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Most people know that eating a balanced diet is required for good health. Astronauts need to stay healthy, too-especially while in space. Yet, some of the body's reactions to microgravity make it hard to get the most out of meals.

"In general, the body works well in space," says Dr. Scott M. Smith. Dr. Smith is a nutritionist and manager at NASA. "Digestion in microgravity is similar to how it is on Earth. There are a few parts in the gut, however, that require special attention."

It takes longer for food to run the course of digestion. Without gravity to push things along, there's a bit of a delay. Beyond that, Smith says that many key nutritional aspects need special attention.

Calcium isn't used as well in space. In space,humans only use about half of the calcium in their diets that they use on Earth. The normal answer to a shortage of nutrients is to get more. This can be done either in the foods you eat, or with a vitamin/mineral supplement. But if the problem is that the body isn't absorbing what you take in, Smith says, taking in more won't help. The challenge is to help the body to use the calcium it has more efficiently. "In space, the body is adapting to the changes. With near-weightlessness, the body feels it doesn't need as much bone mass to support the body, so it starts breaking down the bones"

If you were staying permanently in space, that wouldn't be a problem. You truly wouldn't need that same bone mass as you do on Earth. But since humans always come back home, the problem arises when they need to stand and walk when they're back on the ground. With decreased bone mass, their bodies are too weak to hold them up.

This is a big concern for the crew on longer spaceflights. Bone loss takes much longer to recover from than other symptoms of spaceflight. Instead of a couple weeks to get back to normal, bone can take up to 2 years. The calcium that astronauts lose while in space makes them have symptoms like those of osteoporosis. Research is showing, Smith says, that the best way to prevent osteoporosis is to have strong bones in the first place. Most bone structure is formed before the age of 20. "Age 12 is when you prevent osteoporosis, not age 60," Smith says.

Salt is a problem area in astronauts' diets. Most of the salt in their diets is there because salt is used to help make foods last longer. There are no food refrigerators on the Shuttle or the Space Station. An additional concern of high sodium intake from salt is that it will speed bone loss. "All the more reason to try to develop foods for spaceflight that contain less salt and do not need the saltshaker," Smith says.

On Earth, people are often concerned with getting too little iron. A shortage of iron can lead to anemia. In space, the opposite is true. The body pumps blood and gets oxygen to tissues easier. The body doesn't have to work as hard. Because the body does not have to work as hard, the amount of blood drops. The iron in astronauts' diets that would have gone to produce blood is now stored in the body. Those increased iron stores can cause tissue damage. "Rather than promoting iron-rich diets, we're trying to help astronauts limit the iron in their food," Smith says. "A typical diet provides twice the iron levels they need. Iron also reacts to radiation in the atmosphere."

Radiation exposure during spaceflight is one of our biggest environmental concerns. Astronauts need a diet that will help that, not make it worse. Most of the focus is on making shields to protect them from too much radiation. Now, studies show that the food we eat may also fight against radiation. "We're still in the early stages of exploring this," says Smith. It looks hopeful that fruit and veggies will help keep astronauts safer from radiation.

Most cases of space-related metabolism are good. They show that the body is adapting to its new environment. The body is adapting as if it will be staying permanently in space. It's doing an excellent job. The problem is that the astronauts come back to Earth. That changes all those adaptations the body just made.

What do you think?

How does microgravity affect an astronaut's body?
What can be done to prevent a loss of bone mass for astronauts'?
How is an astronaut's diet different from yours?