Unintended weight-loss is a consequence of astronauts’ weightlessness
Weight loss in microgravity is unavoidable, in more ways than one. Most directly, anyone on the International Space Station (ISS) will feel weightless thanks to their orbit around the Earth. They’re never in a position where the Earth’s gravity can noticeably pull them “down,” meaning they’d weigh zero pounds if they tried to stand on a scale. However, once astronauts get back to Earth’s surface, NASA’s medical staff has found that they have lost weight in another sense, having lost as much as 10 percent of their overall body mass. This has raised concerns about how people might spend extended amounts of time in space without putting their muscle, bone and cardiovascular health at risk.
Astronauts aren’t eating enough
As it turns out, weightlessness may be contributing to astronauts’ weight loss. On earlier visits to space, astronauts were asked to fill out weekly surveys about what food they were eating, although it’s suspected that those answers weren’t terribly accurate. Astronauts on the ISS are now prompted to record every snack and meal they eat on touch-screen app, giving medical staff on Earth a much better sense of how much food is consumed in space. The resulting pattern is that astronauts unconsciously eat less in space, probably thanks to being weightless all the time.
Living in space dulls appetites in a few different ways. Your muscles need to work less in microgravity, and are thus consuming fewer calories every day. Over time, this can contribute to muscle atrophy, giving you even less muscle tissue to feed at each meal. It’s also suspected that microgravity affects how well your stomach’s stretch receptors can do their job. As organs tend to be reshaped without the constant tug of Earth’s gravity, astronauts’ stomachs may start signaling that they’re full earlier in meal, even if they haven’t hit their nutritional needs for the day. Finally, most of the food on the ISS is carefully packaged in sealed containers, food doesn’t have a chance to stimulate appetites like it does cooking on the stove at home. This isn’t to say that there are no food smells on the ISS— seafood gumbo was actually banned by mission commanders because of its lingering odor. Then again, anyone in an open office probably knows how uninvited fish smells don’t do much for one’s appetite.
Fish and fitness
It’s unfortunate that seafood smells have been a problem, because seafood may be one of the easier ways for astronauts to help keep their bodies healthy in microgravity. Crew members that eat more fish have been found to retain more bone tissue, which is likely thanks to the omega-3 fatty acids found in seafood. The benefit seems most pronounced in astronauts who also skip other kinds of meat, clearly indicating that astronauts should eat a lot of sushi. The second element towards keeping one’s body fit has turned out to be exercise. Some residents of the ISS have managed to avoid unintended weight-loss, and their six-day-a-week exercise program has probably helped keep their muscles and bones in shape, countering the atrophying effects of microgravity.
Source: Astronauts lose weight in space, and it might be because their food is literally floating around inside them by Mary Beth Griggs, Popular Science