Effects of amino acid balance and energy: Protein ratio on energy and nitrogen metabolism in male broiler chickens

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Abstract
1. An experiment was performed with growing broiler chickens (14 to 21 d old) to examine 3 diet characteristics which have been implicated in regulatory elevation of metabolic rate: an unbalanced amino acid mixture, high dietary energy concentration and low protein concentration. 2. Differences in energy expenditure could be explained almost entirely (93%) by differences in quantities, and therefore costs, of protein and fat accretion. There was no indication of regulatory diet‐induced thermogenesis. Heat production was not significandy correlated with CP:TME ratio and was negatively correlated (PP< 0.001) with rate of protein accretion, which in turn was more strongly associated with intake of the first‐limiting amino acid (lysine) than with total protein intake. Heat production on an imbalanced, lysine‐limited, amino acid mixture was no greater than on a balanced amino acid source with the same lysine concentration. 4. There was no indication of a stimulation of heat production by excess amino acids. Heat production, adjusted for body weight by covariance analysis, was similar on paired diets which had identical lysine concentrations but a 1.5‐ or 2‐fold difference in crude protein concentration. 5. There was a strong negative correlation (P< 0.001) between protein retention per g of lysine consumed and lysine :CP ratio, suggesting that, in this case, response to a limiting amino acid was improved by the presence of a super‐abundance of other amino acids.