Garlic’s Allicin: Transient Molecule, Big Effects?

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Allicin is the compound that gives garlic its strong smell when crushed or cut- as well as many of the health benefits of garlic. Antioxidant and immune-boosting effects are reported. Plus, it appears to have a role in the prevention and treatment of diabetes mellitus, cardiovascular disease, and cancer, among other conditions. 

Key Points:

1. Allicin gives crushed or cut garlic its distinct aroma, and according to the research, may have many health promoting effects: antioxidant activity, reduced risk of heart disease and stroke, improved learning and memory, antimicrobial activity and immune system support, and cancer-suppressing potential
2. Allicin is produced from the amino acid alliin when garlic is damaged. It also breaks down quickly. This means there is a small window for it to exert therapeutic effects. 
3. Much of the promising evidence comes from animal and cell culture studies (the latter variety may be less affected by allicin’s quick break down). Future human trials will hopefully confirm its powers.
4. It is unclear what is the optimal dose for general health, or for specific conditions- but, consuming it in garlic seems a superior choice than synthetic forms.

Before garlic is damaged, allicin is not present. Instead you have it’s precursor, the amino acid alliin, tucked away in one compartment of garlic, and the enzyme alliinase in another. When a clove is damaged, these two interact, and alliinase converts alliin to allicin… releasing that bountiful smell that’s so pleasant in the kitchen (and less so on the breath). 

What health benefits go with the smelly territory? 

Reduced oxidative stress and inflammation

The antioxidant impact of allicin is central to its protective effect on cardiovascular, neurological and other systems. It has been observed to reduce Malondialdehyde (MDA) levels, a marker of oxidative stress.

Interestingly enough, allicin is technically an oxidant, but at low doses it induces antioxidant expression. It inhibits chains of inflammatory activity and helps an enzyme (SOD) convert damaging free radicals into less damaging molecules. 

Allicin and heart health

There have been links between garlic and garlic oil consumption with a reduction of blood fats, total- and LDL-cholesterol while levels of HDL-cholesterol (the beneficial one) are maintained. Allicin appears to suppress the body’s manufacture of cholesterol. 

Blood pressure reductions may result from allicin breakdown. The new compounds created and biochemical reactions involved can cause blood vessels to relax and expand. 

Dose for dose, allicin appears to be a more potent platelet-aggregation inhibitor than aspirin (a commonly used medication to reduce risk of blood clots). Less clotting means less risk of heart attack. 

The antioxidant effects may also provide protection from myocardial fibrosis (i.e. where heart muscle is replaced by scar tissue) and myocardial hypertrophy (thickening of the heart walls). 

Allicin and stroke risk

Cholesterol and blood pressure lowering effects also reduce risk of stroke. 

In addition, in a study where mice were given induced strokes, those pre-treated with allicin had smaller areas of the brain affected. This also seems to have been associated with several markers of reduced oxidative stress and inflammation, and increased activity of antioxidant enzymes. It was also shown to promote the creation of new blood vessels adjacent to the area of cell death. This could provide blood flow valuable in the recovery process. Furthermore, 24 hours after the ‘stroke’, those mice who received allicin showed better neurological function than those who did not. 

Whether such promising results apply to humans needs further research. Also, note, that allicin was associated with reduced consequences, but was not sufficient to completely counteract effects of stroke. At best it may be an adjunct to otherwise risk-minimising lifestyles. 

While we’re on the brain…

It could also be protective against memory and learning impairments.

Allicin: a key to garlic’s antimicrobial activity

The ancient Egyptians and many other cultures throughout history have used garlic for its antiseptic and antibiotic properties. Lab based studies have since shown that allicin is largely responsible for garlic’s antibacterial and antifungal activity. 

Lab studies have also indicated that it actually has activity against a broader range of bacteria than many commonly used antibiotics. For example, penicillin is practically ineffective against Gram negative bacteria, whereas allicin works against both Gram positive and Gram negative bacteria. Studies suggest allicin can also be used against some antibiotic-resistant bacteria. Eg: methicillin resistant Staphylococcus aureus (MRSA)—the chief culprit in many hospital infections. 

It seems this antimicrobial activity comes from a thiosulfinate-chemical group in the compound. This group is lost when allicin decomposes into another compound which has reduced antimicrobial activity. 

There is a dose response: lower doses will inhibit cells to a certain extent, whereas large doses can kill invaders. 

Plus- immune modulation effects. Sensible doses over an extended period may stimulate the immune system, thereby boosting its ability to deal with pathogens.

Allicin and cancer-fighting potential

A link between garlic intake and reduced incidences of some cancers has been observed. There is no proof allicin works against all cancers, but lab based cell studies and animal studies have shown it can kill some types of cancer cells or inhibit their spread. It may enhance the benefit of other anti-cancer therapies.

In order to do so allicin has to be generated on the surface of the cells. It is rapidly degraded within the body, so allicin consumed from garlic is unlikely to reach where it’s needed. 

In a stroke of genius, scientists have bound the enzyme allinase to an antibody for antigens displayed by certain cancer cells. Once the allinase-anitbody complex was bound to the cancerous cell, allin was delivered, and converted to allicin at the site. Allicin quickly penetrates and harms the targeted cancer cells where it’s produced. However, the researchers indicated that clinical trials are necessary to be confident that this method will not cause toxicity to human cells.  

The ability to inhibit or kill cancer cells (plus pathogens) seems to pivot on its oxidation of ‘thiol groups’ (organic compounds containing sulfur and hydrogen) that are present in many proteins (eg: glutathione, cysteine-containing proteins). Results can include disturbances in a cell’s structure, which alters its function, and a decrease in a metabolic enzyme (enolase) activity. 

The Verdict

While lab and animal studies show allicin has an impressive range of potential benefits, the evidence coming from high-quality human studies is limited. 

As mentioned, the tendency of allicin to decompose very quickly (in the bloodstream at the latest) limits its opportunities for action. Use of allicin like a medicine- i.e. directly delivering it to an affected area- may be appropriate in certain situations. We’ve mentioned the use of allinase-antibody complexes and alliin delivery to cancer sites. In addition, topical application to skin tumours and direct inhalation have been proposed treatments, but are in great need of study. If you are interested in its potential for a condition you experience, seek the guidance of health professionals. 

For general health promotion and disease prevention, allicin should be treated as a regular nutrient i.e. included as part of a varied diet. A clove of garlic (weighing approximately 10g) produces up to 5mg of allicin. However- assuming allicin can significantly improve certain health factors- it is not clear precisely how much is required to achieve this. Neither is it clear what doses- if any- may be harmful. 

In the absence of this information, regularly including normal amounts of garlic in recipes would seem a wise approach. 

Note that I suggested garlic, rather than allicin supplements. A study has shown that an allicin-containing extract of garlic inhibited the bacteria Staphylococcus aureus twice as well as an equivalent amount of synthetic allicin. It is possible that other phytochemicals present in garlic account for this, rather than a superiority in the properties of allicin in garlic, but that is of no consequence to the outcome.

Furthermore, the unstable nature of allicin means you want to minimise the opportunities for it to break down before it reaches your plate. For this reason the best choice is fresh garlic that YOU crush or cut. Coming in at a close second is garlic paste- as there seems to be a slight loss of allicin during processing and storage. Other varieties such as jars of powder or dried, crushed garlic are likely healthful overall… but they tend to lose a lot of allicin content due to heat treatment. Whatever variety you choose, storing it at a cool temperature can preserve the allicin content. 

Are you going to change how you use garlic?