Submitted to: Growth Development and Aging
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 9, 2003
Publication Date: July 1, 2004
Citation: Rosebrough, R.W., Richards, M.P., McMurtry, J.P. 2004. Further studies on dietary protein reversals and lipid metabolism in the broiler. Growth, Development & Aging. 68, 19-32.
Interpretive Summary: Excess fat production in the modern broiler accounts for an annual loss to the poultry industry of 1000 to 1500 million dollars annually. The original source of this problem relates to selection genetic practices that emphasized rapid growth at the expense of other carcass characteristics. The current managerial trend of sex-separate rearing of male and female broiler chickens encompasses the feeding of higher protein diets to males, followed by a step down regimen of lower protein diets as the bird matures. This practice results in a disproportionate increase in apparent lipogenesis in these males, resulting in excessive carcass fat deposition. The present study irrevocably demonstrated that endogenous intermediary metabolism is altered by this dietary changeover. The literature is of limited value in determining methods to depress fat synthesis and allow lean tissue synthesis to remain at an elevated rate. Future research will center on the genetic component in the hyperlipogenesis that results from the switch from a high protein starter diet to a lower protein grower diet.
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. Measurements taken at these intervals included 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 and less efficient 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. 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 two days after the reversal and maximal changes five days after the reversal. In contrast, switching dietary treatments decreased and increased plasma IGF-I, respectively, as birds were switched from high to low and low to high protein diets. Half-maximal changes were observed two days after the reversal. Of the three hepatic enzymes monitored, malic enzyme activity most closely followed the rapid changes in IVL. Plasma IGF-I may be a more sensitive indicator of changes in dietary protein than changes in intermediary metabolism.