Human Space Adaptation Helps us Understand Aging
In 1963, the U.S. population included 17 million people who were 65 years old
or older-today there are twice as many. Meanwhile, the number of Americans 85
years or older is projected to grow from 3.3 million today to 18.9 million by
2050. Gerontologists-scientists who study the aging process- say that more research
into diseases that afflict older people could help to reduce the number of individuals
who require expensive, full-time medical care in their later years.
Studies of age-related health problems have shown that the process of physiological
adaptation to the low gravity of space induces symptoms also seen in aging (some
effects of aging appear to be due to inactivity rather than the aging process
itself). Hence, gerontologists and space life scientists are collaborating to
determine how people adapt to aging and to the virtual absence of gravity in space
and to develop countermeasures where possible. Space biomedical research could
improve understanding of the basic mechanisms of aging, and aging research could
contribute to a better understanding of physiological deconditioning in space.
Astronauts: Simulating the Aging Process
Life on Earth evolved in the presence of gravity. For this reason, gravity
plays a role in all life processes, and exposure to the microgravity environment
of space affects living things significantly. Certain physiological changes that
occur in space also occur with aging: for example, cardiovascular deconditioning,
balance disorders, weakening bones and muscles, disturbed sleep, and depressed
immune response. An important difference, however, is that these changes are reversible
in astronauts.
Research has shown that insufficient exercise- due to aging, paralysis, weakness,
injury, or prolonged bedrest, for example- can cause a downward spiral in an individual's
health over time, increasing susceptibility to bone fractures and slowing recovery
from injuries and other ailments. What researchers learn about the physiological
effects of the inactivity that accompanies space flight may yield ways of limiting
the deconditioning symptoms of the inactivity that comes with aging.
Are these changes inevitable? Do they result from the same processes? Can people
take steps to lessen, prevent, or reverse them? With the understanding that similar
results may be due to different mechanisms and processes, biomedical researchers
are attempting to gain insights into the aging process by studying physiological
adaptation to space, and vice versa.
A primary goal of NASA's Life Sciences Program is to understand the mechanisms
underlying these physiological changes and to find ways of preventing them in
astronauts. The National Institute on Aging's high-priority research interests
reflect a similar focus, encompassing nervous system function, frailty, osteoporosis
and the effects of physical exercise on bone and muscle in the elderly.
Balance Disorders
Space crew members experience neurosensory disturbances such as dizziness and
inability to maintain their balance upon returning from space flights. Humans
sense gravity on Earth directly through receptors in the inner ear and indirectly
by touch and stretch. In space, these sensing mechanisms do not receive their
usual cues. Studies of the neurosensory system conducted in space offer a unique
opportunity to understand how gravity, and the absence of it, affects the central
nervous system and neurosensory-dependent functions such as hand-eye-head coordination,
posture, balance and gait.
Much space life sciences research focuses on better understanding the mechanisms
involved in the brain's interpretation of the body's orientation in three-dimensional
space. With sufficient information in hand, researchers can develop procedures
to protect space crew members from such disturbances, especially when crews return
to Earth after long space voyages. The results of this research apply to patients
with gait and postural disorders of neurological origin, including elderly people
for whom falls may have especially serious consequences.
Sleep Disturbances
The change in sleep pattern that typically comes with aging is early waking
and fragmented sleep. In space, sleep is also fragmented or otherwise disturbed.
Optimal alertness during the day and sound sleep at night, valuable qualities
on Earth and in space, require proper synchronizing of the human circadian pacemaker
(the "body clock"). Thus, researchers seek to better understand how
aging and space flight affect the mechanisms governing circadian rhythms.
While researchers surmise that aging changes the properties of the body clock,
they are not precisely sure how changes occur. Research has shown that bright
light can reset the body clock. This treatment, originally developed for aging
people, more recently has proven useful to astronauts preparing for space flight.
Bone Deterioration
Loss of bone mass is a problem common to aging and space travel. Although the
results may be the same, the causes may be different. Space life scientists and
researchers studying aging are interested in how exercise affects bones, whether
hormones or drugs can prevent bone loss or promote bone formation, and what mechanisms
translate mechanical loading (physical stress or force) on bones into biochemical
signals that stimulate bone formation and resorption. Normally, the breakdown
of old bone mass (resorption) and the formation of new bone mass occur constantly
in a balanced cycle called remodeling. Mechanical forces (that is, gravity-driven
stresses) appear to coordinate these fundamental bone-shaping processes. Determining
how the body translates these forces into the signals that control bone structure
may reveal whether and how exercise or drugs can prevent osteoporosis in the elderly
and in astronauts.
Cardiovascular Deconditioning and Orthostatic Intolerance
Exposure to microgravity degrades the general condition of the cardiovascular
system and specifically degrades orthostatic tolerance (the ability of the cardiovascular
system to supply the brain with enough blood to maintain consciousness while an
individual stands upright). Since orthostatic tolerance may decline with aging,
whatever space researchers learn about this particular adaptation should help
to solve the problem on Earth, as well as in space, even though the mechanisms
of adaptation may be different.
Immune Response
Both aging and space flight depress the human immune response (though the change
in space is temporary while the change due to aging is not). Reduced proliferation
of infection-fighting cells in the immune system may underlie changes in both
conditions. It is not clear, however, whether aging or other factors that typically
accompany aging, such as declining activity, cause this immune system depression.
Models of age-related changes in immune function are difficult to find, so
microgravity may be a very useful model system to use to increase our understanding
of changes due to aging.
For the Future
Although humans have been traveling into space for three decades,
and in increasing numbers of late, researchers have had limited opportunities
thus far to carry out systematic biomedical research in space.
The dedicated space biomedical research missions of Skylab
in the early 1970s and recent Spacelab Life Sciences missions stand out as exceptions.
An example is Neurolab, the joint mission with the National
Institutes of Health, carried out in April of 1998. Future life sciences research
missions, including research on the International Space Station, and continued
collaboration with the National Institutes of Health will give researchers greater
opportunities to solve the mysteries of space deconditioning and aging.
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