Adult male cats consuming diets ranging in crude protein from 85% to 160% of AAFCO recommendations have different respiratory quotients, energy expenditure, gross, digestible, metabolizable, and net energy, but not physical activity, nitrogen losses, and heat increment of feeding.

Evaluation of dietary energy content in extruded pet food is based on digestible (DE) and metabolizable energy (ME), which accounts for fecal and urinary energy losses, respectively. However, the losses occurring through heat increment of feeding (HIF) and the resulting net energy (NE) have not been estimated in diets where crude protein (CP) content is higher for obligate carnivores such as cats. This study aimed to measure NE in diets increasing in CP in exchange for nitrogen free extract (NFE). Two experimental diets containing 23% CP (23CP) and 41% CP (41CP) dry matter (DM) were formulated with the inclusion of titanium dioxide (TiO2) as an indigestible marker to estimate apparent total tract digestibility (ATTD) of gross energy (GE). These were subsequently mixed to form four additional diets of intermediate CP content (25CP; 26CP; 29CP; 36CP). Adult, male cats (n = 12) were assigned treatments in 12 × 6 Latin rectangle design and fed treatments for 14 d where daily intake was recorded. Accelerometers were placed on day 11 to track physical activity and on day 13 cats were placed in calorimetry chambers for 24 h collection of excreta and measurement of oxygen consumption (VO2) and carbon dioxide production (VCO2). Respiratory gases were then used to calculate energy expenditure (EE), total EE (TEE), respiratory quotient (RQ) and HIF. Dose response to CP was analyzed using direct fit for linear, quadratic, and cubic polynomials with lack of fit inclusion. Interestingly, while ME intake was similar among treatments using Atwater calculation (P > 0.05), ME intake estimated using National Research Council (NRC) equation linearly increased with higher CP content (P < 0.01). Fed EE (P = 0.01) and TEE (P < 0.01) increased nonlinearly whereas fed RQ linearly decreased with greater CP inclusion (P < 0.01). Measures of HIF (in kcal/100 g DM or kcal/kg BW) did not differ among treatments (P > 0.05). Overall, GE (P < 0.01), DE (P < 0.01), ME (P < 0.01), and NE (P = 0.02) intake on feed intake basis (kcal/100 g DM) were all linearly associated with %CP. These results indicate that NE of CP diets ranging from low to high was mostly affected by fecal and urinary energy losses. This study provides the first set of NE available for adult, male cat maintenance functions for diets altering in CP content, and offers insight on utilization of dietary protein as an energy source by carnivores.