Philip J. Goscienski, M.D.
April 2010
Persons who have suffered a serious leg fracture that required several weeks in a cast are aware of how quickly the muscles beneath that cast waste away. The bone becomes less dense because there is no tension from the muscles to which it is attached. When muscular inactivity continues for months, years or decades the result is osteoporosis, a rising epidemic in developed countries.
Astronauts who spend months or weeks in space have measureable bone loss because of the weightless that they experience in zero-gravity conditions. They now exercise using special equipment but NASA officials recognize that the bone-loss problem will remain a high priority as future missions reach to Mars or beyond.
Professional scuba divers spend only a few hours a week in near-weightless conditions but that is enough to reduce their bone density.
If inactivity results in bone loss, why don't bears develop thinner, weaker bones during their 4 to 6 months of hibernation, long periods of nearly total inactivity during which they can lose up to 40 percent of their body weight? Over the years and repeated cycles of hibernation their bones become even stronger. Moreover, the females, who have the increased bone-threatening burden of being pregnant while hibernating, do not develop a weaker skeleton.
Scientists have barely begun to unravel the mystery of how bears protect their bones. It may be due to the interplay of hormones and other factors that modify the remodeling of bone during inactivity. Finding the answer could certainly change the course of osteoporosis for individuals and it could spare billions in healthcare costs.
Their peculiar body chemistry may protect bears but what about the astronauts and scuba divers who lose bone mass? There is no evidence that divers suffer from more fractures than other groups but astronauts provide a different story. In a group of 94 of these explorers during an 8-year period there were 26 fractures, most of them sports-related. Even allowing for their exuberant participation in athletics, that is a disturbing number. Recent studies confirm their higher risk of fracture, approaching that of older women with osteoporosis.
Astronauts comprise a highly-conditioned, well-fed group with no underlying medical problems yet after relatively brief periods during which they fail to use their muscles normally they face the threat of fractures now as well as in the future. Clearly, that should be a warning to us mere earthlings: use those muscles every day.
Philip J. Goscienski, M.D. is the author of Health Secrets of the Stone Age, Better Life Publishers 2005. Contact him at drphil@stoneagedoc.com.