If this whole space-faring civilization business is to work out, we need to know that humans can even handle space-life, physiologically speaking. That's not a guarantee. Life on Earth has spent a cool 4.5 billion years being accelerated downward at 9.8 meters per-second squared and that's a hell of a thing. Take a minute in front of a mirror and it becomes clear enough that, well, we look like the product of eons of gravitational influence. We look weird.
Both space- and Earth-based experiments have roundly concluded that spaceflight is not the most healthy thing. Over long periods, bones and muscles begin to waste away. The cells that line our blood cells become inflamed; our eyes start to deform; our immune systems sink. And that's just what we know from six-month ISS tours. It could get worse.
Space travelers may have another concern, according to research published Wednesday in the journal PLOS ONE. Mice that had spent two weeks orbiting aboard the space shuttle Atlantis in 2011 (the final flight of the US space shuttle program) returned to Earth with early signs of liver damage in the forms of both nonalcoholic fatty liver disease and the beginning stages of fibrosis, an accumulation of cellular waste products that can lead to cirrhosis and-or total liver failure.
While liver damage may not be an immediate danger for ISS crews, it could wind up mattering a great deal for astronauts bound for Mars
"The spaceflight environment impacts many physiological systems, resulting in potentially serious consequences, particularly for longer duration space exploration," the paper explains. "As use of the International Space Station (ISS) is increased, and with the rise of commercial spaceflight and tourism, the systemic effects of microgravity must be carefully investigated to protect human health. Although most research has focused on bone, muscle, brain and cardiovascular function, several studies have shown that exposure to the space environment alters both energy and lipid metabolism in humans and rodents."
This is not an out-of-nowhere result. Astronauts have been found to show diabetes-like symptoms that increase in severity depending on how much time was spent in space. The implication is that the microgravity environment is having some sort of effect on the body's metabolic processes. And the liver is a key organ in regulating metabolism, acting as a sort of battery by way of storing and producing glucose (from the energy-storage molecule glycogen) when demanded by the presence of insulin in the blood.
So, while the space-mice overall lost weight, their livers began to accumulate lipids (fat molecules, generally). Compared with a control group of mice that did not fly on the mission, the space-flown mice lost weight in the form of lean muscle mass. As a result, skinnier mice returned to Earth but with a higher overall percentage of body fat composition.
Meanwhile, Karen Jonscher, the study's lead author, found that her frozen mouse livers had an excess of accumulated lipid droplets—fat. This was accompanied by a loss of retinol, an animal form of vitamin A, and alterations to genes responsible for the breaking down of fats by the liver. Still more ominous was the presence of fibrosis; it should take months or years for mice to develop fibrosis, even given a terrible diet. Here it took two weeks.
The results seem to be an answer to a question posed by Jonscher at the outset of her work. "My research focuses on overnutrition," she told Motherboard. "I was curious about the effect of muscle atrophy on the liver. When muscles atrophy, which happens in flight, the proteins break down and their amino acids go through the bloodstream and into the liver. I wondered if that might 'look' like overnutrition to the liver and, if so what might the response be?"
Mice are used as models in microgravity experiments because they do a good job of representing human bodies, but in fast-forward. Two weeks for a mouse might translate to many months or years for humans. So, while liver damage may not be an immediate danger for ISS crews, it could wind up mattering a great deal for astronauts bound for Mars. That trip could take over a year."Whether or not this is a problem is an open question," Jonscher offered in a separate statement. "We need to look at mice involved in longer duration space flight to see if there are compensatory mechanisms that come into play that might protect them from serious damage."