Here's to keeping your health on a tight leash: New research suggests that having a dog might boost a single person's life span.

The study tracked more than 3.4 million Swedes, middle-aged and older, for 12 years. All were free of heart disease at the beginning of the study.

The researchers reported that dog owners who lived alone were 11 percent less likely to die of heart disease and a third less likely to die from any cause, compared with those who lived alone and didn't have a dog.

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The study couldn't prove cause-and-effect, but its lead researcher said there are many reasons why having a pooch might do a body good.

"We know that dog owners in general have a higher level of physical activity, which could be one explanation to the observed results," said Tove Fall, an associate professor in epidemiology at Uppsala University in Sweden.

"Other explanations include an increased well-being and social contacts or effects of the dog on the bacterial microbiome in the owner," she said in a university news release.

A person's "bacterial microbiome" consists of the trillions of "good" microbes living within the body that help keep it healthy.

Experts in the United States agreed that the findings made sense.

"Stress relief through companionship has an inherent benefit to people's overall health, so it is not surprising that dog owners display a lower risk of heart disease," said Dr. Satjit Bhusri, a cardiologist at Lenox Hill Hospital in New York City.

Dr. Gisele Wolf-Klein directs geriatric care at Northwell Health in Great Neck, N.Y. She agreed that Fido or Rover can force their humans to get more active.

"The responsibilities associated with dog ownership impose mandatory daily exercise -- a schedule which cannot be impacted by adverse weather conditions, personal commitments or mood swings," Wolf-Klein said.

That may be especially important for single folks, said study junior author Mwenya Mubanga, a graduate student at Uppsala. Prior research has shown that living alone raises the risk for heart disease.

"Perhaps a dog may stand in as an important family member in the single households," Mubanga said in the news release.

Wolf-Klein believes that whatever the reasons behind the health benefit shown in the study, adopting a furry companion from a nearby shelter might be just what the doctor ordered.

"The findings of the largest ever investigation of the association between dog-ownership and human health should encourage all of us to add a four-legged friend in our family circle," she said.

The study was published in the journal Scientific Reports.