Diet is now considered a major weapon against cancer. The National Cancer Institute estimates that about one-third of all cancers are linked to diet, and recent reaearch indicates that what you eat may help to significantly reduce your risk.

Cancer develops over a long time, which means that you have years - typically decades - in which to hinder or promote it. Researchers are finding that what you eat may interfere with cancer growth at various stages. For example, certain foods can block the chemicals that initiate cancer. Antioxidants, found in some vitamins and minerals, can snuff out oxygen free radicals, substances that are thought to make cells more susceptible to cancer, and they can even repair some of the cellular damage that has been done. And some food - wheat bran in particular - has been shown to shrink precancerous cells.

A recent review of 170 studies from 17 nations reveal that people who eat most fruits and vegetables have about half the cancer rates of those who eat the least. That includes cancers of the lung, colon, breast, cervix, stomach, bladdwer, pancreas and ovary. In fact, some research suggests that frequent consumption of fruits and vegetables can cut the risk of lung cancer even in smokers. "It is almost mind-boggling," says Tim Byers, an epidemiologist with the U.S. Centers for Disease Control and Prevention, "that ordinary fruits and vegetables can be so effectuive against such a potent carcinogen as cigarette smoke."

One of the most studied antioxidants in vegetables and fruits thought to protect against cancer is beta-carotene, concentrated in deep green, yellow and orange vegetables such as carrots, sweet potatoes and spinach. Fruits high in beta-carotene include apricots and cantaloupes. In test-tube studies at Harvard University, beta-carotene had a direct toxic effect on cells taken from malignant tumors. It also reduced the growth of lung cancer cells and altered the proteins needed for tumors to grow.

Research also shows that beta-carotene can change in the body to retinoic acid, a substance used in clinical trials to treat certain cancers.

Hree are some of the foods that contain cancer fighting chemicals:

Tomatoes. One of the compounds in tomatoes that is thought to reduce the risk of cancer is lycopene, the pigment that makes tomatoes red. Lycopene, an antioxidant that is also found in watermelons and apricots, quenches certain cancer-triggering oxygen free radicals.

Having little lycopene in your blood is associated with a higher risk of pancreatic cancer, according to a Johns Hopkins University study. People with pancreatic cancer showed lower levels of lycipene compared with healthy individuals. Those with the least blood lycopene had over five times the risk of pancreatic cancer as healthy people with the most blood lycopene.

Lycopene is present in tomato products, including sauces, tomato paste and even ketchup.

Green Vegetables.A recent Italian study showed that dark green leafy vegetables lower the risk of many cancers. Spinach, broccoli, kale and dark green lettuces are chock-full of antioxidants, including beta-carotene, folate and lutein. A good rule of thumb: the darker the vegetable, the more antioxidants within.

Pungent Preventives. A whole host of chemicals thought to have cancer-inhibiting properties have been identified in allium vegetables, which include garlic, onions and scallions. Animal studies show that many of these chemicals block carcinogens that have been linked to colon, stomach, lung and liver cancer. A study at Pennsylvania State University found that feeding rats various garlic extracts and preparations reduced mammary tumors by as much as 71 percent. In humans, studies suggest that those who eat more onions and garlic are less prone to gastrointestinal cancer.

Research suggests that garlic compounds may even interfere with cancer progression. A recent German study found that ajoene, a garlic compound, is toxic to malignant cells. Garlic may also antagonize existing cancer by boosting immune functions, according to researchers at Loma Linda University School of Medicine. Their study, done on mice, found that garlic's sulfur compounds increased the activity of macrophages and T-lymphocytes, immune system components that destroy tumor cells.

Cirtus Fruit. "Eat oranges, grapefruits, lemons and limes as often as possible," says toxocologist Herbert Pierson, a former project officer with the National Cancer Institute. He calls citrus fruit an all-around cancer package because it possesses every class of natural substances (carotenoids, flavanoids and others) that individually have neutralized powerful chemical carcinogens in animals.

Citrus fruit may be particularly effective in reducing the risk of pancreatic cancer. One study found that in a group of Swedes, those who ate a citrus fruit almost daily reduced the risk of pancreatic cancer by one-half to more than to-thirds, as compared with those who regularly ate citrus fruit less than once a week.

Cruciferous Vegetables. Cruciferous vegetables, such as cabbage, cauliflower, Brussels sprouts, broccoli, kale, mustard greens and turnips, may reduce the risk of breast cancer. In fact, researchers at Eppley Institute for Research in Cancer and Allied Diseases at the University of Nebraska at Omaha found that feeding animals cabbage and collard greens (also a cruciferous vegetable), along with a low-fat diet, reduced the incidence of mammary cancers. Eat cruciferous vegetables raw or lightly cooked. Overcooking may destroy the indoles, compounds found in these vegetables that may protect against cancer.

Cruciferous vegetables may also work to head off stomach and colon cancers. Research conducted in Utah revealed that men who ate the most cruciferous vegetables had a 70 percent lower risk of colon cancer than those who ate the least of such vegetables.

Soybeans. Soybeans contain at least five compounds believed to inhibit cancer. In fact, one of the compoounds is chemically similar to the drug tamoxifen, which is routinely used to treat estrogen-dependent breast cancer and is not being tested in a large clinical trial to see if it can prevent the disease. In animal studies, soybean constituents have been found to block colon, skin and other cancers by possibly slowing the growth and division of cancer cells.

Dried and canned beans - chick-peas, lentils, kidney, pinto, navy, black, pink and white - contain possible anti-cancer agents as well. A group of Seventh-Day Adventists in California showed that those who ate beans weekly were 40 percent less likely to die of pancreatic cancer than those who ate them less than once a week.

Wheat Bran. Wheat bran may lower the risk of colon cancer. A double-blind study of patients at The New York Hospital-Cornell Medical Center found that two one-ounce servings of wheat bran cereal a day caused premalignant colon polyps to shrink within six months. Most remarkable, say researchers, is that such a small amount of food could have such an impact within a short period of time, illustrating that dietary intervention may work even after precancerous warning signs have appeared.

Low-Fat Milk.While the saturated fat in milk seems to promote certain cancers, another substance in milk may deter them. Researchers at Roswell Park Cancer Institute in Buffalo, N.Y., discovered, not surprisingly, that drinkers of whole milk had higher odds than non-milk-drinkers of developing cancers of the oral cavity, stomach, colon, rectum, lung, bladder, breast and cervix. But they also found that those drinking low-fat milk were less likely than non-milk-drinkers to develop cancers of the oral cavity, stomach, rectum and cervix. Why? Scientists suspect that elements such as calcium, riboflavin, or vitamins A, C, and D (present in whole milk, too, but apparently less effective) may act as anti-cancer agents in ways not yet understood.

To get the most cancer-protective compounds from your diet, strive for five or more servings of fruits and vegetables daily, advises the National Cancer Institute. One serving means one half cup of most cooked or raw fruits and vegetables, one cup of raw leafy vegetables, one medium piece of fresh fruit, or six ounces of fruit or vegebatle juice. The ideal diet is low in fat and calories, high in fiber, and it has a variety of nutrient-rich foods such as fruit, vegetables, whole grains, beans, seeds, nuts and, if desired, low-fat animal proteins.