DIETARY CRUDE PROTEIN-CHANGES RAPIDLY ALTER METABOLISM AND PLASMA INSULIN-LIKE GROWTH-FACTOR-I CONCENTRATIONS IN BROILER-CHICKENS

Although changes in dietary crude protein levels change metabolism in broiler chickens, there is little information concerning the time cour se of the process of adaptation. Therefore, male Indian River broiler chickens were fed diets containing either 120 or 210 g protein/kg from 7 to 28 d of age and then were fed the other level for an additional 12 d. Birds were bled and killed at 0, 2, 5, 7, 9 and 12 d following t he reversals. Measurements taken at these intervals included in vitro lipogenesis, growth and feed consumption, hepatic enzyme activities an d plasma metabolites and metabolites. Birds fed the lower level of cru de protein were smaller and less efficient in growth from 7 to 28 d. F eeding these birds a higher protein diet from 28 to 40 d improved both growth and feed efficiency. Lipogenesis was also greater and plasma i nsulin-like growth factor-I (IGF-I) less in birds fed the lower protei n diet. Switching dietary treatments increased and decreased lipogenes is as birds were switched from high to low protein and from low to hig h protein diets, respectively. Half-maximal changes were observed 4 d after the reversal and maximal changes 7 d after the reversal. In cont rast, switching dietary treatments decreased and increased plasma IGF- I as birds were switched from high to low protein and from low to high protein diets, respectively. Half-maximal changes were observed 2 d a fter the reversal. Of the three hepatic enzymes monitored, malic enzym e activity most closely followed the rapid changes in in vitro lipogen esis. Plasma IGF-I may be a more sensitive indicator of changes in die tary protein than changes in intermediary metabolism.