Further studies on dietary protein reversals and lipid metabolism in the broiler

Although changes in dietary protein levels change metabolism in the broiler chicken, there is little information concerning the time course of the process of adaptation. Therefore, male Hubbard broiler chickens were fed diets containing either 12 or 30% crude protein from 7 to 28 d of age and then were switched to the opposite level for an additional 9 d. Birds were bled and killed at 0, 2, 5, 7and 9 d following the reversals. Data taken at these intervals included those involved in vitro lipogenesis (IVL), growth and feed consumption, hepatic enzyme activities and plasma hormones and metabolites. Birds fed the lower level of crude protein were smaller in growth from 7 to 28 d. Feeding these birds a higher protein diet from 28 to 37 d improved both growth and feed efficiency in comparison to controls. Lipogenesis was also greater and plasma insulin-like growth factor-I (IGF-I) less in birds fed the lower protein diet. Switching dietary treatments increased and decreased lipogenesis as birds were switched from high to low and low to high protein diets, respectively. Half-maximal changes were observed 2 d after the reversal and maximal changes 5 d after the reversal. In contrast, switching dietary treatments decreased and increased plasma IGF-I as birds were switched from high to low and low to high protein diets, respectively. Half-maximal changes were observed 2 d after the reversal. Of the three hepatic enzymes monitored, malic enzyme activity most closely followed the rapid changes in IVL. In the present study, plasma IGF-I may be a more sensitive indicator of changes in dietary protein than changes in intermediary metabolism.