The National Research Council (NRC) nutrient requirements for animals can be defined as nutrient levels adequate to permit the maintenance of normal health and productivity. Failure to provide a diet that fulfills the minimal requirements established by the NRC for any nutrient will ultimately immunocompromise the animal and render it more susceptible to infectious disease. Because nutrient requirements to support optimal productivity are well defined, marked deficiencies in protein, amino acids, or trace nutrients are not likely to occur in animals reared in commercial situations. However, the nutrient requirements for optimal productivity may not equal those for optimal immunity because the NRC requirements have been determined from experiments conducted in laboratory situations where infectious disease is minimal. Thus, an important issue that has been the focus of nutritional immunology research is whether specific nutrients fed at or above NRC-recom-mended levels could be used to modulate the animal's immune system in a beneficial manner.

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