After smoking, a poor diet is one of the most important risk factors for cancer. Estimates are that almost four of every 10 cancer deaths "are related to the way we eat," reports the National Cancer Institute (NCI).

Small wonder, then, that NCI is actively pursuing nutritional studies to understand both how certain foods promote cancer and how dietary manipulation might help prevent -- and perhaps even treat -- this disease.

Current work investigates a range of dietary components -- from what protection the trace element selenium may offer, to the role dietary fat may play in promoting breast cancer, to the effects of vitamins A, B12, C and E in preventing cancer.

Regardless of the outcome of these studies, enough evidence already exists to suggest dietary changes that can help reduce cancer risk, according to NCI. These include lowering total fat in the diet, maintaining proper body weight for height, age and sex, and increasing dietary fiber. NCI also offers this advice:

*Eat at least three to four servings a week of cruciferous vegetables (such as brussels sprouts, cabbage, cauliflower, rutabagas and turnips), which are associated with a reduced risk of colon cancer.

*Limit exposure to aflatoxins -- naturally occurring poisons produced by molds that grow during harvesting, shipping and storage of foods, particularly nuts, grains and seeds. Aflatoxins have been linked to cancer of the stomach, liver and kidney in humans. "You can reduce your exposure to these molds by keeping nuts, grains and seeds in dry, sealed containers," NCI advises. "If these foods become moldy, throw them away."

*Eat a daily serving of foods high in vitamins C and A. Research links diets high in vitamin C with a reduced risk of cancer of the stomach and esophagus. People who consume diets low in vitamin A have an increased risk of cancers of the lung, bladder and larynx. Good sources of these vitamins include dark green leafy vegetables such as broccoli, Swiss chard, kale, spinach and endive. Other sources are yellow-orange vegetables including carrots, sweet potatoes and pumpkins, and yellow-orange fruits such as apricots, cantaloupes, cherries and papayas, plus citrus fruits like lemons, limes, oranges, tangerines and grapefruit.

*Limiting consumption of barbecued, charcoal broiled, grilled and smoked foods. Cooking at high temperatures, even in an oven, seems to promote the formation of chemicals known as mutagens, which help cause cancer. "If people choose to barbecue," says Dr. William DeWys, NCI's coordinator of diet and nutrition research, "they should attempt to limit the temperature by reducing the number of coals or elevating the grill away from the fire."

For the future, nutritional therapies are being investigated as potential cancer fighters. Of particular interest are various forms of vitamin A, which Dr. Michael Sporn of NCI and Richard Moon of the IIT Research Institute in Chicago have shown can prevent the development of breast and bladder tumors in animals. Certain types of vitamin A may be able to prevent the spread of cancer even after it has taken hold in an animal, they theorize.

Also under study by Moon: BHA and BHT, the antioxidants used to preserve bread products, may also be effective in preventing cancer.