Here's What Happens to Your Cat's Insides When They Overeat

America has a problem with obesity—and not just among people.

Roughly 60 percent of cats in the U.S. are classed as overweight or obese, according to data from the 2022 State of U.S. Pet Obesity Report. But how does gaining weight actually affect cats physically?

"It is a huge problem," Kelly Swanson, professor in the Department of Animal Sciences and interim director of the Division of Nutritional Sciences at the University of Illinois Urbana-Champaign, told Newsweek. "While some cats can maintain a healthy body weight on their own, most of them cannot. If we can avoid obesity, many other health issues may be prevented or delayed."

While extensive research has gone into feline weight loss, the process of gaining weight, and the health consequences therein, have been largely understudied. Therefore, Swanson and his team set about learning what exactly happens at a metabolic and gastrointestinal level when cats get fat.

The study, which was published in the Journal of Animal Science, included 11 adult female cats who were allowed to eat as much as they wanted over an 18-week period.

"When cats were given extra food, they quickly overate and gained weight," Swanson said. "We found that as cats ate more and gained weight, gastrointestinal transit time was reduced, and so was digestive efficiency. When the body gets less food, it will be more efficient in extracting nutrients. But when the amount of food increases, it passes through the digestive system faster and fewer nutrients are extracted in the process.

"That also affected the fecal microbiota of the cats, likely due to the reduced digestion [and] more nutrients passing through."

Interestingly, the relative abundance of Bifidobacterium, a group of bacteria known to have antimicrobial activity, increased, while Collinsella, which degrades fiber and has been linked to pro-inflammatory diseases, decreased. This is the opposite of what we see in overweight humans, suggesting a complex relationship between the microbes and obesity.

After the experiment, all cats were put on a food restricted diet to return to a healthy weight.

Further research is needed to fully understand the biology of feline weight gain, but Swanson hopes that their results may help with future prevention and treatment plans for overweight pets. But what should you do if your cat is already a bit squishy?

"One important thing is controlled and monitored food intake," Swanson said. "Another is limiting the amount of treats, table scraps, etc. to 10 percent of total calorie intake. Stimulating exercise and activity is also important.

"Successful weight loss requires several things: a committed owner, an appropriate diet, appropriate feeding method, an exercise plan, owner communication, and patient monitoring. [...] If significant weight loss is required, a prescription diet that has been carefully formulated is recommended. If not done carefully, weight loss can be too rapid and dangerous. Nutrient deficiencies can also be an issue if a diet is not carefully selected. [Therefore] designing a weight loss plan along with your veterinarian is always the best strategy."

Correction 11/16/23 10:59 ET: This article was updated to correct professor Swanson's pronouns.