Nutritional regulation of immune function in low birth weight infants and in the elderly

There are many striking similarities between the two ends of the age spectrum. Both neonates and the elderly have suboptimal immune responses and are susceptible to infection. When nutritional deficiency complicates the picture, impairment of immuno-competence is more marked and longer-lasting.

The immune system develops during fetal life and the first few months after birth. If an infant is born preterm or if he or she exhibits growth retardation as a result of a number of environmental factors, including maternal malnutrition or infection, im-munocompetence is reduced. The impact on T lymphocyte numbers and cell-mediated immunity is most discernible. The preterm infant of low birth weight generally recovers its ability to mount immune responses by the age of 3 months. However, the small-for-gestational-age (SGA) infant may continue to show reduced cell-mediated immunity for several months and years. There is a significant difference in the immunocompetence of SGA infants who exhibit a higher level of morbidity and that of infants with lower morbidity. In laboratory animal models of intrauterine malnutrition, immune responses are impaired both in first- and second-generation offspring.

At the other extreme of life, many but not all elderly individuals show a progressive loss of immune function. Cell-mediated immunity is impaired. This may be due in part to a marked reduction in the putative thymic hormone(s). It is interesting that about 35% of subjects above 65 years retain immunocompetence at levels seen in young adult life. Other studies have documented alterations in nutritional status and body composition, including decreased lean body mass, loss of visceral protein, and increase in the relative proportion of body fat. These changes in body constituents may result from a variety of pathogenic factors, such as altered taste acuity, reduced food intake, malabsorption, and the metabolic consequences of concurrent disease. Deficiencies of vitamins and trace elements are observed in almost one-third of all elderly. To date, there are only a few studies that have looked concurrently at both nutrition and immunity in the elderly.

The age-related decline in immunity is similar to that seen in protein-calorie malnutrition. Since the elderly are known to be among the most poorly nourished in industrialized countries and since changes in food intake, body composition and protein metabolism are known to occur with advancing age, the possibility arises that altered immune status in the elderly could be ascribed in part to nutritional deficiency. A number of recent studies have shown the frequent occurrence of nutritional deficiencies in the elderly. When these are corrected even partially, there is a significant improvement in a variety of cell-mediated immune responses and in natural killer (NK) cell activity and cytokine production. Even morbidity may be reduced when such nutritional support is provided. The causal contribution of altered nutritional status in the elderly to deficits in their immune responses needs further study. Recent data indicate that a causal relationship does exist between undernutrition and impaired immunity in many elderly individuals and that this is a correctable abnormality in the majority of them. It is not clear whether maintenance of good nutrition and improved immunocompetence will alter morbidity and longevity. Obviously, long-term prospective studies are required to answer these questions.

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