A Moment of Science

Bed Rest, Space Travel, and Bones

Astronauts and people on bed rest can experience gradual bone loss that increases their risk of fracturing a bone. Here's why.

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Photo: starpause kid (flickr)

That's one reason why getting out of bed to walk as soon as possible after surgery or injury is so important.

What do astronauts and people confined to bed have in common? Both space travel and bed rest take weight off the bones of the legs and spine, which usually carry the body’s weight when we stand upright. As a result, astronauts and people on bed rest can experience gradual bone loss that increases their risk of fracturing a bone. Here’s why.

The calcium matrix in our bones is constantly broken down and built back up again by two different types of bone cells. In an active person who does weight-bearing exercise like walking and climbing stairs, the rate of breakdown equals the rate of buildup, so the calcium content of the bones stays fairly constant.

However, once you take the load off bones accustomed to carrying the body’s weight, the bone cells act differently. The rate of bone formation no longer keeps up with the rate of breakdown, so calcium from the broken down bone enters the bloodstream and eventually is lost in the urine.

If it’s possible to do any standing or weight-bearing exercise during bed rest or space travel, that helps minimize bone loss. That’s one reason why getting out of bed to walk as soon as possible after surgery or injury is so important. If someone’s immobilized for less than 5 to 10 weeks, they’ll likely recover their lost bone mass after they start walking and using their bones again. No one knows yet if returned astronauts recover all their bone mass, but studying astronauts helps find new ways to decrease bone loss in people confined to bed right here on earth!

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