Rate of metabolic decarboxylation of leucine as assessed by a l[1-(13)C(1)]leucine breath test combined with indirect calorimetry of broiler chickens fed isocaloric diets with different protein:fat ratio

Research has shown that broiler chickens reared on a low-protein diet have a more efficient protein digestion. However, information on the fate of absorbed amino acids in relation to the dietary crude protein level in poultry is sparse. Therefore, this study aimed at developing a methodology for a 1-(13)C(1)-leucine breath test combined with indirect calorimetry, and to apply this technique using broiler diets known to induce differences in protein retention. From 14 days of age onwards, broiler chickens were reared on one of two isocaloric diets with substitutions between fat and protein low-protein (LP) vs. high-protein (HP) diet: 130.4 vs. 269 g protein/kg; and 101.8 vs. 27.9 g fat/kg]. Every 4 or 5 days, three chickens per diet were placed in the respiratory cells for 48 h. The broilers were intubated with 40 mg 1-(13)C(1)-leucine/kg body weight, followed by breath sampling for 4 h at 15-min intervals and mass spectrometric analysis of the (13)C:(12)C ratio in the samples. The CO(2) level in the respiratory cell air was monitored and excreta samples were collected. The methodology to study l1-(13)C(1)]leucine decarboxyation in chickens using a breath test combined with indirect calorimetry was accomplished. Results of the nitrogen balance test indicated that the LP broilers had an improved dietary protein retention compared with the HP animals. Moreover, LP chickens decarboxylated a significantly lower percentage of l1-(13)C(1)]leucine, demonstrating several 'protein- or amino acid-sparing' mechanisms in animals reared on a diet with lower protein level, both at the digestive and at the postabsorptive level.