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 (these statements do not apply to all people):
Heart: It grows slightly larger 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. However, cardiac output stays nearly the same as the heart pumps more efficiently.
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 cells (neurons) and others become damaged. However, it adapts by increasing the number of connections between cells -- synapses -- and by regrowing the branch-like extensions, dendrites and axons, that carry messages in the brain.
Kidneys: They gradually become less efficient at extracting wastes from the blood. Bladder capacity declines. Urinary incontinence, which may occur after tissues atrophy, can often be managed through exercise and behavioral techniques.
Body Fat: The body does not lose fat with age but redistributes it from just under the skin to deeper parts of the body. Women are more likely to store it in the lower body -- hips and thighs -- men in the abdominal area.
Muscles: Without exercise, estimated muscle mass declines 22 percent for women and 23 percent for men between the ages of 30 and 70. Exercise can prevent this loss.
Sight: Difficulty focusing close up may begin in the 40s; the ability to distinguish fine details may begin to decline in the 70s. From 50 on, there is increased susceptibility to glare, greater difficulty in seeing at low levels of illumination, and more difficulty in detecting moving targets.
Hearing: It becomes more difficult to hear higher frequencies with age. Hearing declines more quickly in men than in women.
Personality: After about age 30, personality is stable. Sudden changes in personality sometimes suggest disease processes.
Undernutrition without malnutrition extends life spans in laboratory animals.
The findings in this UCLA laboratory, headed by Roy Walford, are not isolated ones. In studies in other laboratories, again and again, undernutrition has increased the life spans of nearly every animal species studied -- protozoa, fruit flies, mice, rats, and other laboratory animals. Now researchers are investigating whether and how caloric restriction will affect aging in primates, human's closest relatives in the animal kingdom.
Particularly intriguing to many gerontologists are findings that animals on restricted diets have reduced rates of disease. In one of the largest studies to date, Roderick Bronson at Tufts University found that caloric restriction not only extended life span in mice, but also prevented or slowed down development of every disease and all types of tumors. These results, described as stunning by gerontologists, have raised hope that further study of caloric restriction will help uncover the mechanisms responsible for disease in old age.
However, whether or not caloric restriction would have the same effect in humans remains a major question. Studies with monkeys are underway at the National Institute on Aging, where rhesus and squirrel monkeys are growing up on a calorically restricted diet. At the University of Wisconsin, preliminary results in Richard Weindruch's laboratory show some promising early signs of improved health in aged monkeys kept on restricted diets: Caloric Restriction in Primates