With the exception of those living in caves, I suspect that everyone has heard the current and widely disseminated advice regarding the health benefits of walking or jogging 10,000 steps a day. In addition to improving cardiorespiratory health and acquiring some sunshine-derived vitamin D, the repeated impact of feet on pavement is critical for maintenance of bone health. Although swimming and cycling are great for the heart and lungs, they do not jolt the bones sufficiently for them to maintain their density and strength.

The heart is entirely dependent on a steady supply of calcium for its health, and if there is not enough calcium coming in through intestinal absorption, hormones release calcium from bone to the heart’s benefit but to the skeleton’s detriment. Bones attempt to counter this wanton thievery by constantly remodeling themselves and strategically adding new calcium crystals in areas where the bone cells sense mechanical stress. This is particularly critical in the hips and spine, since fractures here can be permanently life-changing if not life-ending. For women, menopause brings added peril, because the absence of critical hormones diminishes bone density, i.e., the shortage causes bones to become porous, i.e., osteoporotic.

Regular impact exercise counters this tendency and underlies the recommendation for achieving 10,000 steps a day. It is hard enough to do this on Earth but is nearly impossible in the near-zero gravity of space. Weight lifting is futile, because barbells float. Magnetic shoes might work but would risk interfering with the complex electronics on board. Futurists have also considered creating artificial gravity by spinning part or all of the spacecraft, making it a large centrifuge. Theoretically this would work, but the cabin would either have to be 100 feet in diameter or spin at a rapid, energy-​consuming speed, neither of which seems practical.

Presently, International Space Station inhabitants fasten a harness around their shoulders and waist and hook it with bungee cords to a treadmill to keep from floating away while they jog. Nonetheless, astronauts loose bone mass at ten times the rate seen in postmenopausal women, and a six-month stay on the ISS causes a 10 percent loss of bone. This rate would be unsustainable on a three-to-four-year roundtrip journey to Mars.

The Red Planet, courtesy European Space agency

Maybe once they get to Mars, could travelers bulk up their bones in preparation for their zero-gravity trip back to Earth? Gravity on Mars is one-third that of Earth’s. Nobody knows whether that is enough to maintain or restore bone health. Then if travelers wanted to take their families along or have families en route, they would have to deal with the unknown effects of low gravity on growing bones. Nonetheless, kids would undoubtedly have a blast doing cartwheels and flips during the entire adventure.

courtesy National Park Service, Wikimedia

Solutions might come from a surprising source—hibernating bears. It’s Spring, and bears are beginning to rouse themselves from four to five months of complete indolence, yet they come through without a trace of diminished bone density. Brave investigators have darted grizzlies and similar species and sampled their bone and blood immediately before, during, and after hibernation. (Who said that studying bone hormones had to be dull.) The researchers have found that a bear’s bone metabolism essentially shuts down along with its other bodily functions during hibernation, which proves to be a highly complex, multisystem process affecting the brain, heart, kidneys, muscles, and bone. Investigators understand the process incompletely, so time will tell if bears can offer humans some help with bone health. In the meantime, it’s Spring. Anyone for a walk?


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