23 Feb 2015

Space travel is bad for your health

If we want humans to thrive on Mars, we need to take better care of their immune system while they are travelling there. Viviane Richter reports.

NASA flight engineer Steve Swanson works out on an exercise bike on the International Space Station. Even frequent exercise can't fully protect astronauts from bone-density loss in microgravity.Credit: NASA

With the planned launch of Mars One – a seven-month one-way flight to the Red Planet – only a decade away, the race is on to find ways to keep the astronauts strong while they travel. We already know human muscles and bones waste away in zero gravity – even when astronauts spend up to a quarter of their time exercising. Now research by Michele Goodhardt and her team at the French Institute of Health and Medical Research suggests these brittle bones can weaken the immune system. Their study has been published in the biology journal FASEB.

“This work will make researchers look more closely at how to counteract microgravity,” says Thais Russomano, space medicine expert at King’s College in London.

Our bodies work best when feeling the familiar tug of Earth’s gravity. Without it an astronaut’s femur will lose around 10% of its mass over a six-month stay in space – even with all that exercise.

The effect of spaceflight on the human immune system became apparent during the Apollo missions. Fifteen of the 29 Apollo astronauts caught bacterial or viral infections during their space mission or within a few days of returning to Earth. Previous studies have shown the number of immune cells called T cells, which kill invading cells, can drop during spaceflight conditions. But what about other immune cells - such as B cells, which produce the antibodies that are also a critical part of our immune defence?

In short, bones age faster in microgravity – and the immune system apparently ages with them.

In adults, B cell factories sit in pockets of bone marrow in long bones such as femurs, and flat bones such as the pelvis. So to see what happens to B cell levels in spaceflight, Goodhardt and her team used slings to lift the back legs of three-month-old mice off the ground. (The technique has long been used to mimic the effects of low gravity in the lab.) After only 21 days, the B cell counts of the mice were way down. “We were struck to see B cell levels similar to what we were seeing in old mice,” Goodhardt says. When the researchers looked at the B cell-producing pockets in the bones of the propped-up mice they found them to be less dense, similar to those of elderly 18 to 22-month-old mice.

In short, bones age faster in microgravity – and the immune system apparently ages with them. “The next step will be to see if there is a similar change in B cell functional activity during actual space flight,” says Gerald Sonnenfeld, an immunologist at the University of Rhode Island. Astronauts already take vitamin D and osteoporosis medication to slow their bone loss as much as possible. This might also help keep their B cell population healthy – something else to test.

More broadly, the study shows we still have much to learn about the body’s response to low gravity. These gaps will need to be filled before we can safely send people to Mars. Sonnenfeld predicts the task will still be unfinished before the planned 2024 Mars One launch date. “We have to be prepared,” he says. “We can’t just turn around and take them to the emergency room.”

Also from Cosmos: Will space colonisation cripple our astronauts?

Viviane Richter is a staff writer at COSMOS.