The answer is that spayed and neutered domestic cats ‘tended to be heavier than those that weren’t’. So the answer is, yes, spayed and neutered cats do put on weight but the experts are unsure why.
The quote short above comes from the CBC.com website. The author refers to a massive AVMA study on domestic cat weight. The study involved more than 19 million cats in the US and Canada so the conclusion is very solid although when you read the summary of the study on the AVMA website the authors write:
Mean [body weight of shorthair, medium hair and longhair domestic cats] peaked at 8 years and was subjectively higher for neutered than for sexually intact cats.
The phrase ‘subjectively higher’ is surprising for scientists to use as it indicates that they are unable to provide hard data. They may simply be repeating anecdotal evidence.
It is also surprising that it is hard to find a clean and clear reason why spayed and neutered cats put on weight.
Dr Jennifer Coates on petmd.com honestly writes that she can’t come up with a good explanation as to why domestic cats eat more (as shown in 2 studies) post surgery when given free access to food. The cats don’t need more calories because they are not burning more calories so why were they eating more in the studies? She guesses that it is something to do with their lack of hormones – oestrogen for females and testosterone for males – after the surgery. At least she tells us that weight gain is due to eating more.
“The study on male cats showed a dramatic increase in food intake in some cats during the first week post-surgery, with some cats experiencing a 10 percent increase in body weight.”
Another veterinary source1 says that spaying female cats may slow their metabolism. They don’t make any reference to male cats putting on weight after neutering. This information conflicts with Dr Coates’ statement which does not help me.
However, my distinct impression is that both male a female cats may put on weight post surgery. I wonder if the problem is more prevalent for female cats? It is interesting to note that menopausal women tend to put on weight. During menopause their production of oestrogen is greatly reduced. This cross references what Dr Coates states about weight gain being linked to hormonal change.
The recommendation is food portion control for your cat post surgery if she or he was free feeding before the surgery. Dr Coates recommends restricting food portions for up to five months after the spaying and neutering operations.
For the record, my male cat did not put on weight after he was neutered. He has always be slender and a free eater. I have never needed to do portion control. More uncertainty in trying to answer the question in the title.
1. Cat Owner’s Home Veterinary Handbook page 426 of 3rd edition.