Halfway around a four-mile lake trail last summer, Sasha sat down, her breathing labored, her eyes bloodshot. The 7-year-old beagle was clearly in distress. Rotund even as a puppy, her weight had climbed to 43 pounds hugely overweight for a beagle.

Dorothy Bowers was terrified. It looked as if her dog was having a heart attack. She was afraid her pet was about to die on the spot, and then it hit her: How was she going to carry a 43-pound dog two miles back to the car to get medical help if Sasha couldn’t go on?

People often focus on losing weight as the new year starts. But they might want to glance at Fido and Snowball while they’re stepping on the scales: Lots of pets should shed a few pounds to be healthier.

According to Iams, a pet food company, 61 percent of American adults are overweight, and 40 percent of cats and dogs are too.

Wendy Weirich, a veterinarian, would put the latter figure at closer to 50 percent, if you’re counting pets that are 10 percent over their ideal weight.

Pets generally pick up pounds for the same reasons people do they’re overfed and underexercised. Sometimes weight gain is a symptom of a disease, however, so a vet should look at your pet before you put it on a diet.

Advancing age and declining activity level play a role in weight gain, as does having a pet spayed or neutered; and some breeds, such as Labradors and cocker spaniels, are prone to be portly.

Extra weight puts pets at risk for diabetes and heart disease. It stresses the joints (exacerbating the effects of arthritis, for example), makes breathing difficult and can impede animals’ ability to keep themselves clean, leading to skin problems.

Rebecca Krafft said her chronically overweight cat developed diabetes, and she and her husband ended up having to give insulin shots twice a day.

Jan Martin’s Lab mix started gaining weight when the family moved from a house that backed the woods to a place with a fenced-in back yard. Arthritis was becoming an issue, so Martin thought the small yard would be OK. She got a shock when she took Big Moo in for an annual checkup.

“The vet said we could get the weight off of her or put her down,” she said. “He may have lacked for bedside manner, but he scared the hell out of me.”

Martin slashed the portions she was giving her dog, substituted small rawhide strips for dog biscuits, stopped sharing the Doritos and started taking the dog for walks.

Big Moo moves much better now, Martin reports, which has the side effect of improving her mood and her behavior. Her veterinarians are happy with the success.

Sasha the beagle finished the trip around the lake, and Bowers immediately contacted the vet. He put the dog on prescription diet food; she lost too quickly at the beginning, so they adjusted the amount given. Bowers threatened her husband with a fate worse than death if he slipped Sasha any treats or table scraps.

Vets warn that all the members of the family need to be on board with a pet’s diet. Bowers acknowledges that being strict with Sasha about her diet has been very hard.

“I can’t tell her why she can’t get all that ‘good stuff’ anymore or that I’m doing it for her own good,” Bowers says. “I have to look at her questioning face in the kitchen and say, ‘No, none for you.’ “

People dream of losing 10 pounds a week, but it’s no healthier for animals to drop weight abruptly than for humans. If cats stop eating or lose weight too fast, they can develop a liver disorder (hepatic lipidosis), a potentially fatal disease that occurs when fat cells accumulate in the liver, says Weirich. Dogs can also suffer organ damage from a crash diet.