Karen Collins, R.D.

However, scientists are still learning how these vegetables can promote health.

In one study, about 25,000 people from Italy and Switzerland were grouped according to how much onion and garlic they routinely ate. Some of these people had recently been diagnosed with cancer, and some formed a healthy comparison group.

Researchers controlled for known cancer risks, including BMI and calorie intake. People who used the most onion or garlic about a half-cup of chopped onion daily and a self-assessed "high" garlic consumption  were from 10 percent to 88 percent less likely to have various types of cancer than those who said they used little or none.

Cancers compared included cancers of the esophagus, mouth and throat, colon, breast, ovary, prostate and kidney. High onion intake, for example, was associated with a 56 percent lower risk of colon cancer and a 25 percent lower risk of breast cancer compared to no onion intake.

Protection against colon, stomach cancer
Colon cancer is one cancer research has associated with protection from onion and garlic consumption, but how much protection is unclear. In a study among more than 35,000 women in Iowa, participants who ate one clove of garlic a week had a 32 percent lower colon cancer risk than those who ate garlic once a month or less. An analysis of several studies worldwide linked a 31 percent lower risk of colon cancer with consumption of about four to five cloves of garlic weekly.

Stomach cancer is another of the cancers that research suggests may be reduced by onions and garlic, although the evidence for this association is less clear. In a study of more than 521,000 Europeans, about one tablespoon of chopped onion or three cloves of garlic eaten daily was linked six-and-a-half years later with a 30 percent lower chance of cancer developing in the lowest part of the stomach. Due to the relatively few cases of stomach cancer the participants developed, researchers couldn't determine whether the link is due to onion and garlic or might have occurred by chance.

A wide variation in protection seen may reflect differences between onion and garlic, in how they were prepared, or in individuals' sensitivity to their compounds. Other studies, often from China where stomach cancer is relatively common, show that people averaging five cloves of garlic a week have about half the stomach cancer as non-garlic eaters.

The power of antioxidants
Like many vegetables, onions and garlic contain antioxidants that can block highly reactive free radicals from damaging cell DNA and starting the cancer process. Laboratory studies have shown that onion and garlic compounds can increase enzymes that deactivate carcinogens in the body, enhancing our ability to eliminate carcinogens before they do any damage. Furthermore, in the laboratory onion and garlic compounds slow the growth and stimulate the self-destruction of cancer cells that form. Given this protective potential, the challenge now is to identify amounts that will provide optimal effects.

Some research hints garlic and onion's protective compounds may work more effectively when combined with other compounds that work through different pathways. For example, animal studies show even greater reduction in cancer development from garlic and tomato than from either alone.

The protective effects of onion and garlic seem related to wherever cancer cells grow in the body and not to any specific tissue, such as breast or thyroid. That leads many scientists to say that although research offers more proof of onion and garlic's impact on some cancers than others, they are likely to offer protection against a wide range of cancers.