For millennia, food has been at the center of social events, in times of joy and in times of sorrow. Protein-energy malnutrition is associated with a significant impairment of cell-mediated immunity, phagocyte function, complement system, secretory immunoglobulin A antibody concentrations, and cytokine production. Deficiency of single nutrients also results in altered immune response: this is observed even when the deficiency state is relatively mild. Of the micronutrients, zinc, selenium, iron, copper, vitamins A, C, E and B 6 , and folic acid have important influences on immune responses. Overnutrition and obesity also reduce immunity. Low-birth-weight infants have a prolonged impairment of cell-mediated immunity that can be partly restored by providing extra amounts of dietary zinc. In the elderly, impaired immunity can be enhanced by modest amounts of a combination of micronutrients. These findings have considerable practical and public health significance.

The effect of exclusive breast feeding in the first few weeks after birth on infant morbidity due to infectious and allergic disorders was investigated in three separate prospective studies. In a rural community in India, breast-fed infants had a significantly lower incidence of respiratory infection, otitis, diarrhoea, dehydration and pneumonia. In an urban population in Canada, breast feeding was associated with a marked decrease in the occurrence of otitis and respiratory disease and to a lesser extent of diarrhoea and dehydration. In newborn siblings of children with atopic disease exclusively breast-fed for a minimum of six weeks, the incidence of eczema, recurrent wheezing, elevated serum IgE-antibodies to cow's milk, complement activation in vivo after milk challenge and hemagglutinating antibodies to beta-lactoglobulin was significantly lower compared with formula-fed matched group. These observations provide clinical data attesting the immunologic advantages of human milk.

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.

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