Parallel Processes?
The Study of Human Adaptation to Space 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.

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.

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.

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.

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.

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.

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.

Life Sciences Division Outreach Program Mail Stop 19-15 NASA Ames Research Center Moffett Field, CA 94035-1000 Fax: 650-604-2112