Applications of chemically defined diets to the solution of nutrition problems.

Chemically defined amino acid diets have been developed for most laboratory and meat-producing animal species as well as for humans. In many cases, growth performance of animals fed these diets equals that obtained with standard intact-protein diets. The pattern of both essential and nonessential amino acids is critical to obtaining excellent voluntary food intake. Other factors such as carbohydrate and fat type and level, acid-base balance (i.e., cation-anion ratio), and texture are important to the success of purified diets. Chemically defined diets provide amino acids, mineral elements and vitamins in forms that are maximally bioavailable. Also, virtually any nutrient can be manipulated at will for studies of a) requirements, b) bioavailability, c) factors affecting requirements and bioavailability, d) nutrient-nutrient interrelationships, e) nutrientdrug or nutrient-toxin interrelationships, f) absorption phenomena and g) efficiency and priority aspects of nutrient utilization. Requirements for essential nutrients are generally lower with purified diets than for practical diets because the nutrients in the former are more bioavailable, but also because purified diets generally lack antagonizing factors such as phytate and soluble fiber. That chemically defined diets for pigs, rats and dogs yield such excellent rates of growth suggests that a specific peptide requirement many not exist for these species. Also, this suggests that all known nutrients necessary for maximal growth must be present in the diet. Whether additional nutrients, or different levels, may be necessary for optimal health and immunocompetency, or for maximal life span, needs further study.