Weightlessness sure looks like a lot of fun, but prolonged exposure to zero-G in space can have some negative side effects - like osteoporosis and the weakening of human bones!

by Doug Hullander and Patrick L. Barry

Everybody knows space is dangerous.

Some of the perils are obvious: hard vacuum, extreme cold, and unpredictable blasts of radiation from the Sun.

Other perils are less conspicuous. The effects of prolonged weightlessness on the human body, for example, can be slow and subtle - yet no less dangerous if astronauts fail to take proper precautions.

Weakening of the bones due to the progressive loss of bone mass is a particularly serious effect of extended spaceflight. Studies of cosmonauts and astronauts who spent many months on space station Mir revealed that space travellers can lose 1 to 2 percent of bone mass each month -- a loss doctors don't yet know how to prevent. "The magnitude of this effect has led NASA to consider bone loss an inherent risk of extended space flights," says Dr. Jay Shapiro, team leader for bone studies at the National Space Biomedical Research Institute.

Space travellers aren't the only ones who worry about bone loss. At least 10 million people suffer from bone loss in the U.S. and untold numbers worldwide -- it's called osteoporosis. Postmenopausal women are especially prone to osteoporosis, but most of us contract the disease as we age, including men. Researchers hope that solving the riddle of bone loss in space will reveal important clues about what causes osteoporosis right here on Earth.

Spacefarers typically experience bone loss in the lower halves of their bodies, particularly in the lumbar vertebrae and the leg bones. Diminishing bone mass also triggers a rise in calcium levels in the blood, which increases the risk of kidney stones.

Researchers suspect the root cause of bone loss in space is weightlessness.

In this mutual free-fall, bones no longer have to provide support for locomotion or even for maintaining body posture. As a result, little or no stress (i.e., mechanical strain) is applied to the skeletal system. Scientists think the lack of stress on the bones may be responsible for the progressive bone loss seen in long-term residents of space. (Lack of stress on bones among sedentary Earthlings, such as those confined to beds due to illness or old age, also contributes to osteoporosis.)

People 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. (This is how archaeologists can tell whether skeletal remains belonged to a labourer or an aristocrat, for example. The incessant pull of a labourers muscles causes the bones to reshape themselves slightly where the muscles were attached.)

This reshaping is performed by two types of bone cell that are constantly depositing and extracting calcium phosphate minerals from the structural matrix of the bone. The actions of these two cell types - "osteoblasts," which deposit calcium phosphate, and "osteoclasts," which remove it - usually balance each other out. When the body has a calcium deficiency or during pathological osteoporosis, the removal of the structural calcium phosphate crystals outpaces replacement, leading to a weakening of the bone.

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.

Why weightlessness should inhibit the development of osteoblasts is the subject of a current study at Vanderbilt University. A key chemical in the development of osteoblast cells from precursor cells is an enzyme called "creatine kinase-B." Investigators are trying to figure out which molecules in the body regulate the activity of this enzyme and how those chemicals are affected by low gravity, in the hope that this knowledge will point to a way to boost osteoblast formation in space.

Image courtesy National Space Biomedical Research Institute. The main weight-bearing bones of the body -- indicated with light-purple shading in this drawing -- are also the ones most affected by space-induced bone loss.

Another study at the Medical College of Georgia is investigating a possible connection between eating and bone destruction. Ingestion of food causes levels of a certain hormone - called "glucose-dependent insulinotropic peptide" -- to increase in the bloodstream. The main function of this hormone is to stimulate the production of insulin after a meal, which in turn triggers cells to absorb energy-providing glucose from the blood.

Bone cells are sensitive to this hormone, too. Researchers have found that when this hormone attaches to "receptor" molecules on bone cells, osteoclast (bone destroying) activity goes down and osteoblast (bone creating) activity goes up.

Could hormones like this one be given to space travellers as a supplement to prevent bone degradation? Scientists don't yet know.

Genetic make-up might also play a role, as suggested by the variation of bone loss observed between individual astronauts and cosmonauts.

"The 1 to 2 percent per month loss is an estimate of bone loss - an average value," Shapiro says. "Certain individuals on six month flights have lost as much as 20 percent of bone mass in their lower extremities, while a few have lost none during the same period in space."

"Bone loss of this magnitude leads to a significant increase in fracture rate, which may be as much as five-fold that expected with normal bone mass on Earth," he added. "A limb fracture involving, say, one of a six-person space crew could seriously compromise a mission's objectives."

Indeed, adds Shapiro, "the problem of bone loss must be overcome before people are placed in the position of performing physically hazardous tasks [after a long voyage in zero-G]." Future astronauts who visit Mars, for instance, will need strong healthy bones when they step out of their spaceship and onto the Red Planet.

Humans won't be striding across Mars for some time, but bone loss is hardly a far-off concern. Right here on our own planet millions suffer from osteoporosis - a malady that strikes ordinary people and far-out explorers alike. Solving the problem in space, say researchers, will likely bring welcome relief back home to Earth.