Why do sterilized pets gain weight after surgery?

Spayed and neutered dogs and cats are more likely to become overweight than intact ones, and most of the reason is simple energy balance. After surgery many animals eat more, some burn slightly less, and the portion that kept them lean before the operation becomes too generous. In a decade of records from the UC Davis veterinary teaching hospital, about four in ten dogs were overweight or obese, and neutered dogs carried a higher risk. The gain is avoidable if the food bowl changes when the hormones do.

What changes in the body after surgery

Removing the ovaries or testes removes the main source of estrogen and testosterone. These hormones influence appetite and how the body handles fat, not only reproduction.

Appetite seems to be the faster effect. In a study of adult male cats kept on their usual ration for the first days after castration, energy expenditure did not fall. Once the same cats were given free access to food, they ate more and gained weight. Their levels of ghrelin, a hormone that drives hunger, rose after neutering. The authors concluded that early weight gain in neutered male cats came from eating more rather than from a slower metabolism.

Metabolic rate can still play a part. An older controlled study found that neutered male cats gained more than twice as much weight as intact males over three months, mostly as fat, and that spayed females had a lower fasting metabolic rate after surgery. Food intake rose in the neutered group as well.

The practical result is an animal that asks for more food at the point where it needs no more, and sometimes less. A cat that starts crying at the bowl a few weeks after surgery is not being dramatic. Its appetite has changed.

How quickly weight comes on

Fast enough to notice within months. In the older cat study above, neutered males had gained significantly more than intact males within three months, and in the newer one appetite rose soon after surgery once food was freely available.

Age at neutering seems to matter for cats, though not in the direction many owners expect. A large study of client-owned cats in the Veterinary Record found no difference in long-term weight or body condition between cats neutered at four months or younger and those neutered at five or six months. Cats neutered at seven to twelve months had a lower risk of gaining. The authors still advised weight management advice for every neutered cat, tracked with both body condition score and the scales.

Prevention is far easier than weight loss. Adjusting portions in the first months after surgery avoids the slower and harder work of taking weight off later.

Changing how the animal is fed

There is no single correct cut in calories, because needs vary with age, breed, activity and the food itself. A reasonable approach is to weigh the animal and score its body condition at the time of surgery, trim the portion modestly once it has recovered, and then adjust every two to four weeks based on what the body is doing.

Measure every meal

Free-feeding makes weight gain very likely in a neutered animal whose appetite has just gone up. Weigh the daily ration on a kitchen scale, which is more accurate than a scoop, and split it into meals. Small daily surpluses add up over months.

Consider the food itself

Foods sold for neutered pets are usually lower in energy and higher in protein or fiber, which lets the animal eat a satisfying volume for fewer calories. There is some evidence behind the protein part. In a 26-week study, neutered dogs fed a lower-protein diet gained body fat even though their weight stayed the same, while those on a higher-protein diet kept their energy needs and body composition steadier. A switch is not mandatory. A measured, smaller portion of a complete adult food also works.

Count the treats

A common rule is that treats should supply no more than 10% of daily calories, with the rest coming from the main food. For a cat that needs 200 calories a day, that is 20 calories of treats. Most cat treats hold one to three calories each, but some hold much more, and VCA's weight plan for cats suggests reading the label and budgeting treats in advance. Dental chews for dogs vary widely in energy content, so check the calories per chew before making one a daily habit. More options are in our guide to healthy treats for dogs.

Exercise for dogs and cats

Diet does most of the work, but activity helps keep muscle while the calories come down. Dogs do well with daily walks at a pace that suits their age and build, plus play such as fetch or swimming.

Cats are built for short bursts rather than long sessions. Several brief play periods a day with a wand toy, food balls or puzzle feeders suit them better than one long session. Puzzle feeders also slow down a fast eater.

Checking body condition

Weight alone can miss fat gained while muscle is lost. Current AAHA nutrition and weight management guidelines recommend scoring body condition and muscle condition at every veterinary examination. Owners can check body condition at home on the common 9-point scale:

• Ideal (4 to 5): ribs easy to feel with light pressure but not visible, a waist seen from above, and a tuck behind the ribs seen from the side.
• Overweight (6 to 7): ribs hard to feel under a layer of fat, waist faint or absent.
• Obese (8 to 9): ribs cannot be felt, no waist, belly sagging or rounded.

In the first six months after surgery, check every two to four weeks. If the score creeps above 5, reduce the portion a little and recheck. More detail on scoring is in the body condition score guide.

When to involve the vet

If a dog or cat is already clearly overweight, have it examined before starting a weight-loss plan. Weight gain is not always about food, and the vet can set a target weight and a safe rate of loss.

Cats need particular care. Losing weight too fast costs muscle and puts a cat at risk of hepatic lipidosis, a serious fatty liver disease. VCA advises a loss of about 1% to 2% of starting body weight per week, and closer to 0.5% for very obese cats. A cat that stops eating for more than a day or two, whether dieting or not, should be seen promptly.

Sources

1. Prevalence, risk factors, and disease associations of overweight and obesity in dogs that visited the Veterinary Medical Teaching Hospital at the University of California, Davis from January 2006 to December 2015. Topics in Companion Animal Medicine, 2022.
2. Early effects of neutering on energy expenditure in adult male cats. PLoS One, 2014.
3. Effects of neutering on bodyweight, metabolic rate and glucose tolerance of domestic cats. Research in Veterinary Science, 1997.
4. Long-term effect of neutering age on body condition score and bodyweight in domestic cats. Veterinary Record, 2025.
5. Effect of dietary protein intake on the body composition and metabolic parameters of neutered dogs. Journal of Nutritional Science, 2017.
6. 2021 AAHA nutrition and weight management guidelines for dogs and cats. Journal of the American Animal Hospital Association, 2021.
7. Creating a weight reduction plan for cats. VCA Animal Hospitals.