Insufficient dietary supply of any of the essential nutrients will reduce animal growth performance and is likely to influence the animal's body composition. For this reason, practical animal diets are generally overfortified with vitamins and minerals, which are relatively inexpensive nutrients. Special consideration should be given to amino acids and energy-yielding nutrients.
At the tissue level, animals require amino acids for the synthesis of body proteins. Of the approximately 20 amino acids present in body protein, 12 amino acids can be considered essential or semiessential and must be supplied in the diet or derived from microbial protein that is generated in the rumen of ruminant animals. Animals must be supplied with sufficient quantities of nitrogen for synthesis of indispensable amino acids as well. Insufficient supply of amino acids at the tissue level will limit the growing animal from expression of its protein deposition potential.
Based on carefully controlled animal experiments, the relationship between intake of a specific indispensable amino acid and body protein deposition may be represented using a broken line linear plateau model (Fig. 1). The linear increase in body protein deposition with amino acid intake represented by the sloped line in Fig. 1 indicates that the marginal efficiency of using available amino acid intake for body protein deposition is constant
over a rather wide range of amino acid intake levels. The marginal postabsorptive efficiency of using the first limiting amino acid for retention in body protein is less than 1, due to inevitable amino acid catabolism. The plateaus in body protein deposition represented in Fig. 1 may be determined by the intake of another essential nutrient, energy intake, or the animal's operational body protein deposition potential. The latter reflects the body protein deposition potential that an animal can achieve under practical conditions and when fed ad libitum a palatable diet that is not limiting in essential nutrients. The animal's operational body protein deposition potential is influenced by body weight and approaches 0 when the animal reaches maturity.
Energy-yielding nutrients are required as fuels to support a variety of processes associated with the maintenance of body function and integrity and growth. Energetically, growth may be represented as body lipid and body protein deposition (Fig. 2). When energy intake exceeds requirements for maintenance and maximum body protein deposition, additional energy intake is used only to support lipid deposition and fat tissue growth. However, even when energy intake is insufficient to maximize body protein deposition, some of the absorbed energy-yielding nutrients are partitioned toward the deposition of (essential) body lipid (Fig. 2). Only at extreme low levels of energy intake can animals mobilize some of the body fat reserves to support body protein deposition. This implies that the fatness of the animal's body increases with energy intake level, and this increase in fatness is greatest once energy intake exceeds requirements for maximum body protein deposition. The relationship between energy intake and body protein deposition appears to be influenced by body weight as well. At the same level of energy intake above maintenance energy requirements, more energy is partitioned toward body lipid deposition with increasing body weight, even when energy intake is insufficient to maximize body protein deposition. The efficiency of using dietary energy for various body functions is addressed elsewhere in this encyclopedia.
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