals in question. Interestingly, alliinase, the enzyme which catalyzes the conversion of alliin to allicin, has been shown to be irreversibly deactivated at pH 3 or below, an acidic environment typically found in the stomach.6 Further, a 99% loss in allicin production was observed following consecutive exposure to simulated stomach fluids and simulated intestinal fluids which would occur when one takes a garlic powder orally.4 Therefore, it appears that unless a garlic powder bypasses the stomach, the amount of allicin produced is negligible.

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What about Enteric Coatings?

To prevent the destruction of alliinase by stomach acid, some manufacturers have enteric-coated garlic powders in order to bypass the stomach. Assuming that a high quality garlic powder is used initially (one that is not exposed to high temperatures which can deactivate alliinase), this form of garlic could potentially deliver alliin and alliinase to the intestinal tract. However, simulated intestinal fluids have been shown to reduce allicin production almost 40%.5 (See Table Below). The remaining allicin may exert anti-microbial effects on bad bacteria, however, it may also destroy friendly bacteria.7 Both allicin and raw garlic preparations which contain allicin have been shown to decrease the bacteria flora.7-9 Further, being a strong oxidizing agent10 allicin may damage sensitive cells which line the intestinal tract11 as it has been shown to damage the cells lining the stomach.12  

Intestinal Effects Table

Finally, current research has shown that allicin derived from enteric-coated garlic is not bioavailable.11,13

Allicin in Body from Commercial Products

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Allicin Absorption

No allicin found after consumptionAllicin is not bioavailable. A study in which participants consumed a large amount of allicin (approximately 90,000 mcg) via crushed raw garlic (25 grams; roughly 10 cloves) revealed that neither allicin nor 16 of its daughter compounds could be detected in blood or urine from one to 24 hours after consumption.10

 No allicin found subjects after intake

Due to its high reactivity allicin was shown to be completely metabolized in the liver.14,15 If allicin could even make it to the blood (to be delivered throughout the body), studies have shown that it changes into other compounds within five minutes and in the process may oxidize the blood cells causing them to lose their ability to carry oxygen.5,12 Since allicin is rapidly metabolized in human blood and other tissues, it is doubtful it contributes to any antithrombotic, or blood thinning, actions in the body.16

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Allicin Activity in vivo

Allicin not bioavailableThough allicin was considered to be a key compound in garlic in the past, recent scientific findings, including the pharmacokinetics and metabolism of organosulfur compounds in garlic, have revealed that allicin is not biologically active inside of the body.12-14

Allicin not alone responsable for benefits Contrary to the popular myth that a garlic product must contain allicin to be beneficial, allicin has not been conclusively proven to be responsible for garlic's known health benefits. Most of the garlic or garlic products that have been based to demonstrate garlic health effects do not contain significant amounts of allicin. (Allicin is an odorous and transient garlic compound.)17 Equally untrue is the myth that if garlic or a garlic product does not have a garlic odor it does not provide benefits. A major part of the data on the benefits of garlic is derived from studies of cooked garlic, pickled garlic, aged garlic, and aged garlic extract. All of them have little typical garlic odor.17

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Other Compounds in Garlic

Other Compounds Various forms of garlic, which contain no allicin (e.g., cooked, steamed, microwaved and aged garlic extract), have demonstrated an array of benefits in studies. Therefore, it is logical that compounds other than allicin are responsible for such benefits. To date, well over 100 compounds have been identified in garlic preparations.18 Presently, S-allyl cysteine appears to be a very promising compound with good absorption.19 The pharmacokinetic studies of S-allyl cysteine demonstrated rapid absorption and almost 100% bioavailability after oral administration. In addition, since both the safety and effectiveness of S-allyl cysteine have been reported, this compound appears to play an important role in garlic's medicinal effects.12,13

Synergism Though individual compounds, such as S-allyl cysteine, have shown activity in studies and are absorbed by the body, it is likely that a synergism of various compounds provide the benefits of garlic. This is in agreement with Dr. Koch, a renowned Austrian scientist who stated that the activity of various sulfur compounds could not alone be responsible for the benefits of garlic and fixation on a single group of components can lead to mistakes and wrong conclusions.20 

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References

1.  Cavalitto, C.J. et al. 1944. J. Am. Chem. Soc. 66: 1950.
2.  Allicin. The Merck Index. 1989. (Budavari, S. ed.), 11th ed. p. 244.
     Merck and Co. Rahway, New Jersey.
3.  Brodnitz, M.H. Pascale, J.V., and Derslice, L.V. Flavor components
     of garlic extract. J. Agr. Food. Chem. 19(2):273-275, 1971.
4.  Yu. T-H, and Wu, C-M. Stability of allicin in garlic juice.
     J. Food Sci. 54(4): 977-981, 1989.
5.  Freeman, F. and Kodera, Y. Garlic chemistry: stability of
     s-(2-propenyl)-2-propene-1-sulfinothioate (allicin) in blood, solvents, and
     simulated physiological fluids. J. Agric. Food Chem. 43: 2332-2338, 1995.
6.  Lawson L.D. and Hughes B.G. Characterization of the formation of allicin
     and other thiosulfinates from garlic. Planta Med. 58: 345-350, 1992.
7.  Shashikanth, K.N., Basappa, S.C., and Murthy, V. 1985. Allicin concentration
     in the gut of rats and its influence on the microflora. J. Food Sci. Technol.
     22(6): 110-112.
8.  Subrahmanyan, V., Krishnamurthy, K., Sreenivasamurthy, V. and
     Swaminathan M. The effect of incorporation of garlic in the diet on the
     intestinal microflora of rats. Ann. Biochem. Exp. Med. 18(3): 85, 1958.
9.  Shashikanth, K.N., Basappa, S.C. and Murthy, V.S. 1986. Effect of 
     feeding  raw and boiled garlic (Allium sativum l.) on the growth, caecal
     microflora and  serum proteins of albino rats. Nutr. Reports Int. 
     33(2): 313-319.
10. Lawson, L. D., Ransom, D. K. and Hughes, B. G. Inhibition of whole blood
      platelet-aggregation by compounds in garlic clove extracts and commercial
      garlic products. Throm. Res. 65: 141-156, 1992.
11. Amagase, H., Petesch, B., Matsuura, H., Kasuga, S. and Itakura, Y. Intake
      of garlic and its bioactive components. J. Nutr. 131(3S): 955S-962S, 2001.
12. Kodera, Y. 1997. Dietary Tolerance/Absorption/Metabolism of Garlic.
      Ch. 11. In: Nutraceuticals: Designer Foods III Garlic, Soy and Licorice
      (Trumbell, Ct: Food & Nutrition Press), Paul Lanchance, ed., pp. 95-105.
13. Rosen, R. Determination of allicin and S-allyl cysteine in human plasma and
      urine after consumption of garlic and garlic products. Phytomed. 7(2): 51,
      2000.
14. Egen-Schwind C., Eckard R, and Kemper F.H. Metabolism of garlic
      constituents in the isolated perfused rat liver. Planta Med. 58: 301-305, 
      1992.
15. Egen-Schwind C., Eckard R., Jekat F.W, and Wirterhoff, H. 
      Pharmacokinetics of vinyldithiins, transformation products of allicin.
      Planta Med. 58: 8-13, 1992.
16. Agarwal, K.C. Therapeutic actions of garlic constituents.
      Med. Res. Rev. 16(1): 111-124, 1996.
17. Lin, R.I.S., Ph.D., chairman: "First World Congress on the Health
      Significance of Garlic and Garlic Constituents." Sponsor: Nutrition
      International Company. Cosponsors: Pennsylvania State University and 
      United States Department of Agriculture. August 28-30, 1990.
      Washington, D.C.
18. Block, E. The organosulfur chemistry of the genus Allium - implications
      for the organic chemistry of sulfur. Angew. Chem. Int. Ed. Engl. 31: 
      1135-1178, 1992.
19. Nagae, S., Ushijima, M., Hatono, S., Imai, J., Kasuga, S., Matsuura, H.,
      Itakura, Y. and Higashi, Y. 1994. Pharmacokinetics of the garlic compound
      S-allyl cysteine. Planta Med. 60: 214-217.
20. Koch, H.P. Saponine in Knoblauch und Khchenzwiebel. Deutsch Apotheker
      Zeitung 133 Jahrg Nr.41(14.10):63-75, 1993.

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