Dietary fat and protein interactions in the broiler.
An experiment was conducted to study the interrelationships between dietary fat and protein levels in the regulation of lipid metabolism in the broiler chicken. Birds were fed diets containing 300, 600, or 1,200 kcal ME from fat (corn oil) with either 124 or 190 g CP/kg. Two additional experimental diets contained 234 or 285 g CP and 300 kcal ME from fat. Regardless of fat level, birds fed the diets containing 124 g CP/kg weighed less and were less efficient than birds fed diets containing 190 g CP/kg. The diet containing 600 kcal as fat decreased lipogenesis and malic enzyme activity (P < 0.05) in birds fed the diet containing 190 g CP/kg diet, but not in birds fed the diet containing 124 g CP/kg. Birds fed the latter level of protein required at least 1,200 kcal as fat to express any significant decrease in lipogenesis or malic enzyme activity (P < 0.05). Dietary fat did not affect plasma levels of triiodothyronine (T3), thyroxine (T4), or insulin-like growth factor-I (IGF-I). Feeding diets containing 124 g CP/kg resulted in decreased plasma T4 and IGF-I and elevated T3 (P < 0.05). Increasing dietary protein (compared to increasing dietary fat) increased body weights, IGF-I, T4 and decreased lipogenesis, malic enzyme activity, and T3. 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.