Nutrition in health and immune function of ruminants

Nutrition, an important modulator of immune function, can often influence the balance between health and disease. Diets influence immunity through meeting substrate needs of immune system cells, deprivation of nutrients from pathogen, direct regulatory effects on cells of immune system, changing the balance of hormones that regulate immunity, reduction of collateral damage induced by an immune response, and physical and chemical immunomodulation action of non-nutrient components of feeds. Poor immune-competency can result in greater incidence and duration of infections causing decrease in food intake, nutrient losses, and impaired animal health and well-being. Substrates (energy, amino acids and all nutrients) are necessary for the anabolic activity of immune system's cells (leukocytes), such as proliferation and antibody production as well as the secretion by the liver of large quantities of immunologically active molecules, the acute phase proteins. The small size of the immune system, its capacity to appropriate nutrients from other tissues, and endowment with high priority nutrient transporters generally indicate that the immune system can usually obtain many of the nutrients that it needs to do its functions over a wide range of dietary levels. However, requirements of some nutrients may not be met by nutrient appropriation mechanism under low dietary intake because of their low concentration in muscle and their relatively high need within the immune system. Role of beta-carotene, vitamin E, copper, zinc and selenium in aiding immunity are mediated not only through maintenance of appropriate amounts of antioxidants, but also by the involvement of enzymes in aiding the production of energy, proteins, cells and hormones needed in defense mechanisms. This review discusses the complex relationship between nutritional status and immune function with special focus on mechanism of action of individual key nutrient.