IF YOU ARE one of those people who have just

about given up on making sense of the conflicting medical advice about what to eat, help--at least of a kind--is on the way. A striking convergence of expert opinion is coming about. More and more evidence shows that diet strongly influences the risk of coronary heart disease, cancer, hypertension and other major killers. And the recommended changes in diet for lowering the risk of each of these diseases reinforce, rather than contradict, each other.

The newest evidence comes from a two-year study of the connections between diet and cancer, issued this week by the National Academy of Sciences. The group found first of all that research into this vast and complex subject has hardly scratched the surface. But it did find enough persuasive evidence to justify issuing what it called four "interim dietary guidelines"-- the first and last words indicating that the evidence is not complete and that these are not absolute rules that will guarantee a cancer-free life.

But in the committee's judgment, following the guidelines will lower the risk of getting cancer. And since diet (not including smoking) is believed to be responsible for at least 30-40 percent of cancers, that should be enough to command attention.

The committee recommends that people restrict their intake of fats--saturated and unsaturated--to 30 percent of total calories. For the average American, that means cutting fat consumption--such things as whole milk and its products, ice cream, peanut butter, cooking fats and oils, beef and other fatty meats--by one quarter. The committee also recommends eating "very little salt-cured, salt-pickled, or smoked foods," which include ham, bacon, bologna and hot dogs. The group urges "moderation"--which it fails to define--in alcohol consumption, especially for smokers, since tobacco smoke and alcohol exacerbate each other's effects.

On the positive side, the group found evidence of a protective effect against cancer among several vitamins, minerals and fibers. It recommends eating fruits (especially citrus fruits), whole grain products, and dark-green, yellow and cabbage family vegetables (such as carrots, tomatoes, winter squash, broccoli, Brussels sprouts and cauliflower) daily. Didn't you always know that Brussels sprouts were going to win out over peanut butter, were going to be your destiny?

There will be criticism of these recommendations on the ground that the experimental evidence is not conclusive. But as the academy's report points out, "we are in an interim stage of knowledge similar to that for cigarettes 20 years ago." (Cigarettes are causing one-quarter of the cancer deaths today.) Since absolutely conclusive evidence will take years to develop, the committee members felt that the evidence justifies action now. Surely they are right.

Agriculture Secretary John Block objects to the government's "telling people what they should or shouldn't eat," so his department no longer distributes the dietary guidelines developed by previous administrations, which conform closely to these recommendations. That strikes us as saying it's fine for the government to pay when people get sick, but not for it to spend a fraction of that amount to encourage them to stay well.