RESEARCHERS studying the often-surprising effects of nutrition on immunity report that dietary manipulation can become a promising new tool to foster recovery or prevent disease in millions of people.
''We are discovering that some nutriments can be used, not so much as foods, but as modulators, manipulators or stimulators of the immune system,'' said Dr. Robert A. Good, a pediatrician and immunologist at All Childrens Hospital in St. Petersburg, Fla. ''This could turn out to be pretty powerful medicine if we can understand it better.''
In recent years, scientists have been teasing out the immunological effects of such nutrients as dietary fats, amino acids, vitamins A, E and B-6 and the minerals zinc, iron, copper and selenium. They are finding, for example, that the amino acid arginine can help prevent postoperative infections and that common polyunsaturated fats can impair immune responses that protect against cancer. The findings are expected to help design diets to prevent heart disease that also boost, rather than suppress, immunity.
At the same time, based on this new knowledge, physicians are increasingly using tests of immunological competence to help determine the nutritional status of patients.
Potential beneficiaries of immune-modifying nutrient adjustments include young children and the elderly, certain cancer patients, patients suffering from severe burns or autoimmune diseases, and people undergoing major surgery, organ transplants or treatments that inadvertently suppress immune responses.
There are even hints that dietary measures might eventually be used to slow the aging of the immune system and to extend the human lifespan by delaying the onset of diseases that result from immunological decline. There is no reason to believe, however, that special diets or specific nutrients can counter the virally induced suppression of immune responses in AIDS patients.
''As we learn more, we should be able to tailor-make diets to the needs of the moment, for example, using one diet to down-regulate the immunological response that leads to rejection of a transplanted organ and another diet to foster rejection of a cancer,'' Dr. Good said.
Preliminary research has already indicated that nutrient changes in formulas given by tube or vein to surgical and burn patients may significantly decrease their risk of life-threatening infections. If such changes prove beneficial in more definitive studies, this measure alone could save thousands of lives and millions of dollars each year.
For example, Dr. J. Wesley Alexander and Michelle A. Gottschlich at the Shriners' Burn Institute in Cincinnati studied 57 burn patients, who usually are not fed for a day or so after injury. They showed, however, that immediate administration of a special liquid diet by gastric tube could reduce the risk of infections in severely burned patients by 50 percent. The nutritious diet also shortened the patients' hospital stay and reduced death rates when compared with patients fed standard formulas. The special diet was rich in protein, vitamins A and E, iron and other vitamins and trace elements and derived half its fat from omega-3 fatty acids, or fish oil.
Other research at Albany Medical Center has shown that highly refined fish oil supplements can suppress the symptoms of rheumatoid arthritis, an autoimmune diseases in which the body's immune system mistakenly attacks its own tissues. And studies are under way by Dr. George Blackburn at New England Deaconess Hospital in Boston in patients with early stage cancers of the breast, colon and prostate to test the ability of fish oils to prevent metastasis, or spread of cancer cells beyond the site of origin.
Animal studies have already shown that fish oil in the diet can increase the activity of white blood cells called macrophages, which detect and attack foreign cells.
Both deficiences and excesses of the same nutrients can be harmful to immunological defenses. For example, a deficiency of the trace element zinc depresses the activity of immunological cells, but Canadian studies have shown that an excess of zinc (20 times the recommended daily amount of 15 milligrams) also causes an immunological decline in otherwise healthy people.
The studies by dozens of scientists also show that polyunsaturated fats, which have been widely recommended as replacements for artery-clogging saturated fats, can, when consumed in amounts now common in American diets, impair normal immunological responses. This impairment, in turn, might increase the risk of developing certain infections or even cancer. This observation, made in animal studies, may account in part for the finding in some large studies that diets high in polyunsaturated vegetable oils diminished heart attacks but not overall death rates. Researchers say that polyunsaturates, even when consumed at the recommended level of 10 percent of total calories, could be hazardous.
''Moderation remains the key to a health-promoting diet and a healthy immune system,'' said Dr. Ranjit K. Chandra, pediatrician and immunologist at the Memorial University of Newfoundland. Dr. Chandra, who studies both children and the elderly, was editor of ''Nutrition and Immunology,'' a text summarizing the state of the science published this winter by Alan R. Liss Inc.