Growing Old: Biological and 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 & Elderly Issues > Biological & Physiological Aging

Overview

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.

Metabolic Theory

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 restriction of calories could produce a similar result in human beings. However, although historically various human populations have been subjected to reduced caloric intake and even starvation, the situations were never performed in controlled settings while still administering supplemental nutrients and vitamins, so no conclusions can be drawn. Actuarial studies performed by insurance companies have found not only that those who are the most overweight within an age span are more likely to die earlier, but have also found that those who are most underweight have a similar probability of dying early. Such findings do not support the caloric restriction hypothesis. Further, other factors can be used to explain the experimental results with rodents, including their sedentary lifestyle in captivity.

Free Radical Theory

Another stochastic theory of aging is the free radical theory. Free radicals are individual or groups of atoms that have at least one unpaired electron, causing them to become unstable and highly reactive. In human tissue, free radicals can cause damage to cells and may accelerate the progression of various diseases including cancer, cardiovascular disease, and age-related diseases. Free radicals are byproducts of the normal metabolism of food and the normal synthesis of bodily chemicals. They can interact with key cellular components (e.g., cell membrane, cell proteins, fatty acids, carbohydrates, DNA), causing irreversible damage. The activity of free radicals occurs in three phases.

• First, free radicals are created as the result of normal physiological functions.
• Second, these free radicals produce other free radicals.
• Finally, free radicals react with each other or with other compounds to end the propagation of these molecules.

The body has natural defenses against the effects of free radicals. However, the actions of free radicals are instantaneous, so they still can cause damage before being counteracted by these defenses. It has been proposed that the use of chemicals (e.g., vitamins C and E, and selenium) to stop the propagation of free radicals would extend the life span of animals. A great deal of research has been dedicated to investigating the relationship of free radical damage to aging and ways to neutralize free radicals before the damage occurs. However, although the results of research studies support the conclusion that free radical damage accumulates with aging, the results are inconsistent among species and sexes, and more research is needed before the action of free radicals is well understood.

Cross Linkage Theory

The cross linkage theory of aging posits that the symptoms we recognize as aging result from the accumulation of cross-linked compounds that interfere with the normal functioning of cells. Cross-linking is the formation of bonds or links between different cellular proteins. For example, it has been found that blood sugar reacts with proteins within the body and cross-links with them, including in the crystallins of the human lens and the membrane proteins of the kidney and blood vessels. It has been suggested that the high elevations of blood sugar associated with diabetes results in increased levels of sugar induced cross-links that cause accelerated diseases of the lens, kidney, and blood vessels. According to this theory, although these cross-links still occur during the aging process even without diabetes, they are produced more slowly.

Applications

Physiological Changes of Aging

No matter what is happening on the cellular level, there are certain observable physiological changes that can be observed as one ages. With modern advances in medicine and today's increased emphasis on physical and mental fitness, some of the signs normally associated with aging are being observed later in life than previously. Globally, an individual's life expectancy at birth increased from 49 years in 1950 to 67 years by 1995. In more developed countries, this latter figure was 75 years. According to the World Health Organization, in 2011 the global life expectancy at birth was 70 years and was 80 years in high-income countries (2013). However, if one lives long enough, some of the physical signs of aging are sure to present themselves. In general, during early adulthood (i.e., 20 to 39 years of age), one continues to grow physically. However, during middle adulthood (i.e., 40 to 65 years of age), this growth trend stops and size and muscle mass decrease while fat concomitantly increases; one's eyesight declines and women's reproductive capability ends. In late adulthood (over 65 years of age), one's physical size decreases and one's organs become less efficient.

In many ways, the common activities of youth and old age are the same, including napping, eating, walking, and conversing with friends. Younger people, of course, also engage in more sports and strenuous activity that typically cannot be well handled by those in late adulthood. However, research has also found that even young adults do not engage in such activities frequently. Despite the commonalities across the ages, however, it is obvious to even casual observers that there are physiological changes that occur in old age. The most obvious of these are changes in an individual's appearance: the thinning, graying, or even loss of hair; wrinkling and folding of the skin; and even a loss of height as the thickness of the disks between the vertebrae decreases. However, there are less obvious signs of the aging of the human body as well. The ability of the sensory organs to do their job decrease and the senses of vision, hearing, taste, and smell all become less acute. Visual acuity decreases, the ability of the eye to adapt to changes in light levels slows, and distance perception diminishes. The pupil of the eye also shrinks with age and the lens become less transparent, thereby reducing the amount of light that reaches the retina. This results in more difficulty seeing at night (such as when driving a car) and the need for more light to read.

In addition, during the later years, reaction time slows and physical stamina decreases. Internally, oxygen intake and the ability of the heart to pump blood decrease, and the body is not able to replenish lost nutrients as quickly as when it was younger. As a result, it takes older adults longer to rebound from physical activity than it does for younger adults. Similarly, muscle strength decreases. As far as general health is concerned, in the later years, one's immune system weakens and leaves the individual more susceptible to life-threatening conditions such as cancer or pneumonia. On the other hand, older people typically are less susceptible to short-term illnesses such as colds and flu because of the long accumulation of antibodies over the course of their lifetimes.

The aging process also brings about changes in the brain. Neural processing is slowed, and information needs longer times to be processed, resulting in longer reaction times, and longer times needed to solve perceptual puzzles, perform complex tasks, or even to remember names. This slowing of reaction times can also result in more problems driving and a concomitant increase in vehicle accidents. In addition, regions of the brain related to memory tend to atrophy as part of the aging process, often resulting in a five percent reduction of brain weight by the age of 80 (although there is often less loss for women than for men).

Conclusion

Biologically and physiologically, the aging process results in the reduction of both mental and physical ability and also leaves the body more susceptible to life-threatening diseases. However, it must be borne in mind that these changes are not things that suddenly occur at the age of 65 (or any other random year later in life), but are part of a continuing process of decline that occurs much earlier in life, in many cases even during young adulthood. However, although these changes may start earlier in life, their effects become obvious after 65.

The news is not all bad, however. Particularly for elders who remain active, the creation of new cells and continued proliferation of neural connections helps compensate for cell loss. This means that older adults who remain physically, sexually, and mentally active, often retain greater capacity for these activities even in their later years. In addition, there are a number of nongenetic factors that may contribute to longevity. Research has found, for example, that a history that does not include heavy drinking is often correlated with longer life. Similarly, research has found that older adults who live independently rather than in nursing homes tend to live longer lives. Some personality traits have also been correlated with longevity: curiosity, conscientiousness, and not dwelling on the negative aspects of life. A healthy diet, regular physical and mental exercise, and a sense of control over one's life have also been shown to be related to longer life. For example, research has shown that sedentary older adults who were randomly assigned to a walking program gained enhanced memory and sharpened judgment. Whether or not these are causal factors has not been proven. However, eating right, keeping fit, and remaining mentally active throughout one's life may prolong it.

Terms & Concepts

Correlation: The degree to which two events or variables are consistently related. Correlation may be positive (i.e., as the value of one variable increases the value of the other variable increases), negative (i.e., as the value of one variable increases the value of the other variable decreases), or zero (i.e., the values of the two variables are unrelated). Correlation does not imply causation.

Dementia: In the generic sense of the term, dementia is a generalized, pervasive deterioration of a person's cognitive abilities (e.g., memory, language, executive function). The deterioration caused by dementia can be very severe and can significantly impact the ability of the individual to perform on the job, function well within society, or even accomplish the daily activities of living. Dementia is a group of symptoms rather than a specific disease. It can be caused by different diseases, including Alzheimer's disease, stroke, Pick's disease, Parkinson's disease, Huntington's disease, and AIDS dementia complex. Brain tumors and other treatable conditions may also cause dementia. Although the onset of dementia may vary, in most cases it occurs later in life, typically after 65 years of age.

Empirical: Theories or evidence that are derived from or based on observation or experiment.

Genetics: A branch of biology concerned with the mechanisms and phenomena related to heredity and inherited traits, in particular the mechanisms of hereditary transmission and variation of inherited characteristics among similar or related organisms.

Immune System: A complex biological system in vertebrates that helps protect the body against pathological effects of foreign substances (e.g., viruses, bacteria). The organs in the immune system include the bone marrow, thymus gland, and lymphatic system. The immune system interacts with the nervous system and the endocrine system.

Neuron: A cell in the nervous system (i.e., the brain, spinal column, and nerves) that conducts impulses. Neurons are the basic cellular unit of the nervous system and comprise a nucleated cell body with one or more dendrites (the part of the cell that conducts impulses from adjacent cells toward the cell body) and a single axon (the nerve fiber that conducts impulses away from the body of the nerve cell). A neuron is also called a "nerve cell."

Random: In statistics, the term random refers to the selection of subjects in such a way that each member of the population has an equal chance or probability of being selected without predictability or bias. Random is also used to refer to the occurrence of phenomena that happen without predictability.

Social Role: A set of expectations placed on members of a group of people with a given social position or status within society.

Society: A distinct group of people who live within the same territory, share a common culture and way of life, and are relatively independent from people outside the group. Society includes systems of social interactions that govern both culture and social organization.

Stochastic: Involving chance or probability. Stochastic variables are random or have an element of chance or probability associated with their occurrence.

Bibliography

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Thakar, H., Acharya, C., Apte, S., & Saxena, R. (2013). Free radicals and ageing—Is there any correlation?. National Journal Of Physiology, Pharmacy & Pharmacology, 3, 82-86. Retrieved November 13, 2013 from EBSCO Online Database Academic Search Premier. http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=87365371&site=ehost-live

World Health Organization (2013). Global health observatory (GHO): Life expectancy. Retrieved November 13, 2013 from http://www.who.int/gho/mortality%5Fburden%5Fdisease/life%5Ftables/situation%5Ftrends%5Ftext/en/index.html

Suggested Reading

Dupre, M. E., Liu, G., & Gu, D. (2008). Predictors of longevity: Evidence from the oldest old in China. American Journal of Public Health, 98, 1203-1208. Retrieved July 10, 2008, from EBSCO Online Database Academic Search Premier. http://search.ebscohost.com/login.aspx?direct=true & db=aph & AN=32967542 & site=ehost-live

Fiocco, A. J. (2012). Focus on biobehavioral perspectives on health in late life (Annual review of gerontology and geriatrics, vol. 30). Canadian Journal on Aging, 31, 364-366. Retrieved November 13, 2013 from EBSCO Online Database SocINDEX with Full Text. http://search.ebscohost.com/login.aspx?direct=true&db=sih&AN=79222869

Four small lifestyle changes can mean an extra 14 years. (2008). Harvard Women's Health Watch, 15, 7. Retrieved July 10, 2008, from EBSCO Online Database Academic Search Premier. http://search.ebscohost.com/login.aspx?direct=true & db=aph & AN=30733432 & site=ehost-live

Gavrilov, L. A., & Gavrilova, N. S. (2012). Biodemography of exceptional longevity: Early-life and mid-life predictors of human longevity. Biodemography & Social Biology, 58, 14-39. Retrieved November 13, 2013 from EBSCO Online Database SocINDEX with Full Text. http://search.ebscohost.com/login.aspx?direct=true&db=sih&AN=74550530

Gulli, C. (2008). Secrets to longevity. Maclean's, 121(20/21), 60-62. Retrieved July 10, 2008, from EBSCO Online Database Academic Search Premier. http://search.ebscohost.com/login.aspx?direct=true & db=aph & AN=32166942 & site=ehost-live