Growing Old: Biological & Physiological Aging
Although from an outward perspective many of the physical signs of aging may be obvious, the underlying biological mechanisms of the aging process that cause these observable changes are still not well understood. There are two general approaches taken in biological theories of aging. Genetic or programmed theories posit that aging is caused by the programmed actions of specific inherited genes. Stochastic theories posit that random assaults from the external environment and one's body accumulate over time and result in aging. Stochastic theories include the somatic mutation theory, error catastrophe theory, metabolic theory, free radical theory, and cross linkage theory of aging. As with nature/nurture controversies in psychology, however, it is more than likely that both genetic and stochastic factors play a part in the aging process. In addition, research has found some steps that can be taken to increase one's longevity and stave off the effects of the aging process.
Keywords Correlation; Empirical; Genetics; Immune System; Neuron; Stochastic
Aging is a complex process that is still not well understood. Sociologically, at some point most older people retire from job and career and focus more on family and self. As part of this process, they may also take on different roles in society; changing, for example, from obsessed career person and strict parent to relaxed world travel traveler and doting grandparent. Psychologically, there are changes as well. Older people may experience emotional reactions to growing older such as depression or changes in personality. They also may experience cognitive disorders including delirium and dementia. But for most people, it is the physical changes associated with aging that are the most noticeable: eyesight troubles, achy joints, and loss of skin elasticity.
There are a number of theories that attempt to explain the biological processes underlying human aging. However, as the plethora of theories amply demonstrates, these processes are still not well understood. In general, the biological theories of aging can be divided into two categories:
- Genetic or programmed theories which posit that aging is caused by the programmed actions of specific inherited genes;
- Stochastic theories that posit that random assaults from the external environment and one's body accumulate over time and result in aging.
As with nature/nurture controversies in psychology, it is more than likely that both genetic and stochastic factors play a part in the aging process. At this time, none of these theories of aging on its own completely explains all aspects of this phenomenon from a biological point of view.
Genetic Theories of Aging
As can be readily seen by comparing the relative life spans of one's pets to oneself, genetics plays some part in the aging process. Even within species this phenomenon can be seen. For example, whereas the members of one family may live well into their nineties or routinely live to be over 100, members of another may routinely die decades earlier. On a cellular level, researchers have found that many cells are programmed to reproduce only for a limited number of times. For example, individual human cells that are taken from the body and propagated in laboratory conditions typically divide approximately only fifty times. It has been experimentally demonstrated that the regulation of cell division is a dominant trait controlled by a small number of genes. According to scientists who study the evolution of human longevity, there are approximately 200 genes that help regulate the human aging process. A number of organisms demonstrate programmed aging resulting from biological clocks such as the Pacific salmon that ages rapidly following spawning. In human beings, the most obvious example of the aging of a physiological system is the female menstrual cycle which typically begins in between ages eleven and sixteen and typically ends between the ages of forty-five to fifty-five. Similarly, the human immune system continues to grow in competency through the teenage years, and then decreases in ability after the age of twenty. After the age of fifty, this immune clock (located within the thymus gland) is barely detectable. Some scientists propose that this decline in immune function leads to a number of critical age-related conditions including increased susceptibility to infectious diseases (e.g., pneumonia, influenza), increased risk for cancer, and alterations in blood vessel walls leading to arteriosclerosis.
Stochastic Theories of Aging
Although it is clear that genetics plays some part in the aging process, it is equally clear that genetics is not the only factor underlying the aging process. Even within families there are usually exceptions to the rule: the son who dies in his forties or the grandmother who lives to be 105. Stochastic theories of aging attempt to explain why such individual differences occur. There are a great number of stochastic theories of aging. Some of the more popular ones are briefly described in the following paragraphs.
Somatic Mutation Theory
Somatic mutation theory posits that damage to the genetic integrity of the body's cells results in aging. Enzymes in the body catalyze the body's various chemical reactions, including the synthesis of RNA, proteins, hemoglobin, antibodies, and some of the critical hormones that regulate many of the body's organs. The body's DNA, RNA, and proteins are constantly being subjected to the negative effects about the external environment and the body's internal environment. If the DNA of germ cells is damaged, and inherited mutation can be formed and passed on to succeeding generations as specific human genetic diseases or (e.g. familial Alzheimer's disease, sickle cell disease). Some proponents of stochastic theories of biologic aging propose that aging is the result of an accumulation of mutations in the DNA of somatic or non-germ cells. Over time, the situation could lead to the production of increased amounts of altered messenger RNA and altered proteins. As a result, there could be decreased survival of cells, tissues, and organs. According to Kennedy, Loeb, and Herr, research has shown that somatic mutations do play a significant role in aging and in some age-related conditions (2012).
Error Catastrophe Theory
The error catastrophe theory is another stochastic theory of biologic aging. In this theory, it is posited that errors occur in the pathway from DNA to the formation of proteins. Because protein enzymes are required to synthesize additional proteins, these errors can produce defective enzymes that in turn result in even more errors. According to this theory, this accumulation of errors could eventually result in the production of so many deceptive proteins that cells and tissue die and dysfunction occurs. The error catastrophe theory has been empirically tested by comparing sequences of proteins from both young and aged cell populations. Although studies of specific proteins in animals did not show substantial age-related differences in amino acid sequences, studies of cellular proteins with aging have demonstrated increased alterations of the proteins after synthesis.
The metabolic theory of aging posits that every organism has a finite amount of metabolic life and those organisms with higher metabolic rates have a correspondingly shorter life span. For example, when compared with human beings, mice have an extremely high metabolic rate and an extremely short life span. Similarly, it has been found that when the water temperature of the environment for certain types of fish is lowered (thereby lowering their body temperature and their metabolic rate), they live longer than similar fish who live in water with a warmer ambient temperature. Another type of experiment that supports the metabolic theory of aging involves restricting the number of calories that an animal ingests (while still taking in sufficient nutrients and vitamins). In one experiment, the caloric intake of mice was reduced by 40 percent. This resulted in a 35 percent increase in longevity. Other studies have found that phenomenon holds true even when the caloric restriction was begun in midlife. Rodents who were put on restricted calorie diets also experienced a delayed onset of many age-related physical signs of aging and diseases, including cancer. Some proponents of this theory propose that a similar...
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