Aging : Physiologic Clues, Normal Aging

by National Institute on Aging

(Page 11 of 15)

Physiologic Clues

We don't know very much about the few men and women who have lived to 115 years of age or more, but we can assume that they eluded the diseases that kill many people in their 70s and 80s. At 122, Jeanne Calment, for instance, had lived a relatively disease-free life. In fact, escape from infectious disease is the most common reason that all of us can now expect to live longer than our grandparents.

Chronic diseases and disability were once thought inseparable from old age. This view is changing rapidly as one disease after another joins the ranks of those that can be prevented or at least controlled, often through changes in lifestyle.

We now know, for example, that most people can avoid lung disease by not smoking. And heart disease and stroke rates have fallen at the same time that Americans have lowered their fat consumption, begun to exercise more, and quit smoking.

So if chronic disease is not intrinsic to the aging process, as many gerontologists now believe, then what is? Are there universal or "normal" aging processes?

Normal Aging

Unlike most of us, Satchel Paige was never quite sure of his birth year. "My birth certificate was in our (family) Bible, and the goat ate the Bible," he said. But even had he known his chronological age, it may not have shed much light on how old he was physiologically. In fact, gerontologists are discovering that age in years doesn't necessarily correlate with physiological age.

For decades, investigators at the NIA have compiled data on heart function, lung capacity, and numerous other bodily functions in hopes that this information may one day be used to establish definitive measures of physiological aging. In theory, these biomarkers would be more precise indicators of aging than chronological age itself. Once established, these biomarkers could make it easier to study normal aging, diseases, and possible interventions. So far, however, no such biomarkers have been identified in humans.

In fact, normal physiological aging is quite variable, according to investigators involved in the Baltimore Longitudinal Study of Aging, a long-term NIA study begun in 1958 that has tracked the lives of more than 1,000 people from age 20 to 90 and beyond. Not only do individuals age overall at vastly different rates, it is quite likely that agerelated changes in various cells, tissues, and organs differ as well. For instance, kidney function may decline more rapidly in some individuals. In others, bone strength may diminish faster. The organs that age fastest in one person may not age as rapidly in another. This suggests that genes, lifestyle, and disease can all affect the rate of aging and that several distinct processes are involved.

Although this diversity lessens the likelihood of finding biomarkers of aging in humans, the quest for these indicators has yielded many insights into the physiology of two organ systems that may have important roles in the aging process. One of these is the endocrine system.

What is Normal Aging?

Individuals age at extremely different rates. In fact even within one person, organs and organ systems show different rates of decline. However, some generalities can be made, based on data from the Baltimore Longitudinal Study of Aging.

Heart: Heart muscle thickens with age. Maximal oxygen consumption during exercise declines in men by about 10 percent with each decade of adult life and in women by about 7.5 percent. This decline occurs because the heart's maximum pumping rate and the body's ability to extract oxygen from blood both diminish with age.

Arteries: Arteries tend to stiffen with age. The older heart, in turn, needs to supply more force to propel the blood forward through the less elastic arteries.

Lungs: Maximum breathing (vital) capacity may decline by about 40 percent between the ages of 20 and 70.

Brain: With age, the brain loses some of the structures (axons) that connect nerve cells (neurons) to each other, although the actual number of neurons seems to be less affected. The ability of individual neurons to function may diminish with age. Recent studies indicate that the adult nervous system is capable of producing new neurons, but the exact conditions that are critical for this have yet to be determined.

Kidneys: Kidneys gradually become less efficient at extracting wastes from the blood.

Bladder: Bladder capacity declines. Urinary incontinence, which may occur after tissues atrophy, particularly in women, can often be managed through exercise and behavioral techniques.

Bdoy Fat: Typically, body fat gradually increases in adulthood until individuals reach middle age. Then it usually stabilizes until late life, when body weight tends to decline. As weight falls, older individuals tend to lose both muscle and body fat. With age, fat is redistributed in the body, shifting from just beneath the skin to deeper organs. Women typically have a higher percentage of body fat than men. However, because of differences in how this fat is distributed - on the hips and thighs in women and on the abdomen in men - women may be less susceptible to certain conditions including heart disease.

About the Author

www.nia.nih.gov
NIA, one of the 27 Institutes and Centers of NIH, leads a broad scientific effort to understand the nature of aging and to extend the healthy, active years of life. In 1974, Congress granted authority to form NIA to provide leadership in aging research, training, health information dissemination, and other programs relevant to aging and older people.