Ageing, irreversible biological changes that occur in all living things with the passage of time, eventually resulting in death. Although all organisms age, rates of ageing vary considerably. Fruit flies, for example, are born, grow old, and die in 30 or 40 days, while field mice have a life span of about three years. Dolphins may live to age 25, elephants to age 50, and Galapagos tortoises to 100. These life spans pale in comparison to those of some species of giant sequoia trees, which live hundreds of years.
Among humans, the effects of ageing vary from one individual to another. The average life expectancy for Americans is around 75 years, almost twice what it was in the early 1900s. Although some people never reach this age, and others are beset with illnesses if they do, more and more people are living healthy lives well into their 90s and older. The study of the different ageing processes that occur among individuals and the factors that cause these changes is known as gerontology. Geriatrics is a medical specialty concerned with the prevention, diagnosis, and treatment of diseases in the elderly.
CAUSES OF AGEING
Although the exact causes of ageing remain unknown, scientists are learning a great deal about the ageing process and the mechanisms that drive it. Some of the most promising research on the ageing process focuses on the microscopic changes that occur in all living cells as organisms age. In 1965 American microbiologist Leonard Hayflick observed that under laboratory conditions, human cells can duplicate up to 50 times before they stop. Hayflick also noted that when cells stop normal cell division (see Mitosis), they start to age, or senesce. Since Hayflicks groundbreaking observations, scientists have been searching for the underlying cause, known as the senescent factor (SF), of why cells stop dividing and thus age.
Different theories have been proposed to explain how SF works. One theory is based on the assumption that ageing, and diseases that occur more frequently with advancing age, are caused by structural damage to cells. This damage accumulates in tiny amounts each time the cell divides, eventually preventing the cell from carrying out normal functions.
One cause of this damage may be free radicals, which are chemical compounds found in the environment and also generated by normal chemical reactions in the body. Free radicals contain unpaired electrons and so carry an electric charge that makes them highly reactive. In an effort to neutralize their electric charge, free radicals constantly bombard cells in order to steal electrons in a process called oxidation. Free radicals are thought to greatly increase the severity of or perhaps even cause such life-shortening diseases as diabetes mellitus, strokes, and heart attacks. Researchers have observed that free radicals exist in smaller amounts in those species with relatively long life spans. Increasing human life span may depend on our ability to prevent free radical damage, and scientists are currently examining the role of chemical compounds, called antioxidants, that prevent or reverse oxidative damage in the ageing process.
Another theory suggests that SF is genetically regulated that is, cells are genetically programmed to carry out about 50 cell divisions and then die. Researchers have identified at least three genes that are involved with human cellular senescence. They have also discovered a protein on the surface membranes of senescent cells that inhibits production of deoxyribonucleic acid (DNA), the essential molecule that carries all genetic information.
Another theory proposes that extra, useless bits of DNA accumulate over time within a cell's nucleus. Eventually this so-called junk DNA builds up to levels that clog normal cell action. If this idea is correct, scientists may be able to find ways to prevent accumulation of junk DNA, thereby slowing down the process of senescence in cells.
Other studies focus on cell division limits. Each time a cell divides, it duplicates its DNA, and in each division the sections at the ends of DNA, called the telomeres, are gradually depleted, or shortened. Eventually the telomeres become so depleted that normal cell division halts, typically within 50 cell divisions. Scientists have found that an enzyme produced by the human body, called telomerase, can prolong the life of the telomeres, thus extending the number of cell divisions. In laboratory studies, cells injected with telomerase continue to divide well beyond the normal limit of 50 cell divisions. These promising results have triggered worldwide attention on telomerase and its relationship to ageing.
A number of other studies are underway to investigate the effects of ageing. Scientists have found, for example, a possible explanation for why women have longer average life spans than men. The difference seems to be biologically determined, and male and female sex hormones are probably responsible. The blood levels of female sex hormones drop sharply during menopause. At that time, the incidence of heart disease and high blood pressure in women increases to match the incidence in men, suggesting that the presence of female sex hormones offers some protection against heart disease.
In developed nations, life expectancy has increased more in the 20th century than it has in all of recorded history. A person born in the United States in 1995 can expect to live more than 35 years longer than a person born in 1900. Today more than 34 million Americans are 65 or older, accounting for about 13 percent of the population. By the year 2030, their numbers will more than double: One in every five Americans will be over age 65. A person who lives 100 years or more centenarian was once a rarity, but today about 60,000 Americans are 100 years or older. By the year 2060, there may be as many as 2.5 million centenarians in the United States. The number of supercentenarians people 105 years of age and olderâ€”will probably be as commonplace in the next century as centenarians are fast becoming now.
In some parts of the world, 16 to 18 percent of the population is already age 65 or older. By the year 2025, Japan is expected to have twice as many old people as children. Also by that time, there will be more than one billion older people worldwide. This increase in life expectancy is the result of better public health measures, improvements in living conditions, and advances in medical care. A marked reduction in infant mortality rates has also contributed to increased life expectancy statistics.
Ageing populations are expected to have profound effects on the way societies care for their elderly members. With a larger proportion of the population over age 65, medical care must become better equipped to deal with the disorders and diseases of the elderly. All health care professionals should have special training in geriatrics. As the percentage of older people in the population exceeds the percentage of young working people, traditional methods for caring for older people may need to be modified. For example, in the United States, workers pay taxes throughout their careers so that when they retire, usually around the age of 65, they can receive money from the federal government to survive. This system, called Social Security, may be in jeopardy as the percentage of retired people increases, placing inordinate demands on the smaller number of people working and supporting them.
In many parts of the world, including the United States, older people who cannot work and have health problems live in long-term care facilities such as nursing homes, where they receive care 24 hours a day. But many families are unable to bear the costs of nursing homes and medical care for the elderly, and health insurance is unable to cover the expense. Other countries face similar problems, and multinational efforts are underway to explore new methods to finance the care of the world's older persons, soon to number one billion. 01 December 2010Comment