More than half -- 57% -- of U.S. cats are overweight or obese, according to a 2008 study by the Association for Pet Obesity Prevention. That’s 51 million fat cats.

How can you tell if your cat is overweight? And what can you do about it? WebMD asked Tony Buffington, DVM, PhD, an expert in feline health and nutrition.

Q: How can I tell if my cat is too fat?

A: You do a body condition score. Look down at your cat. You should be able to see a waist when you look down on it from the top, or when you run your hands from its ribs to its hips. Run your hand along its abdomen from its ribs to its pelvis and it should be indented.

If you put your hands on the side of its chest, you should be able to feel its ribs without a thick layer of fat over them. There are charts that show this. You can find them online - all the pet food sites have body condition scoring charts. Or ask at your veterinarian’s office and they can show you the chart and help you evaluate where your cat falls.

Q: Are some breeds more prone to weight gain?

A: A couple of studies have looked at this, and surprisingly, they found that mixed breed cats are about twice as likely to be overweight than purebred cats.

Q: Why should I care if my cat is fat? He looks happy to me.

A: It depends on why your cat is overweight. If he’s overweight because he eats too much, or you feed him too much, then there are the common beliefs that obesity can lead to shorter life spans, diabetes, or joint and skin problems.

But often, cats are overweight because of environmental causes. So first we need to figure out why your cat is overweight. Is he not moving enough because he’s terrified in your house? Do the dog or the kids attack him every time he comes out of hiding? Or, is he bored because there’s nothing to do in the house so he just sits around and eats? If that’s the case, then changing his diet or the amount of food he eats won’t help his weight problem. He’s not a happy fat cat. He’s just fat. So the issue then is making your home a happier place for your cat. If he’s happier and more active, then he’ll probably lose weight.

That’s why, when I work with fat cats, I look at the environment as well as the food intake.

Q: Will spaying or neutering a cat make it gain weight?

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