THIS WEEK, the prestigious magazine Science declared that "careful study of foods and effects of cooking" offers "probably the best hope for reducing cancer incidence." While that statement is bound to cause controversy among the experts, it is indisputably evidence of a profound shift in the scientific consensus.

During the middle 1960s, the prevailing theory held that most cancers were caused by viruses. A decade later, opinion had swung to favor "environmental" causes of cancer. Over the years, however, the difficulty of sorting out the myriad factors in the total human environment led cancer researchers to concentrate more and more on clearly defined substances, any of which, acting alone, can induce cancer: pesticides, asbestos, vinyl chloride, tobacco smoke and so forth. Gradually, the broader concept of environment, meaning the social and cultural as well as the chemical environment, got lost from the picture. This, a growing number of scientists now believe, led to a misplaced emphasis on the role of chemicals and a serious lack of attention to those substances that affect everyone daily -- in particular, the diet.

Sorting out which foods -- acting alone or in concert -- may be carcinogenic is going to be a monumentally difficult task, requiring decades. Research capabilities are already overwhelmed by the job of testing the more than 50,000 chemicals now in the marketplace, and cannot even begin to keep up with the 1,000 or so new chemicals introduced each year. The possible role of elements of the diet opens up virtually a whole new field of research. Not only may thousands of substances have to be tested, but scientists must also learn how each substance is metabolized by the human body.

So what's for dinner -- tonight? What are people supposed to do? It is easy to understand that one should avoid breathing vinyl c4loride fumes or drinking chemically contaminated water. But you can hardly stop eating indefinitely. The only answer is to act on the basis of the best available knowledge, even if it is incomplete and quite general. The tentative guidelines announced by the National Cancer Institute this week are therefore an important and welcome contribution. The NCI's recommendations -- which it calls "prudent interim principles" -- are, in capsule form, to eat more fiber from cereals, fruit and vegetables, less fats (including unsaturated fats) from both meat and dairy products, only moderate amounts of alcohol and fewer total calories.

Some scientists have criticized the NCI for acting in the presence of so much uncertainty, and indeed 20 years from now these guidelines may appear primitive, even laughable. But in the meantime, the NCI has performed a valuable service in presenting to the public the best of what little is known. It would be good personal policy to listen.