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
Everybody knows space is dangerous.
Some of the perils are obvious: hard
vacuum, extreme cold, and unpredictable blasts of radiation from
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.
Image courtesy NASA's
Johnson Space Centre.
The loss of
bone mass that many people experience in space weakens the
bone, which can present problems when the person returns to
a weight-bearing environment, such as the Earth or Mars.
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.
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.
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 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
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
"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.