YOUR MOTHER was right. Eat your vegetables. They're good for you.

For decades, scientists and health professionals have suspected that one's diet somehow might affect his predisposition to cancer. Such studies combined to give impetus for the National Academy of Sciences' report in June recommending that Americans change their diets to include less fatty meat, butter and whole milk, and more fruits, vegetables and whole grains, in order to reduce their risk of developing cancer.

Many of the scientists behind the studies recently gathered at the Shoreham Hotel for the second annual Bristol-Myers Symposium on Nutrition Research with evidence to suggest several conclusions concerning diet and cancer:

* Studies of cancer and other degenerative diseases do not belong exclusively in the rarefied atmosphere of medical laboratories, but in the labs of nutritionists, biochemists, botanists and perhaps even on the couches of psychologists.

* Good nutrition can no longer be defined as the absence of deficiency diseases (scurvy, rickets, anemia and the like). Optimum health requires maintaining a proper balance of nutrients consistently.

* And while none of the scientists is ready to proclaim a dietary cure for cancer, many of their reports reiterated that a diet heavy in fruits and vegetables--and therefore high in vitamins A, B-complex, C and fiber--is favorably linked with lower risk of degenerative diseases.

Dr. Denis Burkitt, the surgeon who made fiber famous after his work in Africa, lays the blame of many diseases, including cancer, at the doorstep of Western civilization. "You never get appendicitis until after you learn English," he said laconically in his keynote address to several hundred doctors, nutritionists and biochemists. Hiatal hernia, heart disease, hypertension and cancers of the colon, breast, endometrium and prostate occur frequently in Western culture, he says, because its diet is heavy on fat and white flour, high in sugar and low in complex carbohydrates. Fiber, he says, is the "grave casualty" in the American diet; as a result, we spend a fortune on laxatives and face a host of health problems.

Dr. Michael Sporn, chief of the Chemoprevention Laboratory at the National Cancer Institute, took the traditional medical approach to vitamin A--megadoses of the vitamin (or a slightly altered form of the vitamin) have been shown to prevent cancer in animals. In the future, he says, megadoses may prove useful in preventing human disease--not only a variety of cancers, but rheumatoid arthritis, cardiovascular disease and psoriasis as well. While Sporn is "very worried that there would be any promiscuous use" of these chemicals until safety tests are performed (many forms of vitamin are toxic in large doses), he says the potential of megadose treatment "should be given serious consideration."

But vitamin megadoses are on the far end of the nutrition spectrum, and presently practical only in a medical sense. Dr. Paul Newberne's research on B vitamins brings nutrition back to the dinner table, hinting that even minor deficiencies may influence someone's predisposition for cancer.

Dr. Adrianne Rogers, Newberne's co-researcher at Massachusetts Institute of Technology, agreed. "With some deficiency you may not get the deficiency disease" but may get resulting damage to metabolism or the immune system, she said, and that could mean cancer. For instance, she said, a liver might be predisposed to cancer long before liver damage, due directly to vitamin deficiency, develops.

Many of the symposium reports were corroborated by Dr. Elizabeth Bright-See, a researcher at the Toronto Branch for Human Cancer Prevention, in her studies of certain population groups. While trying to discover if vitamin C and E consumption affected a woman's chance for cervical cancer, Bright-See found that diets high in foods containing vitamin C do indeed correlate with low risk of cervical cancer. However, she said, these diets are also high in forms of vitamin A, certain B vitamins and fiber.

In addition, she said, the diets were rich in phenols, a substance prevalent in plants and perhaps relevant to lowered cancer risk. Phenols have traditionally been considered non-nutritive, the way fiber used to be, and very little is known about their relation to health. This situation, she says, further demonstrates the need for cancer research to cross disciplines, and in this case to include even plant scientists.

Then, she said, we may need the help of psychologists to teach us to change our eating habits.

In order to cross the lines of expertise, the federal government is contemplating research cooperation between some nutritionists at the Department of Agriculture Research Station at Beltsville and the National Cancer Institute at Bethesda.

Funding, said Dr. Robert Reynolds, coordinator of the nutrition team, "is still up in the air." But tentative plans are to "look at how the normal human body metabolizes certain of the nutrients which have been implicated in the prevention of cancer," such as vitamin A, beta carotene and selenium.

The key word, said Reynolds, is normal--with less emphasis on rats and megadoses. Research may show what normal human beings with normal dietary intakes do to alter their predisposition to disease.