Want to lose weight? Consider spending a few months on the International Space Station. Astronauts who step on the scales after a Space Station mission usually find that they've shed about 5 percent of their original body weight.
But while the weight loss might sound appealing to earthbound observers, it's a real concern for fit astronauts with little excess weight to spare, says Scott M. Smith, a nutrition scientist at NASA's Johnson Space Center in Houston. He and his colleagues have been studying the health and nutrition of astronauts aboard the space station, and they report some of their results in the March Journal of Nutrition.
Smith says that astronauts lose weight mainly because they don't eat enough. On average, the astronauts eat 20 percent fewer calories than their recommended daily intake. Those are calories they can't spare because, although moving around in zero gravity looks effortless, previous studies have shown that working astronauts exert just as much energy in orbit as they do on the ground.
Smith isn't sure why the astronauts tend to eat less in orbit. Although some astronauts initially feel nauseous in zero gravity, the NASA researcher says that usually lasts only 2 or 3 days. Astronauts give all sorts of answers when asked why they eat less in space. "Some crew members will tell you that they were too busy," Smith says, while others say they weren't very hungry.
How does an astronaut check his or her weight—or that of anything—in space? A contraption with a spring-loaded metal arm applies a small force to a body and measures how much it accelerates as a result. A person's mass can then be calculated precisely using Newton's second law of motion, which says basically that the more massive the astronaut, the less he or she will accelerate.
Though it's not worrisome for astronauts to lose a small percentage of their initial weight during their stay on the Space Station, "5 to 10 [percent] is where I start to get concerned," Smith says. "Over 10 percent is where it's critically significant. We have had folks on the station get to that point."
When that drastic a weight loss happens, the doctor assigned to monitor the mission from the ground brings it to the attention of the astronaut in question and encourages the person to eat more high-calorie foods. The study found that astronauts who get this sort of counseling do change their eating habits and are able to meet their daily energy needs and maintain their weight.
Too little, too much
Other nutritional issues noted in the study seem to have nothing to do with the astronauts' eating habits. Instead, they appear to result from the conditions of being in Earth orbit. For example, vitamin D is normally produced in a person's skin by a reaction that requires ultraviolet light. The Space Station, flying as it does above Earth's ozone layer, is shielded to keep out harmful amounts of ultraviolet radiation. Unfortunately, this also prevents normal vitamin D production in an astronaut's skin. Vitamin D is needed for the maintenance and growth of bones.
The astronauts take vitamin D supplements to compensate, but there is the added problem that weightless bodies seem reluctant to use the vitamin D that they have available. Smith thinks the body's shunning of vitamin D is a natural reaction to weightlessness. "The body realizes that it doesn't need the same skeleton to get itself around," Smith says, so it cuts back on bone maintenance. The loss of bone that many astronauts experience during space flight is a problem that has yet to be resolved.
One thing the astronauts have too much of is iron. The Space Station study confirmed earlier results showing that astronauts accumulate unusually high amounts of iron in a form called ferritin. The careful dietary monitoring of the new study rules out the possibility that excess iron intake is to blame. It isn't clear what the real culprit is, but previous studies have shown that in space the body scales back on some activities that require iron, such as the production of red blood cells. The volume of blood in circulation also decreases, another phenomenon that Smith suggests may be a natural adjustment to weightlessness, which seems to make it easier for the circulatory system to route blood and oxygen through the body.
If the environment of space is easier on the circulatory system, it is much more stressful on other parts of the body. The NASA study found signs that the DNA-repair systems in astronaut's bodies were working overtime. These operations kick in whenever chemicals or radiation damage DNA, and the telltale sign of such action is a byproduct called 8HdG.
By the end of astronauts' missions, 8HdG-urine concentrations typically had shot up 30 percent from preflight readings. "The level of change that we see is the same that you'd see in someone who is smoking a pack of cigarettes a day," says Smith.
Radiation from space is responsible for some of the damage, but Smith says breathing pure oxygen, as astronauts occasionally do, may also be a cause. Astronauts breathe pure oxygen during takeoff and landing and when suited up for work outside the station. Breathing abnormally high concentrations of oxygen is known to damage a person's DNA through the action of types of oxygen atoms called free radicals.
The detected DNA damage is even more worrying in light of the decreased concentrations of the nutrient folate in astronauts' blood after flights. Folate helps repair damaged DNA. The good news is that folate levels improved when astronauts took supplements, which was not the case with some other nutrients.
Clearly, says Smith, space flight can be hazardous to one's health. But with NASA's new plans to build a base on the moon and to eventually send astronauts to Mars, scientists are going to have to figure out how to deal with those hazards. A typical stay aboard the Space Station lasts 4 to 6 months, but Smith says a human mission to Mars could entail a 3-year round trip. "That's where [nutrition] is going to be even more critical," he says.
Having surveyed nutrition-related changes that occur in space travelers, the NASA researchers plan further study of the potential problems from those changes. Although Smith is concerned about all of the hazards highlighted in the study, he says that radiation and cancer risks are among the "highest-priority targets" for further studies.
Scott M. Smith
Human Adaptation and Countermeasures Office
NASA Lyndon B. Johnson Space Center
Houston, TX 77058