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.

Still, many pet owners are -- as I was -- in denial about their pets' weight and the potential threats to their health.

"First, you've got to recognize there's a problem," said veterinarian Carol McConnell, a VPI vice president and the company's chief veterinary medical officer. "When you're standing directly over your pet looking down, you should see a waist."

A healthy-weight pet has a tucked abdomen: It's smaller than the animal's rib cage and doesn't sag. You should be able to feel your pet's ribs. (You can find charts showing healthy and unhealthy profiles, as well as weight charts and calorie recommendations, on the Association for Pet Obesity Prevention's site.)

McConnell thinks our perception of what's a normal weight, either for ourselves or for our animals, has been distorted by the growing obesity epidemic. (One in three U.S. adults is obese, according to the national Centers for Disease Control and Prevention.) And our pets are getting fat for the same reasons we are, for the most part: too much food and too little exercise.

Ward's research has found that the typical dog owner walks his or her pet for about 10 minutes a day -- and that could be at a stroll as the animal sniffs around for a place to poop. The typical dog actually needs daily, brisk walks that last at least 30 minutes, he said. The occasional long hike or trip to the dog park isn't a substitute for sustained daily exercise -- just as being a "weekend warrior" is no substitute for daily physical activity for a human.

Cat physiology is different. Felines are set up for short bursts of energetic activity a few times a day (think cheetahs accelerating across the savannah to seize their prey). Ward recommends scheduling in a few short play periods with your cat each day, using cat toys like the feathers hung from fishing-type rods or a flashlight that shines a beam on the floor. Another idea: hiding a few kibbles from her bowl around the house and letting kitty "hunt" for them.

Cats that aren't kept physically active are especially prone to "stress eating," which will exacerbate a weight problem, Ward said.

Some other tips for helping your fur buddy get trim:

  • Talk to your vet. You need to rule out other potential causes of weight gain, such as a bum thyroid, before restricting food or boosting exercise. Your vet also can offer advice about what kind of pet food to use and how much. Pet food labels are another source of information about how much to feed your pet.
  • Measure food. Once you know how much to feed, keep an appropriate-sized measuring cup or marked scoop with your pet's kibble. If you're supposed to offer three-quarters of a cup of food twice a day, for example, mark a one-cup plastic scoop at the appropriate level. Caution other family members about the importance of not overfeeding your pet, even "a little." "As little as 10 extra kibbles a day" of cat food, Ward said, "added up to an average weight gain of one pound a year."

On the other hand:

  • Don't starve your cat. Severe calorie restriction can cause a type of liver failure in cats, McConnell warned. Once the problem takes hold, the cat typically needs a three- to four-day hospital stay to recover -- a harrowing experience for all concerned and expensive for the owner.
  • Limit treats. Every calorie counts -- and that includes food doled out between meals as treats. Reward your pet with activity and play time, instead. If your dog is really giving you the sad eyes, opt for healthful treats. Our pooch likes carrots, apple slices and frozen green beans. (He'll chew on celery and broccoli, two other vet-approved snacks, but won't eat them; he strews the mashed remains all over the yard.) Steer away from feeding table scraps; that can encourage begging and cause gastric problems.

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  • Ease into exercise. If your pooch is used to walking no farther than the curb, don't take him on a 5-mile hike. Increase exercise gradually to avoid strains and sprains -- for both of you. "Remember the last time you killed yourself (with exercise) on Saturday and Sunday, and by Monday you could barely move," McConnell said. "Any increase in activity should be gradual."

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.