Isoenergetic reduction of dietary macronutrients modulates respiratory quotients and heat increment of feeding but not energy expenditure in cats.

Indirect calorimetry can provide insights into the metabolic processes occurring in cats through substrate utilization and energy expenditure (EE). Additionally, the influence of dietary macronutrients on the heat increment of feeding (HIF) in cats remains unexplored. As such, this proof of principle study aimed to test the short-term effects of 3 test diets formulated for adult maintenance according to AAFCO by comparatively reducing protein, fat, or carbohydrates, to create a low-protein (LP; protein 28%ME, fat 40%ME, nitrogen-free extract (NFE) 28%ME), low-fat (LF; protein 40%ME, fat 27%ME, NFE 27%ME), and a low-carbohydrate (LC; protein 35%ME, fat 40%ME, NFE 20%ME) diet on respiratory quotients (RQ), EE, and HIF in cats. Adult (3.4 ± 0.1 yr of age), male, neutered cats (n = 12) were assigned to 1 of 3 groups and offered each diet at an amount to maintain body weight (BW) for 2 wk in a 3 × 3 Latin square design. The cats BW ranged from 3.94 to 6.33 kg (mean ± SEM BW of 5.08 ± 0.28 kg) and body condition score (BCS) ranged from 4/9 to 7/9 (mean ± SEM BCS of 5.58 ± 0.38). At the end of each test period, 22-h indirect calorimetry was performed to determine RQ, EE, and HIF. Daily food intake was greater for cats consuming the LF diet (61.6 ± 1.0 g/d) compared to the LP (58.3 ± 1.0 g/d) and LC (57.7 ± 1.0 g/d) diets (P < 0.0001), though energy intake was similar between diets (223.8 ± 2.2, 227.9 ± 2.0, and 228.4 ± 2.0 kcal/d) (P = 0.1191). In the fasted, immediate postprandial (0 to 120 min), and postabsorptive (0 to 1,200 min) states, cats fed LC had a lower RQ compared to LF (P = 0.0154, P = 0.0346, and P = 0.0407, respectively), indicating greater fat oxidation when consuming LC. No differences were observed between the LP diet and the LF and LC diets for RQ (P > 0.05). For all cats regardless of diet, the RQ increased from the immediate postprandial to postabsorptive state (P < 0.0001) as expected. Following feeding, EE increased for all cats regardless of diet (P < 0.0001). No diet effect was observed for EE in the fasted, immediate postprandial, or postabsorptive states. Diet did not affect HIF in the immediate postprandial state; however, the LP diet had a lower HIF compared to the LF diet over the entire post-feeding state when using the National Research Council ME equation, though the HIF was similar between the LC diet to the LP and LF diets (P = 0.0360). Future research should explore the long-term effects of low levels of dietary macronutrients in cat foods and their role in energy metabolism under various energy balance conditions.