Introduction
Nature has provided scientists with early opportunities to learn about the effect of nutrition on the immune system. Low-nutrient soils have caused mineral deficiencies in some populations of livestock and humans. By studying these populations, scientists discovered the importance of selenium to the immune system. Famines, natural disasters, severe poverty, and wartime have likewise provided us with many of our early learning opportunities about the relationships among nutrition, immune functions, and health. Many essential micronutrients (vitamins and minerals), such as copper and zinc, were once believed to be unessential to the human diet but are now known to be important for normal immune function.
Thorough examination of the effects of nutrient deficiencies, combined with clinical observations and detailed investigations, is helping us understand the relationship between nutrition and immune function. This chapter briefly highlights the effects of nutrient deficiencies and overnutrition on a competent immune system and mentions some current research.
Nutrient Deficiencies
Generalized malnutrition, historically referred to as protein-energy malnutrition (PEM), often coexists with deficiencies of one or more micronutrients. PEM is one of the earliest forms of malnutrition to have attracted the interest of the medical community. The association between severe PEM and atrophy of the thymus gland (primary lymphoid tissue) was described nearly 150 years ago, even before it was realized that the thymus is a principal body organ of the immune system. Beginning as early as 1911, several investigators documented that thymic atrophy results from malnutrition due to food scarcity or from illness associated with cachexia (severe weight loss due to diseases such as cancer). Malnutrition also commonly causes atrophy of the tonsils (secondary lymphoid tissue).
During the first half of this century, an association between severe PEM and increased infections was described. But it was not until the 1960's, when the important role of the thymus as a primary source of cells (T-lymphocytes) of the immune system became known, that the relationships among nutrition, immunity, and health were established. During the past 20 years, it has become clear that PEM in children and adults reduces the number and function of T-helper immune lymphocytes, which promote an active immune-protective response against infectious and other diseases, such as cancer.
Healthy subjects who are fed balanced meals, but in restricted amounts, show suppressed immunity similar to that of children and hospital patients with PEM. ARS and U.S. Army researchers have found that young, healthy men consuming less energy in the form of food and drink than is required during heavy work and exercise show decreased immune function. In contrast to unhealthy subjects, such individuals show rapid correction of their immune functions when they receive enough calories to balance their energy demands.
The role of micronutrients in the immune system has been studied in individuals suffering from micronutrient deficiencies, such as hospitalized patients receiving liquid diets that lacked an unknown essential micronutrient, children living in regions deficient in select micronutrients, livestock grazing on mineral-deficient grassland, and research animals. Through such studies, the essential role of several minerals (iron, zinc, copper. magnesium, and selenium) and several vitamins (vitamin A, vitamin C, B-group vitamins, and vitamin E) has been demonstrated for normal immune functions. The mechanisms of their functions on the immune system and the safe range of intake are not fully understood. As with many nutrients, micronutrients interact with each other in maximizing their role in immune function.
High-Fat Diets
Consuming too much fat can have a suppressive effect on the immune system. The lifestyle of many people of the industrialized world leads to diets that are high in fat.
From animal and human studies, it is known that both the concentration and type of dietary fats play a crucial role in the function of the immune system. Diets high in fat cause reduced resistance to infectious diseases in animals and suppressed cellular immune function in both animals and humans. At high concentration, polyunsaturated fats that are low in vitamin E appear to be more immunosuppressive than saturated fats.
The immunosuppressive effects of high-fat diets can be lessened by eating foods containing antioxidants (certain vitamins and minerals) that neutralize these effects. Foods that have a stabilizing effect on the immune system include those containing vitamin E (oils, shortening, margarine, fruits, and green leafy vegetables), vitamin C (fruits and vegetables), beta-carotene (brightly colored fruits and vegetables), selenium (meats, cereals, dairy products, fruits, and vegetables), copper (liver, nuts, and whole-grain cereals) and manganese (nuts, whole-grain cereals, dried legumes, and tea).
Current and Needed Work
Early studies on nutrition and the immune system involved severe nutritional deficiencies. Since more sophisticated laboratory techniques are now available, ARS researchers are now able to compile information to establish the effects of moderate nutritional alteration on the immune system. Although our understanding of the role of nutrition in the immune system is increasing, considerably more work is needed before we can use the information to improve health.
Work describing maturation of the immune system in breast-fed and bottle-fed infants is needed. Most milk formulas for infants in the Western World are now considered to meet the conventional nutritional requirements of newborn infants. Despite this, investigation of the effects of breast- and bottle-feeding on select immune functions has shown that breast-feeding has contrasting effects on the development of immunity. In the early neonatal period, up to 6 weeks of age, there are enhanced immune responses in the breast-fed infants. But by 3 months of age, many of the immune responses are higher in formula-fed infants. These results suggest that, during the first 6 weeks, breast-fed infants are receiving enhanced immune protection from breast milk, while formula-fed infants must produce this immunity themselves. Developing immunity is not usually a problem for formula-fed infants in societies where the public health standards are high.
Many studies have documented the frequent occurrence of nutritional deficiencies in the elderly. Observations show that these deficiencies are associated with undernutrition due to reduced calorie intake; overall reduced intake; and lower blood levels of iron, zinc, vitamin C, B vitamins, and vitamin E. Socioeconomic deprivation, physical disability, isolation, dental problems, and increased nutrient needs due to underlying disease are common causes of nutritional problems in the elderly. Although it is logical to try to correct nutritional deficiencies in the elderly in order to improve their immune responses and reduce the risk of infectious disease and other age-related disorders, the desirability of taking megadose supplements of vitamins and minerals is questionable.
During the 150 years since a relationship between severe undernutrition and atrophy of the thymus gland was first described, our understanding of the role of nutrients on the immune system has increased tremendously. ARS researchers are now reaching the point where more can be learned about the effects of marginal changes in nutrition on immune protection against diseases. During this time, it is comforting to know that the body has tremendous capabilities for efficiently using nutrients, even in unbalanced amounts, and maximizing responsiveness of the immune system.