Aging and Life Span

Aging is, simply put, the act of getting older. Aging is part of the natural life cycle of an organism. From birth, through maturation, and eventually to death, aging is the element that ties all segments of life together.

Life Span and the Aging Process

How long an organism lives is called its life span. In 1998, the average life span for a human, worldwide, was sixty-six years. However, life span is a complex trait, meaning that many factors, including family history, lifestyle, disease, and residence in a developed nation, determine how long an individual's life will be. The average life span in a particular population changes as these factors change. For example, the average life span in the United States in 1900 was forty-nine; in 1998 it was seventy-seven.

This increase was likely due to several factors, but perhaps the most important was the improvement of sanitation, hygiene, and public health from 1900 to 1998. These improvements included purification of drinking water, treatment of wastewater, widespread vaccination, and improved access to health care. However, even as these sanitary measures were adopted, other elements of modern life emerged as strong influences on life span, such as diet, exercise, and socioeconomic status. Studies have shown that individuals who exercise regularly, eat a diet lower in saturated fats, and avoid unnecessary risk-taking live longer. This may be because such a lifestyle reduces the risk of developing cardiovascular disease and cancer, the top causes of death in developed countries.

Finally, life span is in part genetically determined. Studies of life span in large families have shown that longevity is, to some degree, inherited. This may be due to shared genetic risks of diseases or behaviors that shorten the life span, or it may reflect direct genetic influences in longevity separate from risk of disease.

The aging process causes many changes, both visible and invisible. In humans, these changes take several forms. In the first two decades of life, from birth to adulthood, aging involves physical growth and maturation and intellectual development. These changes are fairly noticeable and relatively swift compared to the rest of the life span. After reaching physical maturity, humans begin to show subtle signs of physical aging that grow more pronounced over time. Long-term exposure to sunlight and the outdoors may begin to toughen the skin and produce wrinkles on the face and body. The senses change: Sight, hearing, taste, and smell become less acute. Gradual changes in the eye cause many older adults to need glasses to read. Hair begins to thin and turn gray. Individuals with less active lifestyles often begin to gain weight, particularly around the waist and hips. Beginning in their 40s (or, rarely, in their late 30s), many women experience menopause, which marks the end of childbearing years. Less visible or noticeable changes associated with aging are the loss of bone density over time (particularly in women), slower reflexes, less acute mental agility, and declining memory.

Diseases Associated with Aging

Many of the diseases common in older adults, such as cardiovascular disease, cancer, dementia , arthritis, blindness, and deafness, are consequences of the acceleration or distortion of these "natural" changes associated with aging. These complex diseases associated with aging are caused by the interaction of genetic and environmental factors. For example, Alzheimer's disease is an illness caused by changes in the brain that impair thinking and memory much more severely than the natural decline that all humans experience during aging. It is the most common form of dementia in adults over age sixty. Certain very rare alleles are associated with the development of Alzheimer's disease, and other much more common alleles (of different genes) increase the risk of a variety of other diseases. Environmental factors such as exposure to toxins have also been implicated. A more rare example is progeria, a disease in which the tissues of the body age about seven times more rapidly than normal. In this case, a person who is chronologically only a teenager looks much older.

Genetics and Aging

Many scientists have hypothesized that some genes may control aspects of aging separate from the development of disease. These hypotheses are based on experimental studies of non-human organisms and the observation that longevity in humans appears to run in families. Studies of yeast and roundworm have identified over ten genes in each that are associated with longevity and aging, and more recent studies have suggested similar genes exist in the fruit fly. The exact function of these genes is unknown, but one or more may help slow down the metabolic rate. Studies in mice have shown that reducing metabolism by reducing food intake can increase life span. Finally, shortening of the telomeres decreases longevity in some model organisms.

Finding similar genes in humans is more complicated, since scientists cannot experimentally control genes to test their effects on longevity in humans. Therefore, genetic studies of human longevity require a more observational approach. One study design is to examine large numbers of long-lived individuals such as centenarians and see what factors they have in common, such as lifestyle, medical history, and genetics.

Studies of centenarians have suggested that variants in multiple genes, including the human leukocyte antigen (HLA) genes of the immune system, apolipoprotein E (APOE), angiotensin-converting enzyme (ACE), plasminogen activating inhibitor 1 (PAI-1), and p53, are associated with living past age ninety. Forms of several of these genes, such as APOE, ACE, and p53, are associated with increased risk of developing Alzheimer's disease, cardiovascular disease, and cancer, respectively. The association of these genes with longevity may be due to these disease associations, or it may be due to their direct influence on extending the human life span. Regardless, genes clearly influence aging and longevity, whether it is by influencing the development of life span-shortening diseases, or by positively influencing longevity independently of causing disease.

see also Accelerated Aging: Progeria; Alzheimer's Disease; Cancer; Cardiovascular Disease; Complex Traits; Telomere.

William K. Scott

Bibliography

Anderson, Robert N. "United States Life Tables, 1998." National Vital Statistics Reports 48, no. 18. Hyattsville, MD: National Center for Health Statistics, 2001.

Finch, Caleb E., and Rudolph E Tanzi. "Genetics of Aging." Science 278 (1997): 407-411.

Schächter, François. "Causes, Effects, and Constraints in the Genetics of Human Longevity." American Journal of Human Genetics 62 (1998): 1008-1014.