Interactions between diet composition and genotype of broiler chickens.

Day-old male broiler chickens of fat line (FL) and lean line (LL) (Leclercq et al., 1980) were obtained from INRA (Nouzilly, France). From two weeks of age onwards, both lines were divided into 2 groups, each receiving 1 of 2 isoenergetic diets with substitutions between fat and protein and a constant gross energy and carbohydrate level (low protein diet (LP): 126 vs 242 g protein/kg and low fat diet (LF): 106 vs 43 g fat/kg). There were no effects of genetic background on the carbohydrate (e.g. glucose oxidation rate as measured by stable isotope breath test) or protein metabolism of the broilers. The line differences in carcass fat content are therefore likely to be due to differences in fat metabolism. The theory linking diet-induced thermogenesis to feed intake could not be corroborated nor refuted. Introduction Research in mammals has shown a hierarchy in the extent to which recently ingested nutrients are combusted. First proteins are oxidized, followed by carbohydrates and finally fat, which corresponds to their ability to induce satiety but is reciprocal to their relative storage capacity (Stubbs et al., 1997). Causal relationships were observed between diet composition, dietinduced thermogenesis, nutrient balance and the satiety power of the different macronutrients. However, for avian species, information is scarce. The use of stable isotope tracers for studying a wide variety of metabolic processes and pathways in vivo has remarkable potentials. Stable isotope breath tests offer a safe and noninvasive method to study several aspects of physiological functioning. A substrate labeled with C is ingested, followed by serial measurements of the C:C ratio in the exhaled CO2. Recently, the methodology to perform stable isotope breath tests in chickens has been developed (Buyse et al., 2004). Models for genetic obesity are numerous for mammals, but are rare for birds. Genetically fat and lean lines of broiler chickens have been developed by Leclercq et al. (1980) in order to investigate the mechanisms controlling fattening in chickens. These lines differ in carcass lipid content, but exhibit a similar live body weight, feed consumption and energy expenditure. Another difference between both lines is related to the plasma glucose-insulin balance. In this experiment, fat and protein were substituted in two isoenergetic diets to determine the role of these macronutrients in regulating voluntary feed intake in two genetically diverse lines of broiler chickens. Important objectives of the study were to explore the role of diet-induced thermogenesis in the regulation of voluntary feed intake and to determine the glucose oxidation of these broiler chickens, known to differ in glucose-insulin balance. Materials and Methods Day-old male broiler chickens of fat line (FL) and lean line (LL) (Leclercq et al., 1980) were obtained from INRA (Nouzilly, France). The animals were divided over floor pens and raised under standard conditions. Until 14 days of age, the chickens received a commercial starter diet. At 14 days of age, each line was divided into 2 groups (4 groups in total), each receiving