Although all essential nutrients are likely needed in some measure for effective immune function, certain micronutrient deficiencies and excesses have been recognized as particularly important. These include the following:
- Vitamin A. As early as the 1920s, vitamin A was called “the anti-infective vitamin” because it is needed to maintain the mucosal surfaces of the respiratory, gastrointestinal, and genitourinary tracts and for differentiation of immune system cells. More than one hundred clinical trials have shown that vitamin A supplementation in populations with low vitamin A status reduces the incidence and fatality of infections of measles, malaria, and diarrheal diseases. However, animal studies suggest that excessive vitamin A can suppress the immune response and increase susceptibility to pathogens. Thus, screening for vitamin A status before administering supplements has been recommended as part of public health efforts to combat deficiency.
- Vitamins C and E. The immune activities of defensive cells such as macrophages require oxygen and generate a highly reactive molecule, called a reactive oxygen species, that can damage the cell membrane if there is insufficient antioxidant protection. Both vitamin C and vitamin E provide this protection.
- Zinc. The importance of zinc to immune function was suggested by the observation that zinc-deficient dwarves in the Middle East died of infections by their early twenties. Zinc is now known to be necessary for gene expression and enzyme activation for B and T cell proliferation. Even marginal zinc deficiency impairs immune response. However, excessive zinc supplementation depresses immunity, possibly by causing copper deficiency.
- Copper. Even a marginal copper deficiency reduces a growth factor needed for immune cells to multiply. Lack of circulating neutrophils is a classic sign of copper deficiency in humans. Lack of copper also impairs the ability of both the neutrophils and macrophages to kill pathogens.
- Iron. Iron has a complicated relationship with immune function. It is now clear that a mild iron deficiency impairs immune function, perhaps because activated T cells produce a receptor on their surface that can take up the iron they need for multiplication. However, severe deficiency does impair the function of T and B cells, as well as neutrophils. Macrophages take up and store iron during infection and seem unaffected by deficiency. This storage is thought to be beneficial because it keeps iron away from invading microbes, which require iron to multiply. This may explain why some studies show that iron supplementation given to children during infection is detrimental, and why iron toxicity increases the rate of infections. Also, excessive iron is a potent oxidant that can damage immune-cell membranes.
- Selenium. In trace amounts, selenium is necessary for the synthesis of thirty-five body proteins, many of which are important enzymes. Selenium has two roles in immune function. It is a required coenzyme for glutathione peroxidase, an important antioxidant enzyme in neutrophils and other immune cells. It also promotes proper B and T cell proliferation and antibody production. Selenium deficiency in an infected host also permits viruses to multiply over a longer period and to mutate into more pathogenic strains. However, selenium excess also impairs immune cell activity.
- Thus, as with iron and vitamin A, both excess and deficiency of selenium compromise the ability to resolve infection.