While participation in ultramarathons — which cover distances beyond the standard 26.2-mile marathons — has seen a surge of popularity in recent years, not much is known about the health effects of this extreme exercise.

Eswar Krishnan, assistant professor of medicine at Stanford, and Martin Hoffman, professor of physical medicine and rehabilitation at UC-Davis, recently published a study in the journal PLOS ONE that addresses the issue.

“I’m tired of my family telling me that all this running is bad for me, without some science,” said Hoffman, who is an ultrarunner himself.

The Ultrarunners Longitudinal Tracking Study consisted of an online questionnaire in which ultrarunners answered questions about the last 12 months of competitions as well as questions about their training, general health and running-related injuries.

The study is geared towards helping understand how much exercise is best, and how far runners should push their bodies.

“We want to understand the more general concept of physical function, meaning why are some patients able to perform well, while others are not,” Krishnan said.

The effect of ultrarunning on the body of athletes remains unclear from the study due to difficulty in isolating the effects of ultrarunning. However, Hoffman said that the biggest concern for these runners — in terms of adverse health effects — is cardiac arrhythmia, which has been shown as a risk factor in cross-sectional studies.

Krishnan also noted the possibility of kidney failure, due to an increase in muscle breakdown products; however, when breakdown occurs in ultrarunners, it usually resolves quickly.

While Krishnan and Hoffman plan to continue studying their pool of 1,200 runners for 20 years, their latest work did reveal some health statistics specific to ultrarunners.

The latest study showed that over 75 percent of ultrarunners had some injury in the past year, with injuries more common in younger, less experienced runners. Most of the injuries were related to knees or other parts of the leg.

Stress fractures were found to be less common in ultrarunners than other runners. However, stress fractures are still important to consider, especially in women who are more prone to osteoporosis in older age. An upcoming study by Krishnan and Hoffman will look further into osteoporosis.

Additionally, the incidence of asthma and allergies was 11 and 25 percent respectively in ultrarunners, compared to the 7-8 percent of both cases in the general population. The higher incidence of allergies is not a new observation, according to Krishnan, given the theory that runners are more exposed to allergens by being outdoors more often. The asthma cases were not permanent, making it not such a significant health problem, Krishnan added.

Krishnan and Hoffman are now looking to shift their focus to former runners who may have had to quit due to injury, personal reasons or medical problems. This will allow for a comparison between healthy ultrarunners and those who have had to stop running.

Krishnan and Hoffman will also work with sports psychologists to understand peoples’ attitudes towards competition.

“I think it’s in their mind, that fact that they can stay focused for so many hours,” Krishnan said.

Hoffman added that people’s motivations toward intense exercise like ultrarunning is a key factor they want to address.

“Part of it is sorting through what motivates people to exercise this much, and if we could get a better handle on that, maybe it would have some application to the general population, where only a very low percentage [of people] actually exercise,” Hoffman said.

Contact Julia Turan at jturan ‘at’ stanford.edu.