Zero Gravity and Bone Loss During Space Flight

The Effects of Weightlessness on Calcium Levels in Astronauts

Nov 12, 2009

As an astronaut's body adjusts to the weightless environment of space where strong bones aren't necessary it requires less calcium for the skeletal system.

Since humans began exploring space the weightless environment has been found to induce physiologic changes with bone mass and calcium levels in the body significantly decreasing. Bone loss induced by microgravity has come to be a critical issue effecting astronaut health and safety with implications for future long-duration space flight.

Calcium and Vitamin D Absorption

On Earth the human body typically absorbs from 40 to 50 percent of the daily recommended 1000 mg of calcium. During space flight that amount drops to only 20 to 25 percent. While in space astronauts lose calcium from all their bones but primarily from the weight-bearing bones like the hips, spine, ankles and upper femur. This occurs at a rate of about 1 percent per month for the duration of the space flight with some sites, such as the heel, losing calcium faster than others.

It was noted on the Skylab missions of the 1970s and by the crews of the Russian space station Mir that calcium loss begins about ten days into the flight. The loss was slow to start, about 140 mg per day, but increased to approximately 250 to 300 mg per day by 84 days into the flight.

While some of this calcium being shed by the bones is eliminated through the urinary tract some of it enters into the bloodstream where levels become high enough to potentially cause kidney stones.

Vitamin D levels, which promote the proper use of calcium, also decrease in zero gravity. Because ultraviolet radiation from the sun is a major source of vitamin D it is suspected that the heavy shielding used on spacecraft to protect astronauts from harmful radiation may cause the significantly decreased vitamin D levels in the body.

Post Space-Flight Findings

A further problem for those traveling in space is the amount of time it takes for calcium and bone levels to return to normal after returning to Earth. One group of Mir space station crew members, after a 115-day flight, took three months for their bodies' calcium and bone levels to return to normal.

Scientists have calculated that if the rate of bone loss experienced during flight is constant and the rate of recovery on Earth is constant then it would take 2.5 times longer than the duration of the flight to recover lost bone.

Because so few people have flown into space it is not known if recovery from bone loss is complete or if some bone loss is irreversible. Multiple missions to space might very well increase this risk. Irreversible bone loss would put astronauts at risk for bone injuries for the remainder of their lives.

Zero-gravity Research Continues

NASA and other space agencies continue studies aimed at understanding how the human body reacts to extended periods in micro-gravity. With plans being considered for long-term missions back to the Moon or to explore Mars it becomes critical to find ways for astronauts to maintain their health in the alien environment of space.

Sources

Rice, Barbara L., Lane, Helen W., Dietary studies in the joint US-Russian space program

http://www.jsc.nasa.gov/sa/sd/facility/labs/nutritionalbl/currentproj/cal_kin.htm

The copyright of the article Zero Gravity and Bone Loss During Space Flight in Space Exploration is owned by Lawrence Koppy. Permission to republish Zero Gravity and Bone Loss During Space Flight in print or online must be granted by the author in writing.