Image: Girl with dog © Rob Melnychuk, Digital Vision, Getty Images

I was deeply offended when the vet said our dog was getting fat.

It's true his trim little waist had disappeared soon after he joined our household. But he certainly hadn't broadened out into the fur-covered coffee tables that our friends' golden retrievers had become.

"He's still a puppy," I protested, though he was nearly 2. "He's a growing dog."

The vet wisely didn't respond, "Yes, he's growing sideways." Instead, she suggested I feel his ribs.

I tried. I couldn't. Point taken.

Half of our dogs and cats are overweight, according to the latest Association for Pet Obesity Prevention study, and one in five is obese, with a body weight 30% or more above normal.

We may think our portly pets are cute, but vets tell us we're setting them up for a host of weight-related diseases and conditions, including:

  • Arthritis and other joint problems.
  • Disc disease and other back problems.
  • Torn and ruptured ligaments.
  • Diabetes.
  • High blood pressure.
  • Kidney disease.
  • Liver inflammation.
  • Asthma.
  • Lipomas (fat tumors).

Weight issues can shorten our pets' lives, and they cost us tens of millions of dollars in unnecessary pet bills every year, said veterinarian Ernie Ward, one of the association's founders.

"People come to me complaining about the high cost of veterinary care," Ward said, "and I tell them, 'Look no farther than the food bowl.'"

No one has done a definitive study of exactly how much fat pets cost their owners -- unlike the studies done about obese adults, which I covered in "What being fat is costing you." But Veterinary Pet Insurance, or VPI, the largest pet insurer and one that processes 1.1 million claims a year, estimates the nine most common weight-related diseases and problems cost its policyholders more than $28 million last year.

Not all of the conditions VPI tracked are exclusively weight-related. Just as you can have a normal-weight human diabetic, you can have a normal-weight animal diabetic. But you also should consider that VPI-covered pets constitute only one half of 1% of all U.S. pets. (Only about 1% of pets have medical insurance, and VPI has about half of that market.) So the real toll of weight-related vet costs actually could be in the hundreds of millions of dollars.