A study published Wednesday in the scientific journal PLOS ONE found that neutered male cats gained weight not because they became lazy and expended less energy, but because they ate more food after the surgery.
Scientists at the University of California Davis set out to determine why cats tend to put on weight after neutering. They carefully controlled the diets of a small group of adult male domestic shorthair cats for a couple of weeks before the cats were neutered. The procedure, which the study authors refer to as a gonadectomy, involves surgical removal of the animals' testicles under general anesthesia.
Body composition and weight were monitored both before and after the procedure. From about two weeks post-surgery until 6 months after the procedure, scientists monitored the cats' activities to track how much energy they expended. The cats did what most house cats do - roamed around their climate-controlled abode, eating as much food as they desired.
After 6 months, the cats' body weight increased by about 20 percent, though their activity levels remained similar to what they were prior to neutering.
According to the authors, an estimated 35 percent of pet cats are overweight, and, they say, "Neutering is recognized as one of the major factors contributing to obesity in male cats, although the mechanisms that promote weight gain following this procedure are not well understood."
The scientists concluded that male cats gain weight after neutering due to increased food intake, which they say could be partially due to elevated blood levels of ghrelin, a hormone believed to stimulate hunger and make high-calorie foods look more appealing. The authors note that the study only examined weight gain during the first six months after surgery, and that cats' food intake needs to be moderated long term to prevent excessive weight gain.
The ASPCA recommends neutering of male cats before they reach 6 months of age to prevent the start of spraying, roaming and other undesirable behavior common to intact male cats. Cats should also be sterilized, the organization says, to avoid contributing to pet overpopulation, because "even a cat who lives indoors may escape and produce kittens if not sterilized. Each year, millions of homeless cats are euthanized or end up in shelters due to a lack of good homes."