Metabolic Adaptations to Weight Loss in Obese Senior Dogs: Effects on Energy Expenditure, Protein Turnover, and Voluntary Physical Activity

Implications of obesity and weight loss (WL) for energy and protein metabolism are not fully understood, particularly in senior dogs. This study evaluated changes in body composition, energy expenditure (EE), protein turnover (PT), and voluntary physical activity (VPA) in senior dogs in maintenance and after 10% and 20% WL. Twenty obese, neutered, client‐owned dogs without comorbidities (8.05 ± 0.44 years, 33.41 ± 1.79 kg, body condition score [BCS] median 8/9; range: 8–9) received the same diet (3.2 kcal/g; per 100 kcal: 10.4 g protein, 4 g fat, 7.8 g fibre), with intake adjusted (±5%–10%) to maintain a WL rate of 1%–2%/week. At baseline, after 10% WL, and after 20% WL, body composition and EE were assessed using the doubly labelled water, and PT was measured using the 13 C‐leucine method. VPA was evaluated at baseline and after 20% WL using triaxial accelerometers. Data were analysed by repeated measures ANOVA. Dogs achieved a final BCS of 6 (5–6) ( p < 0.01) but exhibited a lower WL rate in the second phase of WL (0.85 ± 0.05% vs. 0.70 ± 0.04%/week; p < 0.05). Fatty mass (FM) decreased from 40.57 ± 1.23% to 310.1 ± 1.50%, while lean mass (LM) increased from 59.43 ± 1.23% to 68.90 ± 1.5% ( p < 0.01). LM (kg) decreased in the first phase ( p < 0.05) but was preserved thereafter ( p > 0.05). Energy intake from food decreased over time (from 61.09 ± 2.06 to 52.95 ± 1.3 kcal/kg 0.75 /day; p < 0.001), whereas EE increased during WL (from 98.23 ± 5.1 to 111.26 ± 4.18 kcal/kg 0.75 /day; p < 0.01), raising the caloric deficit from 37% to 58% ( p < 0.05). PT rates declined by 20%–27% when expressed per kg 0.75 or kgLM 0.75 ( p < 0.05), while VPA remained unchanged ( p > 0.05). A progressive predominance of fatty over lean tissue oxidation may explain the increasing need for energy restriction and the reduced WL rate as WL progresses, despite higher EE in dogs. Strategies to preserve LM and protein metabolism are warranted to optimise WL in dogs.