Exercise is tough, particularly if you want to keep at it regularly. Even something as simple and effective as a daily walk often gets derailed by bad weather, work, or—let's face it—plain old laziness. But walking a dog can knock off as much as 14 pounds in a year, two studies have found. And once in the exercise habit, these dog walkers have gone on to join gyms, modify their diets, and make other changes to help them lead healthier lives.

"We're desperately looking for treatments to combat our obesity epidemic. For exercise, one of the most important things I know about is 'buddying up,' " says Robert Kushner, medical director of the Wellness Institute at Northwestern Memorial Hospital in Chicago. "We have gym buddies, make dates to walk with our wives, and that keeps us motivated." Basically, you have a sense of responsibility to meet the other person, not to skip out, he says. So a few years ago he started wondering about dogs as exercise companions and started a trial to see if hounds could reduce pounds. "When you buddy up with your dog, you have even more responsibility than you have with a person," Kushner says. "The dog depends on you."

Rebecca Johnson agrees. A nurse and director of the Center on Human-Animal Interactions at the University of Missouri's veterinary school, Johnson initiated a similar experiment. "People kept telling us, 'The dogs needed us to walk them,' " she says.

Johnson took 13 inactive, overweight people who did not own dogs. She paired them each with a volunteer therapy dog–a well-trained, gentle animal–and started them on a graduated walking program. At first, the participants, who had not been exercising at all, walked for 10 minutes, three times a week. They kept increasing their time and eventually walked up to 20 minutes, five times a week. After 50 weeks, the walkers lost an average of 14 pounds. "And the benefits continued after the study. Four members of our group went to the local Humane Society and volunteered to walk their dogs, so they kept at it. That was wonderful," Johnson says.

Kushner took a different tack, monitoring 36 dog owners who were already doing a little bit of walking with their pets. "Our basic charge to them was: Do more," he says. Kushner also offered them classes on nutrition, focusing on things like controlling the size of their food portions. After a year, people lost 11 pounds, on average.

Kathleen O'Dekirk, a Chicago lawyer, and Winston, her King Charles spaniel, took part in the program. "I was just turning 50, and I'd ballooned up to 151 pounds, and I'm only 5 feet, 3 inches tall," she says. "I'd been 112 pounds for most of my life. And I was having trouble bending over." Winston was also piling it on. He weighed 31 pounds and needed to drop about 7 of them. "That was really the impetus. He needed it as much as I did. So we started to walk. I used to take him out for 25 minutes. But now I'm doing an hour in the morning, 25 minutes at lunch, and another 20 minutes at dinner." O'Dekirk dropped 13 pounds, and Winston hit his weight-loss target. The lawyer was also inspired to join a gym for the first time in her life, and she still takes yoga classes.

But it wasn't just the jaunts with her pooch that led to O'Dekirk's trimmer frame. The nutrition classes were a big help. "I got advice on using beans and using ground turkey instead of ground beef. And I found out I don't really need more than a tablespoon of salad dressing," O'Dekirk says. And in fact, a comparison group of people who don't own dogs but who walked by themselves and took the nutrition classes lost just as much weight. So Kushner says he can't specify how big a factor the dogs really were.

O'Dekirk, however, is sure they were key. "It gets cold here in the winter, and you don't want to go out. But when you have this little face looking up at you, you can't resist."