Although space travel exposes astronauts to forms of radiation that are uncommon on Earth, and that are linked to cancers and heart problems, a U.S. study suggests this doesn't significantly shorten their lives.

Researchers compared nearly 60 years of data on U.S. male astronauts and a group of men who are similarly extra-fit, affluent and receive elite health care: pro athletes. They found that neither group has higher rates than the other of death overall or of early deaths. Both groups do tend to outlast the

rest of us, however.

Astronauts are generally well-educated, more affluent and more physically fit than the typical American, and some previous research has linked this career to a lower risk of premature death, the study team notes in Occupational & Environmental Medicine.

But much of the previous research on mortality rates in astronauts hasn't accounted for the mental and physical demands of this career, or the so-called "healthy worker effect" that leads people with employment of any kind to typically have fewer medical issues than individuals who are unable to work, said study co-author Robert Reynolds of Mortality Research &

Consulting Inc. in City of Industry, Calif.

Comparable group needed

"The challenge has always been to understand if astronauts are as healthy as they would be had they been otherwise comparably employed but had never gone to space at all," Reynolds said by email. "To do this, we needed to find a group that is comparable on several important factors, but has never

been to space."

The researchers compared mortality rates for male U.S. astronauts to those of professional athletes from Major League Baseball and the National Basketball Association between 1960 and mid-2018.

Both athletes and astronauts had a lower risk of premature death than the general population, the study found. And there was no meaningful mortality difference between NBA and MLB players.

Astronauts were more likely to die of accidents and other external causes, and less likely to die from heart disease and all other natural causes, the study also found.

"We cannot be sure from the data we have, but we speculate that cardiovascular fitness in particular is the most important factor in astronaut longevity," Reynolds said.

NASA astronauts Reid Wiseman (R) and Barry Wilmore FILE - NASA astronauts Reid Wiseman, right, and Barry Wilmore work inside the International Space Station on October 1, 2014. FILE - NASA astronauts Reid Wiseman, right, and Barry Wilmore work inside the International Space Station on October 1, 2014.

The results suggest that radiation exposure in space might not lead to a premature death for astronauts due to heart problems or certain cancers, the study authors conclude. In fact, astronauts had a lower rate of death from heart disease than the NBA and MLB players, and had cancer mortality similar to the athletes' rates.

The study wasn't designed to prove whether or how space travel may directly impact human health. It also didn't examine mortality among female astronauts or athletes.

Lower radiation exposure

Radiation exposure may also have been much lower during early missions to the moon and not reflect what would happen with the current generation of astronauts, said Francis Cucinotta, a researcher at the University of Nevada-Las Vegas, who wasn't involved in the study.

"The missions in the past were low dose, while in the future the dose would be 50 to 100 times higher for a Mars mission," Cucinotta said by email.

Astronauts have typically never smoked, leading to a lower risk of heart disease than the general population, Cucinotta added.

Diet and exercise also set astronauts and professional athletes apart from the rest of the population, said Michael Delp, a researcher at Florida State University in Tallahassee who wasn't involved in the study.

"When physical fitness is a requisite part of a job, such as with astronauts and professional athletes, this is a major determinant of the healthy worker effect," Delp said by email.

Even for the rest of us, "remaining or becoming physically active and maintaining a well-balanced diet greatly improves overall health and well-being, and can enhance successful aging," Delp said.