Americans can markedly reduce their risk of getting cancer by eating less, by eating a balanced, low-fat, high-fiber diet, and by drinking only moderate amounts of alcohol, the government's National Cancer Institute said yesterday.

The pronouncement, first of its kind by the government's main cancer-fighting agency, was made in response to a growing body of evidence that seems to link diets and cancers.

For example, many authorities believe there is more breast cancer in women -- and more cancer of the colon in both men and women -- who eat large quantities of meat and dairy fats.

The statement, before a Senate nutrition subcommittee headed by Sen. George McGovern (D-S.D.), was also a response to strong pressure from McGovern and Sen. Bob Dole (R-Kan.), the subcommittee's ranking minority member, to say something to "help the public" avoid cancer.

McGovern hailed the recommendations as a major and "precedent-setting act," since both the public and the scientific community look to the National Institutes of Health -- of which the National Cancer Institute is a part -- for research leadership.

Because the advice, if taken, would mean consuming fewer animal and vegetable fats -- less meat, fewer fatty dairy products and less vegetable oils and margarine -- the NCI view can be expected to win few friends in the food industries.

Opponents, and some critical scientists, are certain to say the recommendations are based only on inconclusive evidence.

Dr. Arthur Upton, NCI director, conceded this. He said the recommendations represent only "prudent interim principles" -- "tentative" guides -- in a field in which "the exact role that diet plays remains unclear" and much research must still be done.

Nevertheless, he argued, the time has come for "a modest beginning" in applying the best available knowledge to save lives.

These dietary recommendations, it also turns out, are not very different from the "prudent," low-fat diet that most authorities advise to help avoid heart disease. The main differences, Upton said, are the addition of more fiber -- from fruits, vegetables, bread and cereals -- and the warning against too much alcohol.

"Too much" alcohol, Upton said, is certaionly any more than two drinks a day. "Two are probably in the borderline region," he said, calling "a glass of wine or beer or highball" or "no more than one or two ounces of hard alcohol" daily safely moderate.

There is still another difference, however, between this new anticancer diet and the usual prudent heart diet. Most heart specialists advise against overeating saturated -- usually solid -- fats, like meat fats and butter, and instead advise use of unsaturated, usually more liquid fats, like some oils and margarines.

Upton advised going easy on all fats, based on recent experiments showing the number of cancers in rats fed unsaturated fats was even larger than that in rats fed saturated fats (which include coconut oil).

In short, he said, "don't overuse margarine just because it doesn't contain animal fat." And the average American who now consumes an amazing 40 percent of his or her calories in fats could well reduce fat intake to 33 percent, he advised.

Upton recommended a balanced diet, including "ample fresh fruits and vegetables," to provide important vitamins and minerals without resort to vitamin pills. He advised "generous intake of dietary fiber" on the basis of reports of fewer colon -- bowel -- cancers in people and some experimental animals who eat fibrous foods.

But, he emphasized, one can only call such advice "prudent," since the evidence isn't all in. The Department of Health, Education and Welfare, NCI's parent agency, has shunned firm pronouncements on diet and cancer.

However, Dr. Ernest Wynder, head of the American Health Foundation in New York, urged such advice last June after his group's conference on cancer prevention. Dr. Mark Hegsted, head of the Agriculture Department's new Human Nutrition Center, and Dr. Anthony Miller, head of epidemiology at the National Cancer Institute of Canada, concurred.

Another panel of scientists -- among them, Dr. Barry Commoner of Washington University -- yesterday urged reform of the way NIH reviews scientists' research proposals. The panel said that without reform, establishment scientists will keep blocking the new kinds of nutrition research needed to get the answers Upton said are required.