Assessing the risk of becoming overweight or obese among spayed and neutered dogs

Obesity is an epidemic in the US pet population, driven by a multitude of variables including caloric intake, genetics, activity level, underlying disease, age, sex, and reproductive status. In fact, new data from Banfield found a 13% increase in dogs diagnosed as overweight or obese over a two-year period, jumping from 38% of dogs seen in 2020 to 43% in 2022.

Studies have shown sterilization – which is a common practice in U.S. veterinary medicine that has many health benefits and can prevent reproduction and help mitigate overpopulation – can also be one of the various factors that may contribute to pet obesity. Banfield data powered a new study that yielded detailed information about risk profiles for weight gain after surgical sterilization, which can help care teams tailor plans for individual pets to minimize the risk of obesity.

What this means for veterinary teams
Among other key study findings, sterilized dogs were almost twice as likely as intact dogs to become overweight or obese (O/O), and the heightened risk of O/O after surgical sterilization was greater for males than for females of the same age and breed size. By providing a clearer view of which dogs are more at risk of becoming O/O after surgical sterilization, this research can help veterinarians and owners better manage each pet’s medical care.

Surgical sterilization and obesity
Surgical sterilization alters hormone balance, which can slow metabolism and lead to weight gain if not addressed. O/O in dogs can reduce longevity and quality of life in the long term and is associated with serious health conditions including endocrinopathies, metabolic abnormalities and cardiovascular disease, as well as functional alterations, such as joint disorders and decreased immune functions.

Although many studies have addressed the association between surgical sterilization and O/O in dogs, they have tended to include only certain breeds or breed groups and/or classified mixed breeds into a single category. In addition, much of the literature used a cross-sectional approach to evaluate factors associated with O/O. Cross-sectional studies can misclassify dogs that return to normal weight and exclude those that die, distorting estimates of risk. Furthermore, existing research has raised unresolved questions about how characteristics such as sex, breed, or age at sterilization influence the risk of O/O in later life.¹ More precise and comprehensive assessments of the risk of O/O after surgical sterilization are needed to inform the care of each patient.

We collaborated with the Alliance for Contraception in Cats and Dogs who convened researchers at several academic institutions on a retrospective cohort study of the risk of O/O in spayed or neutered (S/N) dogs who visited Banfield’s network of primary care veterinary practices in 2014 through 2019.² Unlike prior studies, this analysis treated mixed-breed dogs comparably to purebred dogs, by classifying them in size categories (Table 1), and followed dogs over time to assess their risk of developing O/O. Like other retrospective cohort studies, these analyses alone cannot establish causation but can suggest association. Associations between O/O and S/N status, sex, breed size, and age at sterilization were analyzed.

Table 1. Size categories for dogs in this analysis.

Risk of becoming overweight or obese after surgical sterilization
More than 150,000 dogs representing 347 breeds were included in the study; 45,732 dogs were S/N in 2014 and 109,467 remained intact throughout 2014 (some of them were sterilized later). S/N dogs were almost twice as likely as intact dogs to become O/O (incidence of O/O was 1.91 times higher in S/N dogs compared to intact dogs).

Characteristics that influence risk of becoming overweight or obese
The heightened risk of O/O after surgical sterilization was greater for males than for females of the same age and breed size. For example, standard-size males sterilized at 1 year of age were 2.62 times more likely to become O/O than were their intact counterparts, whereas standard-size females sterilized at the same age were 1.53 times more likely to become O/O compared to their intact counterparts.

O/O risk varied according to breed size, but not linearly. Toy and small dogs had the highest risk of becoming O/O after S/N, followed by large dogs; medium dogs had somewhat lower risk; and giant dogs had the lowest risk. A follow-up study is planned to determine whether particular breeds within a size category are driving specific results.

Compared to intact counterparts of the same age, O/O risk was greater for most S/N dogs regardless of age at sterilization. For both sexes and all breed sizes, risks of O/O after sterilization tended to increase with age at sterilization until leveling off at the age of ~1.5–3 years.

A separate analysis assessed the impact of age at sterilization on O/O among S/N dogs. Dogs sterilized at 1 year of age had a lower risk of O/O compared to dogs sterilized after 1 year of age. Dogs sterilized at 6 months or younger generally had a similar or lower risk of O/O compared to counterparts sterilized at 1 year of age, although the relative risks varied across breed sizes. A pronounced exception was for large dogs of both sexes, which had a significantly higher risk of O/O when sterilized at 6 months of age. 

Summary and applications of the findings
Like several prior studies, this analysis found that surgical sterilization increased obesity risk for most dogs. The extent of the increased risk differed with sex, breed size, and age at sterilization. Veterinarians and owners can discuss these findings and other factors when considering or managing surgical sterilization of an animal.³ These data will also be useful for comparing the effects of other sterilization alternatives, including potential non-surgical options, on obesity risk.

By integrating relevant research evidence and the unique circumstances of an individual case, veterinarians can recommend personalized medical care for each and every pet. In the case of reducing the risk of O/O, care could include developing personalized nutrition plans, critically appraising caloric requirements, discussing options for exercise plans, and other measures to balance calorie intake with caloric expenditure. 

1. Reichler IM. (2009). Gonadectomy in cats and dogs: a review of risks and benefits. Reprod Domest Anim. 44 Suppl 2:29-35. doi: 10.1111/j.1439-0531.2009.01437.x.
2. Benka VA et al. (2023). Association between canine gonadectomy and overweight or obese body condition score in United States primary care veterinary clinics: A retrospective cohort study. Journal of the American Veterinary Medical Association. doi: https://doi.org/10.2460/javma.22.12.0596
3. Mckenzie B. (2010). Evaluating the benefits and risks of neutering dogs and cats. CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources. 5. doi: 10.1079/PAVSNNR20105045.