There have been hundreds, maybe thousands, of studies on garlic, but its health effects remain something of a mystery. Garlic is a key part of the Mediterranean diet and many cuisines around the world. And it has been used medicinally since ancient times.
Some people still eat garlic, at least in part, for the potential health benefits. This has boosted sales of garlic supplements, which are marketed to lower cholesterol, as well as fight cancer, hypertension, diabetes, arthritis, Alzheimer’s disease and the common cold.
Potential Benefits of Allicin in Garlic
Garlic contains interesting compounds that have been linked to many proposed health benefits. One is allicin, a sulfur compound formed in raw garlic after a clove is cut or crushed. Allicin is a major source of garlic’s taste and smell. But not all scientists agree that allicin is the key ingredient, since it breaks down quickly into other compounds. And the enzyme that forms allicin can be destroyed if the whole clove is cooked before being cut (that’s why cooked cloves taste less garlicky). In fact, no one knows which, if any, component is most important.
Lab and animal studies suggest that garlic (or compounds from it) has a range of health benefits. For example, garlic keeps blood platelets from sticking together, which reduces the risk of clots, and may have anti-cancer, anti-inflammatory and cholesterol-lowering effects. But what happens in people is unclear. Nearly all human studies have been small, short and/or poorly designed. Plus, they have used different garlic preparations and doses, making comparison difficult.
Here are some main areas of interest:
- Cholesterol. While some studies (many using supplements) have found that garlic reduces LDL (“bad”) cholesterol somewhat, others have shown little or no effect. A well-designed 2007 study from Stanford University found no benefit when it tested raw garlic and two popular supplements (one containing powdered garlic, the other an aged-garlic extract) for six months in people with high LDL. More recently, two analyses concluded that clinical trials have not shown consistent or significant improvements. Regardless of its effect on cholesterol, there’s no evidence that garlic prevents heart attacks (unlike statins).
- Blood pressure. Small, short-term studies have found that garlic can lower blood pressure slightly in people with hypertension. But various garlic preparations may have different effects.
- Cancer. The evidence is mixed, at best. Some (not all) population studies have found that people who eat a lot of garlic have a lower risk of certain cancers, notably stomach, colon and prostate. But there have been few large, long-term randomized trials, which are needed to prove that it’s really garlic, and not something else about garlic eaters, that affects cancer risk. Two such studies, done in China a few years ago, reached opposite conclusions about the effect of garlic pills on the risk of stomach cancer.
- Other claims. For other conditions, such as upper respiratory infections, diabetes and arthritis, there’s no good evidence of benefit.
Garlic Supplement Problems
Garlic supplements vary widely, depending on the age of the garlic and how it was 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. Claims such as "allicin-rich" or "high potency" don't mean much. Testing of garlic supplements by ConsumerLab.com found that nearly half had problems—for instance, they did not meet label claims or, even worse, were contaminated with lead.
Some garlic supplements may reduce blood clotting, which could theoretically be a problem if you have a bleeding disorder, are planning to have surgery or are taking a drug that affects blood clotting, such as warfarin (Coumadin). The supplements may interact with some medications for diabetes, HIV, hypertension, cancer and high cholesterol. Their effect on many drugs has not been studied. Like raw garlic, supplements may cause nausea, heartburn, stomach upset, bad breath and body odor.
BOTTOM LINE: Don’t take garlic supplements. Even if they do lower blood cholesterol or blood pressure or thin the blood, which is uncertain, the effect is small, so the supplements can’t replace medication. In any case, no one knows what form or dose would be best. Still, there’s no harm in eating more garlic in your food, if you like it.
Adapted from The University of California, Berkeley Wellness Letter (January 2012)