In chickens, metabolism is influenced by environmental factors and of particular interest nutritional factors, such as diet quantity and composition. With respect to the dietary macronutrients, literature clearly shows that in isoenergetically formulated diets, the protein level has a pronounced effect on metabolism, whereas fat and carbohydrate concentrations play a limited role. A decreased dietary protein level results in depressed growth rates, with reduced carcass gains in both water and protein content. In spite of the lower protein retention in chickens fed low protein diets, the efficiency of utilization of dietary protein appears to be increased. This indicates an inverse relationship between protein intake and utilization, which is reflected in decreased circulating uric acid levels. In addition, feed intake is increased, at least when protein levels are slightly reduced, leading to an involuntary overconsumption of energy compared to protein. The chickens deal with this excess energy consumption by increasing de novo lipogenesis and fat deposition, which is supported by increased plasma triglyceride concentrations, and heat production is increased. In contrast to adult mammals, the dietary macronutrients have no effect on diet-induced thermogenesis, nor is there any relationship between diet-induced thermogenesis and feed intake. Plasma growth hormone levels are higher in chickens reared on a low protein diet, indicating a causal relationship between growth hormone secretion and protein efficiency. In spite of the increased growth hormone levels, plasma insulin-like growth factor concentrations are lowered in low protein reared chickens, and may be partially responsible for the reduced growth rate observed in these animals. Plasma corticosterone concentrations are augmented in chickens on low protein diets, despite a decline in the ACTH concentration. Finally, plasma T-3 and T-4 levels are well known to increase and decrease, respectively, with decreasing protein content, whereas the effects of dietary macronutrients on circulating leptin levels require more research.