Scientists have long recognized that people who live in poverty and are malnourished are more vulnerable to infectious diseases. Whether the increased rate of disease is caused by malnutrition’s effect on the immune system, however, is not certain. There are still relatively few studies of the effects of nutrition on the immune system of humans, and even fewer studies that tie the effects of nutrition directly to the development (versus the treatment) of diseases.

There are studies of the effects of nutritional changes on the immune systems of animals, but again there are few studies that address the development of diseases in animals as a result of changes in immunity. For example, one group of investigators has found that in mice, diets deficient in protein reduce both the numbers and function of T cells and macrophages and also reduce the production of immunoglobulin A (IgA) antibody.

There is some evidence that various micronutrient deficiencies — for example, deficiencies of zinc, selenium, iron, copper, folic acid, and vitamins A, B6, C, and E — alter immune responses in animals, as measured in the test tube. However, the impact of these immune system changes on the health of animals is less clear, and the effect of similar deficiencies on the human immune response has yet to be assessed. But the research at this stage is promising, at least for some of the micronutrients.

So what can you do? If you suspect your diet is not providing you with all your micronutrient needs — maybe you don’t like vegetables or you choose white bread over whole grains — taking a daily multivitamin and mineral supplement brings health benefits of many types, beyond any possibly beneficial effects on the immune system. Taking mega doses of a single vitamin does not. More is not necessarily better. Researchers are investigating the immune boosting potential of a number of different nutrients.

Selenium.Some studies have suggested that people with low selenium levels are at greater risk of bladder, breast, colon, rectum, lung, and prostate cancers. A large-scale, multiyear study is currently in progress to look at the effects of combining selenium and vitamin E on prostate cancer prevention.

Vitamin A.Experts have long known that vitamin A plays a role in infection and maintaining mucosal surfaces by influencing certain subcategories of T cells and B cells and cytokines. Vitamin A deficiency is associated with impaired immunity and increased risk of infectious disease. On the other hand, according to one study, supplementation in the absence of a deficiency didn’t enhance or suppress T cell immunity in a group of healthy seniors.

Vitamin B2.There is some evidence that vitamin B2 enhances resistance to bacterial infections in mice, but what that means in terms of enhancing immune response is unclear.

Vitamin B6.Several studies have suggested that a vitamin B6 deficiency can depress aspects of the immune response, such as lymphocytes’ ability to mature and spin off into various types of T and B cells. Supplementing with moderate doses to address the deficiency restores immune function, but mega doses don’t produce additional benefits. And B6 may promote the growth of tumors.

Vitamin C.The jury is still out on vitamin C and the immune system. Many studies have looked at vitamin C in general; unfortunately, many of them were not well designed. Vitamin C may work in concert with other micronutrients rather than providing benefits alone.

Vitamin D.For many years doctors have known that people afflicted with tuberculosis responded well to sunlight. An explanation may now be at hand. Researchers have found that vitamin D, which is produced by the skin when exposed to sunlight, signals an antimicrobial response to the bacterium responsible for tuberculosis,Mycobacterium tuberculosis. Whether vitamin D has similar ability to fight off other diseases and whether taking vitamin D in supplement form is beneficial are questions that need to be resolved with further study.

Vitamin E.A study involving healthy subjects over age 65 has shown that increasing the daily dose of vitamin E from the recommended dietary allowance (RDA) of 30 mg to 200 mg increased antibody responses to hepatitis B and tetanus after vaccination. But these increased responses didn’t happen following administration of diphtheria and pneumococcal vaccines.

Zinc.Zinc is a trace element essential for cells of the immune system, and zinc deficiency affects the ability of T cells and other immune cells to function as they should. Caution: While it’s important to have sufficient zinc in your diet (15–25 mg per day), too much zinc can inhibit the function of the immune system.

Excerpted fromThe Truth About Your Immune System, a Special Health Report from Harvard Health Publications