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 imbalanced 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 significantly correlated with CP:TME ratio and was negatively correlated (P < 0.01) with dietary TME concentration. 3. Heat production was closely correlated (P < 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.
