Submitted to: British Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 29, 1998
Publication Date: N/A
Interpretive Summary: Excess fat production in the modern broiler accounts for an annual loss to the poultry industry of 800 to 950 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 literature is of limited value in determining methods to depress fat synthesis and allow lean tissue synthesis to remain at an elevated rate. Additions of fat and increases in the dietary protein of diets fed to broilers depressed the innate ability of these chickens to synthesize fat. It should be noted that diets excessively low in protein promote a rate of fat synthesis that cannot be attenuated with additional dietary fat. In addition, these low protein diets depress circulating concentrations certain hormones that are responsible for the regulation of lean tissue synthesis. Although replacement of dietary carbohydrates with either fat or rprotein reduce precursors for fat synthesis, both energy sources have additional unique effects on metabolism. Dietary protein levels modulate metabolic effects of dietary fat. Moderate amounts of fat can be added to broiler diets to depress the synthesis of carcass fat from dietary carbohydrates. Cost-benefit data must be derived for each producer before a blanket recommendation can be made for high fat diets for broilers.
Two experiments were conducted to study the interrelationships between dietary fat and protein levels in the regulation of broiler lipid metabolism. Birds were fed diets containing either 1.25, 2.77 or 4.65 MJ of energy as fat with 124 or 190 g protein/kg diet or 185, 234 & 285 g protein containing 1.25 MJ of energy as fat or 124 g protein containing 1.25, 2.77 or 4.65 MJ of metabolizable energy as. Birds fed the lower level of crude protein were smaller and less efficient in growth from 7 to 28 d. The diet containing 2.77 MJ as fat decreased lipogenesis and malic enzyme activity (P<0.01) in birds fed the diet containing 190 g protein/kg diet, but not in birds fed a containing 124 crude protein/kg diet. Birds fed this level of protein required at least 4.65 MJ as fat to express any significant decrease in lipogenesis or malic enzyme activity (P<0.01). Dietary fat did not affect plasma hormone levels (T3, T4 or IGF-I). The lower level of crude protein decreased plasma T4 and IGF-I and increased plasma T3 (P<0.05). Increasing dietary protein increased body weights, IGF-I, T4 and decreased lipogenesis, malic enzyme activity and T3 compared, to increasing fat. Both of these regimens involve decreasing dietary carbohydrate at equal rates, but results differed. Although replacement of dietary carbohydrates with either fat or protein reduce precursors for fat synthesis, both energy sources have additional unique effects on metabolism. Dietary protein levels modulate metabolic effects of dietary fat.