This is another in my series of evidence updates on the risks and benefits of neutering in dogs and cats. I will be updating the evidence and conclusions of my original 2010 review as relevant new findings become available. This update concerns risk of being overweight or obese, which is one of the most common medical problems faced by veterinary patients.
Lefebvre, SL. Yang, M. Wang, M. Elliott, DA. Buff, PR. Lund, EM. Effect of age at gonadectomy on the probability of dogs becoming overweight. Journal of the American Veterinary Medical Association. 2013;243(2):236-43.
Objective-To determine whether gonadectomy or age at gonadectomy was associated with the risk that dogs would subsequently become overweight.
Design-Retrospective cohort study. Animals-1,930 dogs gonadectomized between 1998 and 2001 at ? 6 months of age (n = 782), > 6 months to ? 1 year of age (861), or > 1 to ? 5 years of age (287) and 1,669 sexually intact dogs.
Procedures-Dogs were followed-up through medical records for ? 10 years or until a diagnosis of overweight (defined as overweight, obese, or having a body condition score ? 4/5) was recorded. Information extracted included age at study entry, sex, breed, breed-size category, hospital visit frequency, and diagnosis (yes or no) of overweight or diseases that might affect body condition. Relative risk of a diagnosis of overweight was assessed among age groups of gonadectomized dogs and between gonadectomized and sexually intact dogs.
Results-No difference was detected among dogs grouped according to age at gonadectomy with respect to the risk of being overweight. This risk was significantly greater in gonadectomized dogs than in sexually intact dogs, but only during the first 2 years after gonadectomy. Sexually intact male dogs were approximately 40% less likely to have this diagnosis (hazard ratio, 0.61; 95% confidence interval, 0.52 to 0.72) than were sexually intact female dogs; no difference in risk between the sexes was evident for gonadectomized dogs.
Conclusions and Clinical Relevance-Gonadectomized dogs had a greater risk of being overweight than did sexually intact dogs, but this risk was not influenced by age at gonadectomy
Previous research discussed in my original review has indicated that neutered animals are at greater risk of becoming overweight than intact animals. Interestingly, there is some indication that this is not due to a change in the calorie needs or activity or neutered animals but possibly a change in the satiety mechanism, the system which tells an animal when it is no longer hungry. Neutered animals appear to eat more after neutering, which leads to weight gain. In any case, regardless of the true mechanism, the weight gain associated with neutering can be avoided by proper restriction of the calories fed to our pets.
What has been less clear is whether the age of neutering influences the risk of obesity. This study has added some evidence to help answer that question, and the results appear to show that there is no effect of the age of neutering on the obesity risk.
There are a number of limitations in the conclusions we can draw from retrospective studies in general, and from the particular data set used here, and the authors discuss some of these in the paper. Overall, however, this study provides yet another bit of useful data to help inform neutering decisions. If there are compelling reasons to neuter early (less than 6 months of age), as is often the case for unowned animals in shelters, we can be fairly confident that this will not greatly increase the risk of later obesity so long as the dogs are appropriately fed.
A somewhat surprising finding was that neutered animals appeared to be less likely to be diagnosed with osteoarthritis than intact dogs. Even more surprisingly, dogs diagnosed with arthritis in the study were less likely to be diagnosed as overweight. These findings contradict the usual understanding that obesity can predispose to osteoarthritis and that arthritis can reduce activity and possible worsen obesity. However, due to a number of methodological limitations, it is not possible to say if there is truly an increased risk of arthritis in intact dogs or, if so, why that might be. This provides an interesting question for further research, but not a reason to change current neutering practices.