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The domestic dog is one of the most diverse mammalian species in the world. However, knowledge regarding potential differences in metabolism and disparities in nutritional considerations due to body size is limited. The primary focus of this study was to investigate differences in energy needs of small dogs compared with medium and large dogs by analyzing longitudinal intake for individuals of varying body sizes. A secondary aim was to assess age-associated differences in the energy needs of small dogs by comparing longitudinal intake between young and old small dogs. Caloric intake (kcal/d), metabolic body weight (MBW)-adjusted caloric intake (kcal/kg0.75/d; MBW-Kcal), and BW were assessed in a population of mature adult dogs dogs (10.8 ± 1.9 yr; n = 58) categorized into three size groups based on ideal body weight; small (≤11 kg; n = 19), medium (11 < x < 22 kg; n = 29), nd large (≥ 22 kg; n = 10). The second study population was made up of small dogs less than seven years old (1.2 ± 0.8 yr; ≤11 kg; n = 46). All dogs were fed to maintain ideal BW. Intake was measured daily; BW was recorded monthly. The activity factor (AF) of each dog was calculated by dividing daily caloric intake by their resting energy requirement. ANOVA was used to assess effects of size and age on all data. Nonparametric test of medians was conducted to evaluate the distribution of MBW-Kcal. As expected, caloric intake increased as size increased (P < 0.001); however, this effect was not attenuated by adjusting for MBW (P < 0.001). Mean MBW-Kcal was greater in small dogs than medium (P < 0.001) and large dogs (P = 0.031). The variance of MBW-Kcal in small dogs was greater than in large (P < 0.001) and medium dogs (P < 0.001), indicating that small dogs had a larger range in the distribution of caloric intake even when adjusted for MBW (Figure 1). Mean AF of small dogs (1.7) was also greater than large (1.5; P = 0.030) and medium dogs (1.5; P < 0.001), but the AF of medium and large dogs were similar (P = 0.650). MBW-kcal and AF were similar between small old and young dogs (P > 0.05). These data demonstrate that even when adjusted for MBW, small dogs may require more calories to maintain BW than their medium- and large-breed counterparts, and this increased caloric need appears to be sustained in both dmall young and old dogs. These results suggest that small breed dogs have different metabolic factors driving caloric needs compared with larger dogs and further emphasize that nutritional guidelines should be updated to better reflect the energy needs of small dogs across life stages. [ABSTRACT FROM AUTHOR]