Theories of Aging

Theories of Aging

Sue E. Meiner, EdD, APRN, BC, GNP

Theories of aging have been debated since the time of the ancient Greeks. In the twelfth century thoughts were centered on predetermination and an unalterable plan for life and death. The philosopher Maimonides thought that precautions and careful living might prolong life. In the late 1400s, Leonardo da Vinci attempted to explain aging as physiologic changes while studying the structure of the human body. Studies were few until the late 1900s when world populations began to have increasing numbers of older adults. Scholars have sought to embrace a theory that can explain the entire aging phenomenon. However, many scholars have concluded that no one definition or theory exists that explains all aspects of aging; rather, scientists have found that several theories may be combined to explain various aspects of the complex phenomena we call aging.

Theories function to help make sense of a particular phenomenon; they provide a sense of order and give a perspective from which to view the facts. Theories provide a springboard for discussion and research. Some theories are presented in this chapter because of their historical value; for the most part, they have been abandoned because of lack of empiric evidence. Other theories are the result of ongoing advances made in biotechnology and, as such, provide glimpses into our future.

Human aging is influenced by a composite of biologic, psychologic, social, functional, and spiritual factors. Aging may be viewed as a continuum of events that occur from conception to death (Ignatavicius & Workman, 2005). Biologic, social, and psychologic theories of aging attempt to explain and explore the various dimensions of aging. This chapter explores the prominent theories of aging as a guide for developing a holistic gerontologic nursing theory for practice application. No single gerontologic nursing theory has been accepted by this specialty, which requires nurses to use an eclectic approach from other disciplines as the basis of clinical decision making (Comfort, 1970) (Box 2–1).

By incorporating a holistic approach to the care of older adults, nurses can view this ever-increasing portion of the population more comprehensively. Interactions between gerontologic nurses and older adults are not limited to a specific disease or physiologic process, absolute developmental tasks, or psychosocial changes. Nurses have the ability to synthesize various aspects of the different aging theories, and they visualize older adults interfacing with their total environment, including physical, mental/emotional, social, and spiritual aspects. Therefore an eclectic approach provides an excellent foundation as nurses plan high-quality care for older adults.

Theories of aging attempt to explain this phenomenon of aging as it occurs over the life span, which is thought to be a maximum of approximately 120 years (Cetron & Davies, 1998). Several basic assumptions and concepts have been accepted over the years as guiding research and clinical practice related to aging. Human aging is viewed as a total process that begins at conception. Because individuals have unique genetic, social, psychologic, and economic factors intertwined in their lives, the course of aging varies from individual to individual. Senescence, defined as a change in the behavior of an organism with age, leading to a decreased power of survival and adjustment, also occurs. The recognition of the universal truths is what we attempt to discover through the theories of aging.

Biologic Theories of Aging

Biologic theories are concerned with answering basic questions regarding the physiologic processes that occur in all living organisms as they chronologically age. These age-related changes occur independent of any external or pathologic influence. The primary question being addressed relates to the factors that trigger the actual aging process in organisms. These theories generally view aging as occurring from a molecular, cellular, or even a systems point of view. In addition, biologic theories are not meant to be exclusionary. Theories may be combined to explain phenomena (Hayflick, 1996; Hayflick, 2007).

The foci of biologic theories include explanations of the following: (1) deleterious effects leading to decreasing function of the organism, (2) gradually occurring age-related changes that are progressive over time, and (3) intrinsic changes that can affect all members of a species because of chronologic age. The decreasing function of an organism may lead to a complete failure of either an organ or an entire system (Hayflick, 1996; Hayflick, 2004; Hayflick, 2007). In addition, according to these theories, all organs in any one organism do not age at the same rate, and any single organ does not necessarily age at the same rate in different individuals of the same species (Warner, 2004).

The biologic theories can be subdivided into two main divisions: stochastic and nonstochastic. Stochastic theories explain aging as events that occur randomly and accumulate over time, whereas nonstochastic theories view aging as certain predetermined, timed phenomena (Box 2–2).

Stochastic Theories

Error Theory

As a cell ages, various changes occur naturally in its deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), the building blocks of the cell. DNA, found in the nucleus of the cell, contains the fundamental genetic code and forms the genes on all 46 human chromosomes (Black & Hawks, 2005).

In 1963, Orgel proposed the Error Theory, sometimes called the Error Catastrophe Theory. This theory’s hypothesis is based on the idea that errors can occur in the transcription in any step of protein synthesis of DNA, and this eventually leads to either the aging or the actual death of a cell. The error would cause the reproduction of an enzyme or protein that was not an exact copy of the original. The next transcription would again contain an error. As the effect continued through several generations of proteins, the end-product would not even resemble the original cell and its functional ability would be diminished (Sonneborn, 1979).

In recent years the theory has not been supported by research. Although changes do occur in the activity of various enzymes with aging, studies have not found that all aged cells contain altered or misspecified proteins, nor is aging automatically or necessarily accelerated if misspecified proteins or enzymes are introduced to a cell (Hayflick, 1996; Hayflick, 2004; Schneider, 1992; Weinert & Timiras, 2003).

Radical Theory

Free radicals are by products of fundamental metabolic activities within the body. Free radical production can increase as a result of environmental pollutants such as ozone, pesticides, and radiation. Normally, they are neutralized by enzymatic activity or natural antioxidants. However, if they are not neutralized, they may attach to other molecules. These highly reactive free radicals react with molecules in cell membranes, in particular, cell membranes of unsaturated lipids such as mitochondria, lysosomes, and nuclear membranes. This action monopolizes the receptor sites on the membrane, thereby inhibiting the interaction with other substances that normally use this site; this chemical reaction is called lipid peroxidation. Therefore the mitochondria, for example, can no longer function as efficiently, and their cell membranes may become damaged, which results in increased permeability. If excessive fluid is either lost or gained, the internal homeostasis is disrupted and cell death may result.

Other deleterious results are related to free radical molecules in the body. Although these molecules do not contain DNA themselves, they can cause mutations in the DNA–RNA transcription, thereby producing mutations of the original protein. In nervous and muscle tissue, to which free radicals have a high affinity, a substance called lipofuscin has been found and is thought to be indicative of chronologic age.

Lipofuscin, a lipid- and protein-enriched pigmented material, has been found to accumulate in older adults’ tissues, and is commonly referred to as “age spots.” As the lipofuscin’s presence increases, healthy tissue is slowly deprived of oxygen and nutrient supply. Further degeneration of surrounding tissue eventually leads to actual death of the tissue. The body does have naturally occurring antioxidants, or protective mechanisms. Vitamins C and E are two of these substances and can inhibit the functioning of the free radicals or possibly decrease their production in the body.

Harman (1956) was the first to suggest that the administration of chemicals terminating the propagation of free radicals would extend the life span or delay the aging process. Animal research has demonstrated that administration of antioxidants did increase the average length of life, possibly because of the delayed appearance of diseases that may have eventually killed the animals studied. It appears that administration of antioxidants postpones the appearance of diseases such as cardiovascular disease and cancer, two of the most common causes of death. Antioxidants also appear to have an effect on the decline of the immune system and on degenerative neurologic diseases, both of which affect morbidity and mortality (Hayflick, 1996; Weinert & Timiras, 2003; Yu, 1998, 1993).

Cross-Linkage Theory

The cross-linkage theory of aging hypothesizes that with age some proteins become increasingly cross-linked or enmeshed and may impede metabolic processes by obstructing the passage of nutrients and wastes between the intracellular and extracellular compartments. According to this theory, normally separated molecular structures are bound together through chemical reactions.

This primarily involves collagen, which is a relatively inert long-chain macromolecule produced by fibroblasts. As new fibers are created, they become enmeshed with old fibers and form an actual chemical cross-link. The end result of this cross-linkage process is an increase in density of the collagen molecule but a decrease in the capacity to both transport nutrients to the cells and remove waste products from the cells. Eventually, this results in a decrease in the structure’s function. An example of this would be the changes associated with aging skin. The skin of a baby is soft and pliable, whereas aging skin loses much of its suppleness and elasticity. This aging process is similar to the process of tanning leather, which purposefully creates cross-links (Bjorkstein, 1976; Hayflick, 1996; Hayflick, 2004).

Cross-linkage agents have been found in unsaturated fats; in polyvalent metal ions like aluminum, zinc, and magnesium; and in association with excessive radiation exposure. Many of the medications ingested by the older population (such as antacids and coagulants) contain aluminum, as does the common cooking ingredient baking powder. Some research supports a combination of exercise and dietary restrictions in helping to inhibit the cross-linkage process, as well as the use of vitamin C prophylactically as an antioxidant agent (Bjorkstein, 1976).

One researcher, Cerani, has shown that blood glucose reacts with bodily proteins to form cross-links. He has found that the crystallin of the lens of the eye, membranes of the kidney, and blood vessels are especially susceptible to cross-linking under the conditions of increased glucose. Cerani suggests increased levels of blood glucose cause increased amounts of cross-linking, which accelerate lens, kidney, and blood vessel diseases (Schneider, 1992).

Cross-linkage theory proposes that as a person ages and the immune system becomes less efficient, the body’s defense mechanism cannot remove the cross-linking agent before it becomes securely established. Cross-linkage has been proposed as a primary cause of arteriosclerosis, a decrease in efficiency of the immune system with age, and the loss of elasticity often seen in older adult skin. The cross-linkage theory has emerged from deductive reasoning, and aside from the previous examples, there is little empiric evidence to support its claims (Hayflick, 1996).

Wear and Tear Theory

This theory proposed that cells wear out over time because of continued use. When this theory was first proposed in 1882 by Weisman, death was seen as a result of tissues being worn out because they could not rejuvenate themselves in an endless manner (Hayflick, 1988). Essentially, the theory reflects a belief that organs and tissues have a preprogrammed amount of available energy and wear out when the allotted energy is expended. Eventually this leads to the death of the entire organism.

Under this theory, aging is viewed as almost a preprogrammed process—a process thought to be vulnerable to stress or to an accumulation of injuries or trauma, which may actually accelerate it. “Death,” said Weisman, “occurs because a worn out tissue cannot forever renew itself” (Hayflick, 1996; Weinert & Timiras, 2003; Holliday, 2004).

Proponents of this theory cite microscopic signs of wear and tear that have been found in striated and smooth muscle tissue and in nerve cells. Researchers question this theory in light of research demonstrating increased functional abilities in individuals who exercise daily. This effect occurs even in persons with chronic limiting states such as rheumatoid arthritis. If exercise has been found to increase a person’s level of functioning rather than decrease it, critics challenge, how can the wear and tear hypothesis be correct? This theory was developed during the Industrial Revolution, when people were attempting to explain and make sense of events in their world. These people were trying to equate humans with the marvelous machines they were producing. It eventually became clear just how different humans were from these machines.

Nonstochastic Theories

Programmed Theory or Hayflick Limit Theory

One of the first proposed biologic theories is based on a study completed in 1961 by Hayflick and Moorehead. This study included an experiment on fetal fibroblastic cells and their reproductive capabilities. The results of this landmark study changed the way scientists viewed the biologic aging process.

Hayflick and Moorehead’s study showed that functional changes do occur within cells and are responsible for the aging of the cells and the organism. The study further supported the hypothesis that a cumulative effect of improper functioning of cells and eventual loss of cells in organs and tissues are therefore responsible for the aging phenomenon. This study contradicted earlier studies by Carrel and Ebeling in which chick embryo cells were kept alive indefinitely in a laboratory; the conclusion from this 1912 experiment was that cells do not wear out but continue to function normally forever. An interesting aspect of the 1961 study was that freezing was found to halt the biologic cellular clock (Hayflick & Moorehead, 1961).

Based on this 1961 study, unlimited cell division was not found to occur; the immortality of individual cells was found to be more an abnormal than a normal occurrence. Therefore this study seemed to support the Hayflick Limit Theory. Life expectancy was generally seen as preprogrammed, within a species-specific range; this biologic clock for humans was estimated at 110 to 120 years (Gerhard & Cristofalo, 1992; Hayflick, 1996). Based on the conclusions of this experiment, the Hayflick Limit Theory is sometimes called the “Biologic Clock,” “Cellular Aging,” or “Genetic Theory.”

Immunity Theory

The immune system is a network of specialized cells, tissues, and organs that provide the body with protection against invading organisms. Its primary role is to differentiate self from non self, thereby protecting the organism from attack by pathogens. It has been found that as a person ages, the immune system functions less effectively. The term immunosenescence has been given to this age-related decrease in function.

Essential components of the immune system are T lymphocytes, which are responsible for cell-mediated immunity, and B lymphocytes, the antibodies responsible for humoral immunity. Both T and B lymphocytes may respond to an invasion of the organism, although one may provide more protection in certain situations. The changes that occur with aging are most apparent in the T lymphocytes, although changes also occur in the functioning capabilities of B lymphocytes. Accompanying these changes is a decrease in the body’s defense against foreign pathogens, which manifests itself as an increased incidence of infectious diseases and an increase in the production of autoantibodies, which lead to a propensity to develop autoimmune-related diseases (Hayflick, 1996; Weinert & Timiras, 2003) (Box 2–3).

The changes in the immune system cannot be precisely explained by an exact cause-and-effect relationship, but they do seem to increase with advancing age. These changes include a decrease in humoral immune response, often predisposing older adults to (1) decreased resistance to a tumor cell challenge and the development of cancer, (2) decreased ability to initiate the immune process and mobilize the body’s defenses against aggressively attacking pathogens, and (3) heightened production of autoantigens, often leading to an increase in autoimmune-related diseases.

Immunodeficient conditions, such as the human immunodeficiency virus (HIV) and the immune suppression of organ transplant recipients, have demonstrated a relationship between immunocompetence and cancer development. HIV has been associated with several forms of cancer, such as Kaposi’s sarcoma. Recipients of organ transplants are 80 times more likely to contract cancer than the rest of the population (Black & Hawks, 2005).

Emerging Theories

Neuroendocrine Control or Pacemaker Theory

The neuroendocrine theory examines the interrelated role of the neurologic and endocrine systems over the life span of an individual (Box 2–4). The neuroendocrine system regulates and controls many important metabolic activities. It has been observed that there is a decline, or even a cessation, in many of the components of the neuroendocrine system over the life span. The reproductive system, and its changes over the life of an individual, provides an interesting model for the functional capability of the neuroendocrine system.

Research has shown there are complex interactions between the endocrine and the nervous systems. It appears that the female reproductive system is governed not by the ovaries or the pituitary gland but by the hypothalamus. Men do not experience a reproductive event such as a menopause, although they do demonstrate a decline in fertility. The mechanisms that trigger this decline may offer a template for understanding the phenomena of aging (Hayflick, 1996; Weinert & Timiras, 2003).

Another hormone that has been receiving attention is dehydroepiandrosterone (DHEA). This hormone, secreted by the adrenal glands, diminishes over the lifetime of an individual. Administration of this hormone to laboratory mice showed it increased longevity, bolstered immunity, and made the animals appear younger. These mice also ate less, so there is some question whether DHEA-fed mice exhibit the effect of calorie restriction (Cupp, 1997; Guardiola-Lemaitre, 1997; Hayflick, 1996; Hayflick, 2004).

Melatonin is a hormone being investigated for its role as a biologic clock. Melatonin is produced by the pineal gland, the function of which was a mystery until recently. Melatonin has been found to be a regulator of biologic rhythms and a powerful antioxidant that may enhance immune function. The level of melatonin production in the body declines dramatically from just after puberty until old age.

The belief that melatonin has a role in aging comes not only from its effect on the immune system and its antioxidant capability but also from studies on rodents that demonstrated an increased life span when melatonin was administered. These studies also found that rodents fed supplementary melatonin restricted their calorie intake. More research needs to be performed regarding the safety and efficacy of melatonin. However, in the United States melatonin can already be sold as a dietary supplement, so there is little financial incentive for conducting research. In Europe melatonin is considered a neurohormone, so there would be more financial gain to determining its role in the aging process. At this time, no individual should take melatonin without his or her primary health care provider’s knowledge (Guardiola-Lemaitre, 1997; Hayflick, 1996).

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Nov 26, 2016 | Posted by in NURSING | Comments Off on Theories of Aging
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