Jogging regularly into old age may have more benefits than a simple runner’s high — the healthy habit may slow the effects of aging, according to a Stanford study two decades in the making.

In 1984, Emeritus Prof. of Medicine James Fries came up with a hypothesis called Compression of Morbidity, which holds that healthy lifestyles will not only prolong survival, but will also decrease the number of years with disability. He gathered a team of researchers and began his experiment.

The study started with around 1,000 people over the age of 50. Half of the participants were members of a runner’s club, and the other half were otherwise healthy people from Stanford, but not members of the running club. For 21 years up until 2005, the participants rated their disability status on a scale of 0 (no severe health problems at all) to 3 (pretty much unable to do most things).

“Everybody reported increasing disability as they get older, especially since we’re talking about people who were around 60 when they started,” said Prof. of Medicine Eliza Chakravarty. “However, we found that over time, the runners group had very low levels of disability that increased very slowly. The controls, on the other hand, had disability that increased much more quickly. At the end of the study, the difference between the disability levels was very dramatic.”

Participants who were part of a runner’s club also enjoyed lower mortality rates — about half that of the non-participants.

“So not only were they less disabled, but they lived longer as well,” Chakravarty said of the running participants.

One difficulty in this study was getting rid of self-selection bias — the possibility that factors other than running contributed to the experimental results. According to Fries, it was a highly plausible explanation that a positive, healthy attitude — rather than running — produced better outcomes. The research team tried to minimize the self-selection bias in several ways.

“We statistically adjusted for many factors such as obesity and age,” Chakravarty said. “We also chose pretty healthy people in our control group. They were not obese, had normal body weight, had the same education levels as the runners and didn’t drink or smoke much. We tried to restrict it so that the main difference between the two groups was running.”

The basic message of the study, however, is simple: Exercising, at any age, is very important.

“People shouldn’t be discouraged from exercising because they’re older,” Chakravarty said. “Adopting an exercising program — even in the 50s or 60s will be beneficial down the road. We’re just really excited that we’re able to show benefits of exercise, not only in living longer, but living healthier up into the 80s.”

Of course, swimmers, bikers and other exercise fiends need not fret over the study’s emphasis on running; other forms of cardiovascular exercise serve a similar purpose.

“We think it’s probably vigorous exercise in general [that produce these results],” Chakravarty said. “Even though we chose the groups for running and non-running, we don’t necessarily think that there is anything specific to running — that running is better than cycling or swimming or any other form of regular aerobic exercise.”