The National Cancer Institute (NCI) has summarized the latest research findings linking diet with cancer in a booklet with practical suggestions for changing the foods we choose to eat.

The publication, "Diet, Nutrition and Cancer Prevention: A Guide to Food Choices," says diets high in fiber and low in fat, with plenty of fresh fruits, vegetables and whole grain breads and cereals, may reduce the risk of cancer.

The booklet is probably the boldest advice the federal government has yet handed out to the public about diet, because it offers consumers some firm numbers. In what may turn out to be a controversial recommendation -- at least in the nutrition community -- NCI suggests that Americans double their current average fiber intake of 10-20 grams per day. "Populations that consume diets containing twice this amount have a lower rate of cancers of the colon and rectum," says NCI, recommending enough fiber-rich foods each day to total 25 to 35 grams.

"We feel a solid body of evidence all tend to show (fiber's) protective effect," says Dr. Peter Greenwald, director of NCI's division of cancer prevention and control. One example noted by Greenwald: People in Finland have diets similar to those in the United States but eat twice as much fiber. The rate of colon cancer in Finland is one-third that in the United States. Similar findings have been found in other populations, he says.

Other nutrition professionals have preferred more cautious advice about increasing fiber intake. The U.S. Agriculture Department's recently revised "Dietary Guidelines for Americans" suggests increasing fiber but offers no specific amount and notes that while the "habitual intake of diets low in fiber may increase the risk of developing colon cancer, there are no firm data on this point."

In addition, some nutritionists at USDA are concerned that the methods for measuring dietary fiber in foods are not perfect, making it difficult to know just how much fiber is found in different foods. The other problem is that too much fiber in the diet can block absorption of other nutrients.

Since high-fat diets have been linked to cancers of the colon, breast, prostate and endometrium, NCI recommends a cutback in fat in the diet, noting in the pamphlet that the National Academy of Sciences has recommended that people lower fat intake to 30 percent of daily calories. In fact, NCI had been planning to recommend a diet with only 20 percent of calories from fat, a drastic departure from the average fat-laden American diet in which 40 percent of calories come from fat.

When nutritionists at USDA got wind of the impending recommendation last fall, they prevailed on NCI to stick with what they considered a more reasonable 30-percent level. "Only 3 or 4 percent of the adult population have diets with fat contents of 30 percent or less," says Isabel Wolf, former administrator of USDA's Human Nutrition Information Service who advised NCI on the booklet. "To recommend 20 percent would be like recommending the Pritikin diet."

In addition, Wolf says, USDA "saved" NCI from a "major embarrassment" by correcting fat content values of a variety of foods that she says were overstated by as much as 300 percent. According to Wolfe, the tables in some cases presented values for fat content of uncooked, untrimmed meats, which are usually much higher than meat cuts trimmed and cooked. If the inaccurate values had been used in the booklet, says Wolf, NCI "would have been assaulted by commodity groups."

In a section in the NCI booklet on food labels, the agency presents a mathematical formula for calculating the percentage of calories in a food product that comes from fat. (For those with math anxiety, the formula is reason enough to support mandatory fat-content labeling.) The booklet also contains a chart for calculating how much fat to eat each day. At 2,000 calories a day, for example, a diet with a 30 percent fat level would contain 67 grams of fat, with no more than 600 of the calories coming from fat.

Dietary changes don't have to be painful, according to NCI. "You don't have to give up any of the foods you like to protect against cancer risks," NCI says. "The idea is to choose more often the foods that may help reduce your risks of cancer and choose less often the foods that might increase your risk of cancer."

To help consumers meet the challenge, NCI lists the foods for each meal that should be chosen more or less often. For breakfast, NCI says, choose low-fat milk, yogurt and cheeses, and whole-grain cereals. Hold off on whole milk, cream, hard cheeses, white breads and sweet rolls.

Other advice offered by NCI:

* Eat more "cruciferous" vegetables -- brussels sprouts, cabbage, broccoli, cauliflower, rutabagas and turnips. Substances in these vegetables may exert an anticancer effect.

* Eat foods high in vitamins A and C. Some research links diets high in vitamin C with reduced risk of cancers of the stomach and esophagus and diets low in vitamin A with increased risk of cancers of the lung, bladder and larynx. These studies suggest protective effects from foods containing the vitamins rather than from vitamin supplements, NCI said.

* Reduce your exposure to aflatoxins, which are naturally occurring molds that can grow on nuts, grains and seeds and are potent carcinogens. NCI recommends keeping nuts, grains and seeds in dry, sealed containers and throwing such foods away if they become moldy.

* Heavy drinking of alcoholic beverages, particularly when combined with cigarette smoking, increases the risk of cancers of the mouth, pharynx, larynx, esophagus, liver and bladder. NCI says to reduce these risks, if you drink at all, drink only in moderation (two or fewer drinks a day), especially if you smoke.

* Some cooking methods, including barbecuing, grilling or smoking, may produce cancer-causing substances. To protect against them, wrap food in foil or put it in a pan to reduce its direct contact with the smoke and flames. You also can raise the grill further above the coals or flame and cook foods more slowly at a lower temperature. NCI advises baking, roasting, oven-broiling and microwaving rather than barbecuing or frying at high temperatures.

A free copy of the NCI booklet can be obtained by calling 636-5700.