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Can garlic help prevent cancer?

BACKGROUND

Garlic is one of the best-selling herbal remedies around the world. It is most commonly recommended to protect against cardiovascular disease, but is also said to be beneficial in preventing cancer. Different studies have come to very different conclusions as to whether or not garlic does reduce someone’s risk of cancer.

The medicinal use of garlic is ancient. It is mentioned in the earliest books of the Bible, and garlic cloves have been found in Egyptian pyramids, suggesting they were used therapeutically.

EVIDENCE FROM STUDIES

Garlic extracts contain numerous compounds, several of which are chemically bonded to sulphur. Such sulphur-containing compounds are responsible for garlic’s pungent odour and important for its biological effects. Depending on how it is grown and harvested, garlic can contain up to 33 different sulphur-containing compounds. However, these are affected by the way garlic is processed, leading to different types of products having different effects.

Garlic also contains a number of vitamins, amino acids and other antioxidants. It accumulates selenium, an important trace mineral with anti-cancer properties. Selenium plays a role in controlling genes involved in the development of cancer.

Many laboratory and animal studies have shown that garlic extracts and compounds purified from garlic have anti-cancer effects. However, the results have been more variable in human trials. Large population surveys have found that people with larger amounts of garlic in their diets have lower risks of some cancers.

The clearest connections are with stomach or colorectal cancer. However, such studies can only identify possible connections between lifestyle factors, and cannot show if one factor, such as dietary garlic, causes the observed association.

A systematic review identified more than 20 trials studying garlic consumption and risk of stomach cancer. One was a randomised, double-blind study involving more than 3,000 people in China. This found no impact on stomach cancer risk after people took an aged garlic extract and garlic oil supplement for seven years. Three other high-quality studies involved detailed examinations of thousands of case records rather than being randomised controlled studies. None found a significant connection between garlic consumption and risk of stomach cancer. The reviewers concluded that there was “no credible evidence” to support the claim that consuming garlic reduces stomach cancer risk.

The same systematic review found nine studies of garlic and colorectal cancer. One small study randomly assigned people with colon or rectal polyps to placebo or a garlic supplement. Polyps indicate a higher risk of colorectal cancer. Garlic did not reduce the number of polyps, but it did reduce their size. The other studies mostly found no association between garlic consumption and colorectal cancer.

PROBLEMATIC ASPECTS

Garlic is available in many different forms: cloves that can be eaten raw or cooked, capsules containing dried garlic powder, aged garlic extract and garlic oil. Each preparation contains different active ingredients, making it difficult to know exactly what effects to expect.

Garlic is safe to take, but it can cause intestinal problems and sometimes allergic reactions. Anyone with a bleeding problem or taking a blood-thinning agent should get medical advice before taking garlic supplements. Other drugs may interact with garlic, so always discuss herbal remedies you are considering with your doctor or pharmacist.

RECOMMENDATIONS

Laboratory studies have shown that garlic contains compounds that affect cancer cells. Population studies show that there may be a connection between garlic intake and some cancers, with stomach, colon and rectal cancers being the most promising.

However, the evidence from controlled studies is not convincing. Another systematic review published this month concluded that there is some evidence that total allium vegetable intake is connected with stomach cancer risk. Allium vegetables include garlic, onions, shallots, leeks and others. This review found that daily eating 20g of such vegetables reduced someone’s risk of stomach cancer by about 10 per cent.

Perhaps the best that can be said is that garlic is good to include as part of a plant-based diet. Whether garlic supplements help reduce the risk of cancer has not been clearly established.


Dónal O’Mathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a senior lecturer in the School of Nursing, Dublin City University. If you would like to see a herbal remedy or food supplement reviewed e-mail healthsupplement@irishtimes.com