Top and side views of cats (not counting fur). Silhouettes 3 and 4 represent optimal weight cats.

Seriously overweight cats are more likely to suffer diabetes mellitus, lameness and non-allergic skin conditions, a Cornell University veterinarian's four-year follow-up to a feline obesity study has shown.

Most likely to be tubby are neutered, apartment-dwelling, mixed breed cats eating prescription cat food.

"The original obesity study and the follow-up confirm what veterinarians and cat owners have suspected for a long time," said Janet M. Scarlett, D.V.M., associate professor of epidemiology in Cornell's College of Veterinary Medicine. "Too much extra weight is not healthy for the cats, but being very thin can be a problem, too."

Scarlett was a collaborator, along with veterinary nutritionist Susan Donoghue, in the landmark 1989 study of 2,000 cats at 31 veterinary clinics in the Northeast, as well as the principal investigator of the tracking study. The Waltham Centre for Pet Nutrition in the United Kingdom funded the first study, which found that about 25 percent of cats brought to veterinarians are overweight: 20 percent were ranked as "heavy" by veterinarians and 5 percent were considered "obese." Slightly more pet owners -- about 29 percent -- thought their cats were overweight in the same survey.

Ralston Purina Co., the American pet food manufacturer, funded the four-year follow-up, in which Cornell veterinary students called as many of the original pet owners as they could locate and asked about the cats' health. Researchers also reviewed veterinarians' medical records for the animals in the first survey. Among the findings:

• Overweight cats, including those considered "heavy" and "obese," are four-and-a-half times more likely to develop diabetes mellitus, compared to optimal weight cats.
• Obese cats are seven times more likely to require veterinary care for lameness, caused by joint diseases such as arthritis or muscle injuries, compared to optimal weight cats. Heavy cats are three times more likely to suffer lameness.
• Obese cats are three times more likely to be presented to veterinarians for non-allergic skin conditions, probably because the cats cannot reach all parts of their bodies to groom themselves properly.
• Obese cats are twice as likely to die in middle age, which for cats is 6 to 12 years.
• Extremely thin cats are more likely to be presented to veterinarians for diarrhea.

Veterinarians in the survey ranked the cats, from extremely thin to optimal to obese, after physically examining the cats' body condition under their fur and comparing the cats to a series of six standard silhouettes.

Unlike the case with humans, there is no body-mass index for obesity in cats, Scarlett noted.

Most cats refuse to sit still for the electrical impedance test -- one way to measure body mass -- and they really dislike the other method -- being completely immersed in water while exhaling. Ranking body condition by silhouettes is currently the most practical way to survey large numbers of privately owned cats, the epidemiologist said.

In the original study, cat owners completed a three-page survey with information about their animals' breed, housing accommodations and lifestyle, food, age, gender, whether the animal was neutered and other factors that could affect body condition. Analysis of those reports found several factors that are associated with being overweight:

• Cats living in apartments are about two times more likely to be overweight than are cats that roam larger houses or the out-of-doors.
• Neutered cats are 3.4 times more likely to be overweight than those that are sexually intact.
• Obesity is more prevalent among mixed breed cats, compared with purebred cats. However, some of the purebred cats are probably show cats that are kept in prime condition, Scarlett commented, and some of the more popular purebred cats, such as Siamese and Burmese, are thin animals to begin with.
• Cats eating some prescription diets for conditions other than weight are more likely to be overweight. That is because many prescription diets contain more calories in the same volume of food that the cat is accustomed to eating, Scarlett said. "Prescription diets tend to be caloric-dense, and if you feed them the same way you feed grocery store cat food, the cats will gain weight."
• Inactive cats that get virtually no exercise are 16 times more likely to be obese.

The veterinarians were surprised to find cats putting on extra weight so early in life. By one year of age, some 15 percent of cats in the survey were already overweight. And by two years of age, nearly 20 percent had landed in the "heavy" or "obese" categories.

Results of the 2,000-cat obesity study were reported in the International Journal of Obesity (Vol. 18 Supplement 1, 1994). Results of the follow-up study will be presented at the Eighth Symposium of the International Society for Veterinary Epidemiology and Economics July 8-11 in Paris.