Changes in body size and composition, i.e. body weight (BW) gain or loss, affects the daily energy expenditure (EE). To ensure appropriate BW reduction and to find an efficient strategy to reduce and maintain a target BW, regular evaluations and adjustments of energy allowance are important. This study aimed to provide detailed knowledge about possible changes in resting EE, by using the oral 13C-bicarbonate technique (o 13CBT) as a research tool for measurements in 16 overweight pet dogs undergoing BW reduction. Effects of dietary composition (i.e. in % of dry matter being a high protein (33.3), low fat (9.6) and high crude fibre (18.0) diet (LFHFibre), and a high protein (37.9) high fat (52.0), carbohydrate-free diet (HFat)), during 16 weeks of energy restriction were evaluated regarding effects on resting EE, rate of BW reduction, body composition and plasma concentrations of metabolic hormones involved in energy metabolism and appetite regulation. Mean rate of BW loss was higher (P < 0.05) for the dogs fed the LFHFibre diet (1.1%/week), than for dogs fed the HFat diet (0.8%/week), but the total BW reduction of 14.6% and 12.0% of initial BW did not differ significantly (P > 0.05). Resting EE was lower (P < 0.02) after the BW reduction; 414 kJ (99 kcal)/kg BW 0.75/d at the start (w0), and 326 kJ (78 kcal)/kg BW 0.75/d when measured at the end (w16) of the study, respectively. The BW reduction in both groups (P > 0.05) consisted of both fat mass (FM) and fat free mass (FFM). Calculated in relation to amount of FFM, EE was not significantly (P > 0.05) affected by BW reduction. Dietary composition did not significantly affect (P > 0.05) plasma concentrations of insulin, leptin and ghrelin, and no effect (P > 0.05) of BW reduction could be demonstrated on hormone concentrations. In conclusion, the o 13CBT proved to be a useful research method for studies regarding short-term EE in overweight dogs. However, even though all dogs lost BW, most dogs were still overweight at the end of the study and, due to high individual variation among dogs, a longer experimental period with a larger sample size would be desirable.