Garlic: Allicin wonderland?

Eat all the garlic you like -- it can't hurt, and might help.
Despite hundreds of scientific studies, garlic remains something of a mystery. Its use dates back thousands of years as both a traditional medicine and seasoning.

Ancient Egyptians consumed garlic to reduce fever. In the Middle Ages people believed it would protect against the plague. Soldiers in both World Wars are said to have eaten garlic to prevent gangrene. And, of course, legend credits garlic (the "stinking rose") with the power to ward off vampires.

Traditionally, garlic has been an integral part of the Mediterranean diet and many Asian cuisines. Nowadays people around the world eat garlic because they like it and/or think it is good for them. And garlic supplements are top sellers, marketed to lower cholesterol, as well as fight cancer, hypertension, diabetes and infections, including the common cold.

Garlic's ingredients

Garlic is a member of the Allium family, which also includes onions, leeks, shallots and chives. It's not rich in vitamins or minerals, but contains more than 200 chemicals, some of which are thought to confer health benefits -- notably allicin, a sulfur compound.

Formed in raw garlic when a clove is chopped, crushed or chewed, allicin is one of the substances that give garlic its strong taste and smell. But not all scientists agree that it is the main beneficial ingredient, as it breaks down quickly into other compounds when eaten. And the enzyme that forms allicin is destroyed by cooking. In fact, no one knows which, if any, component is most important; different ones may have different effects in the body.

Lab and animal studies suggest that garlic has a range of benefits. For example, it keeps blood platelets from sticking together (which would reduce the risk of blood clots) and may have anti-cancer and cholesterol-lowering effects. But what happens in people is less clear. Human studies have been small and short. Many were poorly designed and used different garlic preparations and doses, making comparison difficult.

Here are some main areas of interest:

Cholesterol. While some studies (most using supplements) have found that garlic reduces cholesterol by about 10 percent over the short term, others have shown little or no effect. A 2003 review by researchers at Brigham Young University found that garlic supplements were modestly effective in lowering total and LDL ("bad") cholesterol in six out of 10 studies.

Cancer. Population studies suggest that people who eat a lot of garlic and other allium vegetables have a lower risk of certain cancers, including stomach, prostate and colon. But it's difficult to do the large controlled clinical trials that are needed to prove that it's really garlic and not other diet, lifestyle or genetic factors that are protective.

One such study, recently done in China and published in the Journal of the National Cancer Institute, found that garlic supplementation for seven years did not reduce the risk of stomach cancer. In addition, there is no evidence to back the claims made that garlic helps prevent breast or lung cancer.

Other possibilities. Garlic may lower blood pressure -- but by fairly small amounts at best. It may "thin" the blood slightly, too. There's no good evidence it helps with upper respiratory infections, diabetes or any other condition it is touted for.

Supplemental problems

Garlic supplements vary widely in their chemical composition, depending on the age of the garlic and how it is processed. There's debate about which form -- powder, oil or aged "deodorized" garlic extract, for example -- may be best; there is no accepted standard dose.

Some products give "alliin" amounts. Alliin is the substance that is converted to allicin by the enzyme alliinase when the pill is swallowed. But unless the pill is enteric-coated, stomach acid can destroy the enzyme. Claims such as "allicin-rich" or "high potency" don't mean much either. And a new report from found that eight of 14 supplements tested had problems -- for instance, they did not meet label claims or were contaminated with lead.

Garlic supplements may increase the risk of bleeding if taken with warfarin (Coumadin), aspirin or other drugs that decrease blood clotting, or possibly with fish-oil pills. They may interact with some medications for diabetes, HIV disease, hypertension, cancer and cholesterol. Some supplements may cause nausea, heartburn, bad breath and body odor.

Bottom line

We don't recommend garlic supplements. Even if they do lower blood cholesterol, which is uncertain, the effect is relatively small, especially compared to medication. And no one knows what form or dose would be best.

But there's no harm in eating more garlic. Keep in mind that cooking garlic at high temperature destroys potentially active components. On the other hand, some people find raw garlic too strong-smelling or irritating to the mouth and stomach. So enjoy it as you like it. A milder option is elephant garlic. It's not really garlic (it's more like a leek), but it has the same types of compounds, only in smaller amounts.

Reprinted, courtesy of University of California Berkeley. For more articles and information, visit

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