Here's what happens when cats get too fat (it involves acidic poop)

A cat getting measured by a vet
Roughly 60% of cats in the United States are overweight or obese. (Image credit: Ryerson Clark via Getty Images)

Feline obesity is a growing problem, with roughly 60% of cats in the United States classified as being overweight or obese, according to the 2022 State of U.S. Pet Obesity Report.

And, just like in humans, packing on additional pounds can often lead to a host of different health problems in cats, including changes to their digestive system, type 2 diabetes and chronic inflammation, according to a study published Sept. 29 in the Journal of Animal Science.

"It is a huge problem," study co-author Kelly Swanson, a professor of human nutrition and the interim director of the Division of Nutritional Sciences at the University of Illinois Urbana-Champaign, told Live Science in an email. "Similar to other species, feline obesity is associated with numerous health problems. If we can avoid obesity, many other health issues may be prevented or delayed."

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To find out what happens when our feline friends habitually overindulge, scientists corralled 11 adult spayed female cats and let them gobble up as much standard dry cat food as they wanted, according to a statement. The study also included several cats that were fed a controlled diet to compare the effects of their food intake.

The overeating cats' average body condition score (BCS) — a measurement system similar to body mass index (BMI) in humans — was 5.41 on a 9-point scale at the beginning of the study, but after 18 weeks of overindulging, that number shot up to 8.27, which corresponded to being 30% overweight, according to the study.

As the felines tipped the scales, researchers observed not only a surge in poop production but also a rise in gastrointestinal transit time, which is the amount of time between eating and bowel movements. (To measure this, cats were fed a bright-green, non-digestible dye that turned their poop green.)

"That led to a reduced amount of time for the body to digest the food consumed, resulting in a reduced digestive efficiency (nutrient digestibility)," Swanson said. "That also affected the fecal microbiota of the cats, likely due to the reduced digestion (more nutrients passing through)."

Because the cats were not getting all of their proper nutrients, the researchers noticed "significant changes" in the gut microbial compositions in the fat cats. For instance, they recorded an increase in Bifidobacterium, bacteria that inhibit pathogens and stimulate the immune system, and a decrease in Collinsella, bacteria that break down fiber and can protect against inflammatory diseases, according to the statement.

Interestingly, this is the opposite of what happens in overweight humans. However, more research is needed to understand why.

The cats' poop also became more acidic due to a decrease in fecal pH, indicating poor absorption of carbohydrates and fat — which is related to "higher food intake and reduced digestibility," according to the statement.

Swanson said that because the study was relatively short and lasted only 18 weeks, none of the cats developed any long-term health conditions, and that once the study commenced, researchers "placed [them] on a weight-loss study so they are all back at a healthy body weight again."

Jennifer Nalewicki
Live Science Staff Writer

Jennifer Nalewicki is a Salt Lake City-based journalist whose work has been featured in The New York Times, Smithsonian Magazine, Scientific American, Popular Mechanics and more. She covers several science topics from planet Earth to paleontology and archaeology to health and culture. Prior to freelancing, Jennifer held an Editor role at Time Inc. Jennifer has a bachelor's degree in Journalism from The University of Texas at Austin.