Numerous studies have linked dog ownership to weight loss:

• One year-long study found that walking an overweight dog helped both the animals and their owners lose weight. Researchers found that the dogs provided support in similar ways to a human exercise buddy, but with greater consistency and without any negative influence.
• Public housing residents who walked therapy dogs for up to 20 minutes five days a week lost an average of 14.4 pounds in a year, without changing their diets.
• A third study found that people who got a dog walked 30 minutes more a week than they did before.

Source: Harvard Health Publications

For additional information on the health benefits of dogs (and cats) check:

http://www.helpguide.org/articles/emotional-health/the-health-benefits-of-pets.htm