Nutrition and immune responses.

Clinical and epidemiologic data suggest a causal relationship between nutritional deficiency and infection. Among other factors, impaired immune responses secondary to malnutrition increase susceptibility to infectious illness. Protein-energy undernutrition and deficiencies of iron, zinc, pyridoxine, and other nutrients depress a variety of immunity functions. Cell-mediated immunity, complement system, microbicidal activity of phagocytes, secretory antibody response, and antibody affinity are often decreased. Recent studies have revealed many metabolic and hormone alterations as well as changes in the number and function of lymphocyte subpopulations. Obesity also is associated with impaired cellular immune functions. Dietary factors may play a critical role in host resistance to disease.