If 65 years of manned spaceflight has taught us anything, it's that space is a very hostile place for human beings. Unfortunately, a study from Kansas State University led by assistant professor of exercise physiology Carl Ade shows that it's even more hostile than previously thought, with data from the International Space Station (ISS) indicating that weightlessness significantly reduces physical fitness due to a decrease in the circulation of oxygen in the body.

In the early days of the Space Age, the lack of gravity in orbit was thought to be more of nuisance than a threat, with cases of space sickness, running fans to keep astronauts from re-inhaling their own exhalations and overcoming the problem of how to deal with the expelling of human waste all things that aren't a big concern on Earth.

But the effects of prolonged periods of weightlessness are even harder to overcome. Decades of studies of astronauts in space show that the human body relies heavily on gravity to function properly. Without gravity, fluids don't distribute properly, causing astronauts' faces to become puffy and the senses of smell and taste become blunted by a permanent case of swollen sinuses. Worse, vision becomes blurry, exercise is necessary or muscles and bones lose mass, the heart becomes weaker, and even cellular processes are disrupted.

In collaboration with NASA's Johnson Space Center in Houston, Texas, the Kansas State kinesiology study looked at physiological data from nine male and female astronauts who spent six months on the ISS. The researchers compared measurements from stationary bike exercise tests before and during their flights as well as a few days after returning to Earth. Some measurements, like oxygen uptake, cardiac output, hemoglobin concentration, and arterial saturation, were indicators of how oxygen was moving from the heart to the mitochondria of the muscles.

What they found was that the astronauts showed a decrease in exercise capacity of 30 to 50 percent based on the maximum rate of oxygen consumed during exercise, which the scientists attribute to the heart and blood capillaries being unable to move enough oxygen to the muscles.

"It is a dramatic decrease," says Ade. "When your cardiovascular function decreases, your aerobic exercise capacity goes down. You can't perform physically challenging activities anymore. While earlier studies suggest that this happens because of changes in heart function, our data suggests that there are some things happening at the level of the heart, but also at the level of the microcirculation within capillaries."

The researchers believe that lack of gravity is the key factor in this decrease, but the exact mechanism will require further research to pin down.

"This decrease is related to not only health, but performance," says Ade. "If we can understand why maximal oxygen uptake is going down, that allows us to come up with targeted interventions, whether that be exercise or pharmacological interventions. This important new information can help these astronauts and prevent any adverse performance changes in their job."

According to the team, finding answers isn't just important for astronauts' general health, but also for combating deteriorating job performance. Though the life of an astronaut may seem like one of floating about pressing buttons and staring at screens, it also involves a number of physical tasks – some of them very demanding, like opening hatches or dealing with emergencies in flight, or after landing on Earth or, one day, other planets.

One bright spot in the study is that in addition to highlighting the perils of spaceflight, it also provides an insight into similar problems associated with heart disease and aging on Earth.

The research was published in Journal of Applied Physiology (PDF).