Introduction

The history of garlic's medicinal uses goes back to 1500 BC. In more recent times, its potential value in protecting against heart disease has been under investigation for at least ten years, but there is still a long way to go before we can say with certainty what role it plays. Does garlic protect against cancer? Researchers from the University of North Carolina recently examined the major studies that looked at that question and have concluded that garlic may guard against stomach and colorectal cancers. Their findings are published in a recent issue of the American Journal of Clinical Nutrition.

Role of garlic in cancer prevention: is the sum of the evidence stronger than the individual pieces?

The scientists analyzed the results of 22 studies from around the world, conducted over the past 33 years, which looked for an association between garlic consumption and cancer. Overall, they found that people who ate 6 to 10 (or more) cloves of garlic weekly had 30% less colon and colorectal cancer and almost 50% less stomach cancer than those who ate less than one clove a week. Despite the conclusions derived from the existing data, the researchers expressed skepticism about their results.

Before you add garlic to the shopping list for its health benefits, look at the recent analysis with a scientist's eye. Most of the studies did not consider dietary differences other than garlic. People eating large amounts of garlic may have also been eating many fruits and vegetables. And a high fruit and vegetable intake is currently believed to protect against cancer. Or, people who pass on garlic may consume diets that are different in other ways. They may eat more high fat foods or drink more alcoholic beverages --both linked to the development of certain cancers. In short, since we cannot say with certainty that the level of garlic consumed was the only difference in the diets of the study volunteers, it is impossible to conclude that garlic was responsible for the lower rates of stomach and colorectal cancer.

The researchers point to another, somewhat more subtle problem that they discovered in a second series of analyses. It is sometimes called "publication bias." In general, studies are more likely to be published if an effect is demonstrated, -- in this case, an association with levels of garlic intake and cancer rates. A finding of "no effect" is an important result. But, because it isn't as exciting, it may be less likely to be published. Using a special mathematical model, the authors demonstrated that in the case of the garlic-cancer connection, publication bias had occurred, and that, quite likely, studies which failed to find an effect had simply not made their way into the scientific literature.

Garlic may indeed contribute to cancer prevention. Both laboratory and animal studies indicate that components of garlic may have cancer-fighting properties, but that conclusion must wait for more evidence.

Add more garlic to your diet... Only if you like the taste!

If you like garlic, an extra clove or two may only heighten your gastronomic pleasure. But remember that garlic is rarely eaten by itself. So, consider it in the company that it keeps. It will not make high-fat cream sauce more healthful, but it certainly can make a low-fat tomato sauce more pungent.