Effects of substitution between fat and protein on feed intake and its regulatory mechanisms in broiler chickens: endocrine functioning and intermediary metabolism.

The aim of this study was to investigate the influence of dietary macronutrient ratio on postprandial (5 or 48 h after refeeding) endocrine functioning and metabolites of the intermediary metabolism and to relate these parameters with dietary-induced alterations in energy, protein, and lipid metabolism. Male broilers were reared from 1 to 7 wk of age on isoenergetic diets with substitutions between fat and protein but similar carbohydrate contents [low protein (LP): 126 vs. 242 g protein/ kg; low fat (LF): 43 vs. 106 g fat/kg]. The LP chickens had significantly increased postprandial plasma triglyceride levels. This was likely the result of stimulated hepatic lipogenesis, as corroborated by their significantly higher respiratory quotients. Plasma free fatty acid concentrations were higher in LP broilers, whereas glucose levels were unaffected by dietary composition, suggesting that these chickens preferred carbohydrates as an energy source over free fatty acids. Plasma uric acid levels were lower in LP compared with LF chickens, indicating a more efficient protein retention in the former group. LP birds that were fasted and refed at 48 h had higher plasma 3,5,3'-triiodothyronine (T3) levels, corroborating their increased heat production. The postprandial T3 increase was more pronounced in the LF chickens, possibly induced by their higher protein consumption. In conclusion, diet-induced changes in heat production and energy partitioning are reflected in circulating levels of intermediary metabolites and hormones. Furthermore, nutritional studies should consider the ability of organisms to habituate to changed diet compositions and that alterations in feeding status follow higher-order responses.