PLOS ONE · 4 authors, 4 centres
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Neutering domestic shorthair kittens causes an upward shift in growth trajectory, especially in females and those neutered earlier. This weight gain is primarily due to increased fat mass, with a smaller increase in lean mass. Veterinarians should monitor body weight closely after neutering to prevent obesity.
**Background:** Neutering is common in pet cats but is a known risk factor for obesity. Growth standards have been developed for sexually-intact kittens, but the impact of neutering on growth patterns is unclear. This study aimed to compare growth in neutered kittens with these standards and to clarify the effects on body composition and shape.
**Methods:** Two studies were conducted. Study 1 was a retrospective observational study using electronic medical records from Banfield Pet Hospitals (USA, 1994–2016). Data from healthy, client-owned domestic shorthair (DSH) kittens in optimal body condition (never received an abnormal body condition rating up to 4 years of age) were included. Kittens were grouped by age at neutering based on quartiles of all neutering procedures: for males, groups were <0.39y, 0.39–0.44y, 0.44–0.54y, >0.54y; for females, <0.40y, 0.40–0.48y, 0.48–0.56y, >0.56y. Growth trajectories were modelled using GAMLSS (BCCG model) and converted to z-scores relative to the growth standards. A secondary analysis compared pre- and post-neutering z-scores within individuals using a linear model with Tukey HSD correction. Study 2 analysed new data from a previously published randomised trial of 11 pairs of female DSH littermates (11 neutered at 19 weeks, 11 sexually-intact). Body composition (DEXA) and zoometric measurements (height, length, girth, ribcage, chest depth, elbow width, forelimb, hindlimb) were taken at 11, 18, 30, and 52 weeks. A Bayesian mixed model with multivariate endpoint was used to compare fold-changes from 11 weeks between groups.
**Key Results:** In Study 1, all neuter groups showed an upward inclination in median growth trajectory after neutering. For males, the upward shift in z-score by the end of growth was 0.34, 0.41, 0.27, and 0.14 for groups 1–4, respectively. For females, the shifts were 1.27, 1.21, 1.39, and 0.51. The increase was larger in females than males and smaller in later-neutered groups. In the within-individual analysis, all groups had a significant increase in z-score post-neutering (all P<0.001), with females showing larger increases than males. In Study 2, by 52 weeks, neutered kittens were heavier (mean difference in fold change vs. 10 weeks 1.34, 95% CI: 1.07–1.72), had greater fat mass (1.91, 95% CI: 1.09–3.21), and greater lean mass (1.23, 95% CI: 1.03–1.48). Abdominal girth (1.20, 95% CI: 1.04–1.39) and rib cage length (1.18, 95% CI: 1.02–1.36) were also greater, but no significant differences were found in other zoometric measurements.
**Clinical Implications:** Neutering, especially early neutering in female kittens, leads to increased weight gain primarily due to adipose tissue accumulation. This effect is more pronounced than in dogs and may be underestimated in this study due to the inclusion of only cats with optimal body condition. Veterinarians should consider this risk when discussing the timing of neutering and should implement close post-operative weight monitoring (e.g., monthly checks) using growth charts to detect upward centile crossing. Nutritional management, including controlled feeding and accurate energy calculation, is recommended to prevent obesity. The findings support the need for lifelong weight management in neutered cats.