Age at gonadectomy, sex, and breed size affect risk of canine overweight and obese outcomes: a retrospective cohort study using data from United States primary care veterinary clinics

OBJECTIVE To examine the risk of developing an overweight or obese (O/O) body condition score (BCS) in gonadectomized versus intact dogs and, separately, the impact of age at gonadectomy on O/O outcomes among sterilized dogs. ANIMALS Dogs were patients of Banfield Pet Hospital in the US from 2013 to 2019. After exclusion criteria were applied, the final sample consisted of 155,199 dogs. PROCEDURES In this retrospective cohort study, Cox proportional hazards models evaluated associations between O/O and gonadectomy status, sex, age at gonadectomy, and breed size. Models were used to estimate the risk of becoming O/O in gonadectomized versus intact dogs and, separately, to estimate risk of O/O BCS according to age at surgery among gonadectomized dogs. RESULTS Gonadectomy increased O/O risk for most dogs compared to intact dogs. Unlike most prior findings, O/O hazard ratios among gonadectomized versus intact dogs were larger for males than females. O/O risk varied according to breed size but not linearly. Sterilizing at 1 year old tended to yield a lower O/O risk compared to doing so later. Comparative O/O risk among dogs gonadectomized at 6 months versus 1 year varied by breed size. Overall patterns for obesity related to size were similar to patterns in the O/O analysis.