Like most cats, Max had a swagger in his walk. But because he was slightly overweight, the 15-year-old Maine coon began having trouble “jumping up on things,” his owner says, the extra pounds worsening his arthritis.

So his owner, Jaime Wilson, decided her pet needed to go on a diet — barely two tablespoons of dry food in the morning and again at night, along with a larger portion of canned wet food once a day and a supervised exercise program that included treadmill work and running through stationary poles.

“He was ravenous all the time,” his owner concedes. But after six months, “he’s very sleek and thin,” says Mrs. Wilson , who works at the University of Florida’s Small Animal Hospital in Gainesville, Fla. “Not having the extra pounds has been great for his joints.”

The Association for Pet Obesity Prevention estimates that in the United States, veterinarians now classify more than 100 million dogs and cats as overweight or obese, up from 80 million five years ago. Sixty percent of cats tip the scales at unhealthy weights, slightly more than the 56 percent of dogs.