Body Composition, Physical Activity and Energy Requirements Estimated Using Two Isotopic Methods in Old and Adult Beagle Dogs.

Energy requirements (ER) are known to decline as dogs age into the senior stage. However, the implications of body composition and physical activity (PA) on this parameter remain poorly investigated. The aim of this study was to evaluate ER, body composition, water turnover rate (WTR), and voluntary PA by triaxial accelerometers in adults (5.2 ± 1.75 years; 11.8 ± 0.65 kg; body condition score (BCS) 5 ± 0.2/9) and old Beagle dogs (14.2 ± 2.65 years; 12.5 ± 0.37 kg; BCS 5 ± 0.31/9). The ER were assessed by the food intake to constant body weight (FI), 13C-Bicarbonate (13C-Bic) and doubly labelled water (DLW) methods. Animals were fed to maintain a constant body weight for 30 days. Food intake was recorded, and the diet's metabolisable energy was determined via total faecal collection method. Masks were used to measure 13C in expired air after 13C-Bic enrichment. Body water was enriched with 2H and 18O, and isotopes elimination was evaluated after 7, 10, and 15 days. ER was similar between methods (p = 0.99), but 30% higher for adult (511.9 ± 4.72 kJ/kg0.75/day) than old dogs (389.0 ± 5.07 kJ/kg0.75/day; p < 0.001). WTR was higher for adult (134.58 mL/kg0.75/day) than old dogs (106.51 mL/kg0.75/day; p = 0.005). Despite similar BCS, old dogs showed lower lean and higher fat mass, suggesting sarcopenia (p < 0.01). Old dogs presented lower PA (p < 0.01), which, may have contributed to their lower lean mass, ER and WTR. Considering that 13C-Bic and DLW resulted in similar estimations of energy expenditure to the FI method, the most appropriate approach might depend on experimental conditions and objectives. The implications of reduced ER on food and nutrient intake and its relationship with lower WTR, sarcopenia and obesity development, highlight the need for targeted nutritional recommendations for ageing dogs.