Let's Talk About: Weightlessness decreases bone mass

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Humans face many hazards while living and working in the dangerous and unforgiving environment of space. Some of the hazards are obvious: hard vacuum, extreme cold, and radiation from the sun. Other hazards are less conspicuous, such as the effects of prolonged weightlessness on human bones.

Weakening of the bones due to the progressive loss of bone mass is a serious side effect of extended spaceflight. Scientists think reduced stress on bones in the weightless environment of space may be responsible for the progressive bone loss seen in long-term space travelers. Astronauts who spend many months aboard the International Space Station can lose on average 1 to 2 percent of bone mass each month.

We often think of bones as rigid, unchanging calcium pillars. But bones are actually dynamic living tissues that constantly reshape themselves in response to the stresses placed on them. Two types of bone cell that are constantly building new bone or destroying old bone perform this reshaping. The actions of these two cell types called osteoblasts and osteoclasts usually balance each other out. But when stresses on bones are reduced, removal outpaces replacement, leading to too little bone, which can more easily break.

In prolonged weightlessness, bone mass appears to decrease because the lack of stress on the bones slows the formation of osteoblast cells. Fewer bone-building cells, along with a constant level of bone-destroying activity, translates into a net loss of bone mass.

Astronauts aren't the only ones who experience bone loss. At least 10 million people suffer from it in the U.S. and more than 200 million worldwide from a disease called osteoporosis. Researchers hope that solving the mysteries of bone loss in space will reveal important information about what causes osteoporosis on Earth.

You can experience what it would be like to live and work in zero gravity at Carnegie Science Center's new "SpacePlace" exhibit.

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