Dietary macronutrients and performance and plasma hormone and metabolite levels of broiler chickens fat by carbohydrate substitution

There is a wealth of information about the effects of feed formulations on performance and body composition, in particular fat content of broiler chickens (e.g. JACKSON et al., 1982; LAURIN et al., 1985; JONES, 1986; SUMMERS et al., 1992; MACLEOD, 1997). In general, diets with high metabolisable energy (ME) contents or with high ME: crude protein ratios promote fat deposition. Increasing the crude protein (CP) content will result in a leaner, but a less efficient bird, as the elimination of excess nitrogen is an energy-consurning process. On the other hand, diets with a suboptimal CP content are accompanied with an augmented fat deposition. In contrast, far less studies were aimed at identifying the underlying physiological mechanisms for the empirically observed effect of diet composition on bird's performance and body composition. These studies were focused on energy and nitrogen metabolism (e.g. BUYSE et al., 1992; MACLEOD, 1990, 1992; NIETO et al., 1997), the endocrine system (e.g. BUYSE et al., 1992; RosEBROUGH et al., 1996; 1999) and the intermediary metabolism (e.g. ROSEBROUGH et al. 1985, 1999; TANAKA et al., 1983a, b). The outcomes of these studies with respect to the effects of dietary ME, ME : CP ratio and macronutrient (protein, fat, carbohydrate) content on endocrine functioning and intermediary metabolism were recently summarized by BUYSE et al. (2001). However, there is a large diversity in the diets employed between these studies and this aspect should be considered with scrutiny. For example, increasing the dietary ME content by adding one specific nutrient (hence a wider ME: CP ratio) has another irnpact on bird's physiological functioning than increasing ME content to a higher level but maintaining ME: CP ratio. Furthermore, it is irnportant