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1.5: What we know thus far

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    The study of biological aging became a topic of great interest only recently compared with areas of biology such as embryology, anatomy, and genetics. Although many fundamental questions about aging have not been addressed and others have been only partially answered, much has been learned in a short time. Research is continuing, expanding, and creating synergistic interdisciplinary bridges as the study of aging - gerontology - is undertaken by experts in more disciplines. Aging studies are also becoming more cross-cultural and international. These studies are revealing similarities and differences in aging among people of different cultures, races, regions, and national origins. Results from these comparisons are beyond the scope of this book. It deals with human aging in advanced Western civilizations, especially in the U.S.

    What Happens during Biological Aging?

    When Aging Begins The ability of the body to maintain homeostasis seems to reach peak capacity peak during the third decade of life, after which age changes begin and bodily functioning starts to decline. (Figure 1.12). There seems to be no plateau period during which the body retains its maximum level of performance. The effects of aging are not immediately apparent, however.

    Figure 1.12 Body capacity. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

    Why Aging Appears Well after It Begins

    One reason for delay in the appearance of age changes is the large reserve capacity present in many parts of the body. The heart, which can increase the amount of blood it normally pumps fivefold, and the respiratory system, which can move six times the amount of air normally breathed, provide two examples. Detrimental age changes draw first on such reserve capacities. The effects become apparent only when the body is called upon to function near peak capacity but much of the reserve is gone. Since biological aging occurs slowly and the body is rarely called upon to function at peak capacity, it takes many years for the reserves to become noticeably low. For example, some individuals reach age 40 or beyond before significant age changes are noticed. Additional normal aging must occur before body capacities become so low that a person seems impaired most of the time. As aging continues after that, impairments become so severe that they are classified as diseases. Eventually, body capacities are so low that homeostasis cannot be maintained even with medical assistance, and the person dies. Death is inevitable, but since the person has made the most of their biological life, they have aged normally.

    The body behaves like a person who develops a large savings account when his or her income is high. That person can continue to live well by drawing on the savings when his or her income goes down. When savings become low, the person may not have enough funds to afford ordinary recreational activities, but essentials are still affordable. As funds dwindle, essentials become unaffordable, and the person may depend upon loans or other financial support. Eventually, funds become so low that the person is bankrupt.

    Aging is like finances in two other ways (Figure 1.12). First, if a person does not develop to his or her peak potential during youth, they enter the declining years of aging with less reserve capacity. Malnutrition, poor health care, or other adverse circumstances during youth may produce this effect. With less reserve capacity when aging begins, less time is needed for the body to reach the impaired or diseased levels. Second, an adult with a high peak capacity may have their bodily reserves ravaged by aging plus by other adverse factors (i.e., misuse, disuse, disease). Again, impairment and disease develop more quickly. This latter scenario is common, so it has been called "usual" aging.

    A second reason for the delay in the appearance of age changes in normal aging and in usual aging is the use of compensatory mechanisms. Some adjustments bolster diminishing functions. For example, greater amounts of chemicals (e.g., norepinephrine) that stimulate the heart are produced. This helps maintain pumping as the intrinsic strength of the heart declines. Some adjustments involve using more efficient ways to accomplish goals. People can learn how to pace themselves or use tools more effectively and thus continue to perform very difficult tasks. Finally, through changes in lifestyles and goals, many people tend to adjust their activities by participating in activities where they are comfortable and capable while shying away from activities that become too difficult and burdensome.

    There is considerable variability among people in both the age at which age changes are noticed and the rate at which these changes progress. This variability derives from several differences in aging (Figure 1.13).

    Figure 1.13 Aging variables: (a) Age of onset. (b) Rates of aging. (c) Rates of aging among people. (d) Average aging rates. (e) Aging rates. (f) Aging rates at different ages (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

    First, aging of a particular part of the body starts at different times for different people. Second, once a part has begun to age, it does so at different rates in different people. For example, bone strength declines faster in some individuals than in others. Third, the parts that age fastest in one person may not be the ones that age fastest in another person. Thus, one person’s heart may have the most age changes, while in another person the lungs may be aging faster than the heart. Fourth, the rate of aging of most of one person’s body parts is faster than the rate in another person. In other words, throughout the body, some people age faster than others. Because of all four differences, one person may begin aging or show signs of aging before a second person does. After some years, however, aging in the second person may surpass that in the first. (Suggestion: Chap 01 - 16-2-1)

    Two other types of variability in aging make this matter even more complicated. First, certain body parts usually seem to age faster than others do; for example, the lungs age faster than the blood. Second, though aging generally progresses steadily, the aging of some body parts in some individuals may speed up for a while, become quite slow for a while, and stop or show a reversal for a while.

    Many factors combine in each person to affect the specific time of onset and rate of aging for each body part. Each individual’s sex, genetically determined condition, and intrinsic compensatory powers when aging begins are unalterable factors. Occasional occurrences such as accidental injury and short-term diseases, along with long-term aspects of a person’s life such as education, diet, exercise, occupation, air quality, and protracted diseases, also play a role both before and after aging begins. The rate and the degree of effects from the progressive changes caused by aging are altered as these factors change.

    Therefore, though the rate of aging is determined in large part by conditions over which a person has no control, it is also heavily influenced by modifiable factors. As we identify and learn more about the factors that can be altered, we can gain more control over the progress of biological aging. We will also be better able to ward off the abnormal changes and diseases that become more likely as aging progresses.

    Heterogeneity among the Elderly

    Every individual is subjected to a unique combination of the factors that affect aging. Each factor acts at various ages to different degrees and for different lengths of time. The complex interactions among these factors add even more diversity. As a result, the older people get, the more different from one another they become. For many body parts, differences among those who have reached age 50 are already great. As more years pass, the heterogeneity among people expands more quickly. Some people become impaired or seriously ill at an early age, while other people remain hardy beyond age 100 (Figure 1.12). The elderly are the most diverse age group. Therefore, this book avoids using numerical values, which may be erroneously interpreted as ideals, norms, or goals. Some averages and ranges of values for the body are included, but only to provide approximations and trends within an expanding and increasingly diverse group.

    An important consequence of heterogeneity among the elderly is the need to provide individualized treatment for them. As the age of a group of people increases, generalities apply less and less well to the individuals in that group. Any planning for elderly persons must consider this individuality. This would include, for example, evaluating eligibility for employment or educational opportunities, designing housing, developing nutritional programs, planning physical fitness programs, and providing health care. More attention to the increased differences among aging people would assure not only that more individuals will receive proper consideration but also that fewer will be subjected to detrimental care and therapy.

    Another significant conclusion derived from increased heterogeneity with aging is that there is no set age at which a person becomes “elderly”. Although this book describes biological age changes observed frequently in people above age 50, that age was selected because most research on human aging has been done on people above age 50 and because many age changes do not become significant until after that age. Many individuals consider “old age” to begin at age 60, age 70, or even age 80 and beyond. Age 65, a figure commonly used to denote the onset of old age, was first used when the Social Security system was established. It was based on estimates of how long people should be fully employed so that there would be enough revenue to pay benefits to those who retired. Choosing age 65 really had nothing to do with aging. With changes in populations and government policies, the standard retirement age under Social Security has been changed to 67. This change occurred for demographic, economic and political reasons, not because it takes two additional years for people born after 1941 to become "old."

    The Concept of Biological Age

    Although there is no specific chronological age at which a person becomes biologically old, some researchers believe that determining it a person’s biological age is possible. While there are several ways to do this, all of them start by attempting to determine average values for normal people at each chronological age. In one method, the levels of functioning of organs or systems are measured under resting conditions. In another technique, the levels of functioning are measured under stressful or maximum operating conditions such as during vigorous exercise. A third procedure measures the ability of the body to maintain normal conditions under adverse conditions, for example, the ability to maintain temperature while in a cold environment. Still another approach is to find the rate at which the body returns to resting conditions after being exposed to an adverse situation such as an excessive intake of salt.

    Once the average normal values have been obtained, the measurements for the individual whose biological age is being determined are compared with those values. Scores for different functions may be considered individually, or a figure calculated from a combination of scores may be employed.

    A simple procedure for carrying out the comparison would be to find the normal group whose average score equals that of the individual being considered. The individual’s biological age could then be said to equal the chronological age of that group. Other types of comparisons of biological status among people of the same chronological age can establish the percentile rank of an individual within the group, as is done in comparisons of intelligence test scores.

    The value of determining an individual’s biological age can be greatly increased by repeating the procedure periodically, such as annually. This will provide information about the individual’s rate of aging.

    While any of these techniques can produce seemingly meaningful results, there is a lack of consensus about the validity of the procedures. Disagreements arise over which approach should be used. There is also the question of whether all the functions tested are equally important. If they are not, attempts to select the useful ones or to rank those that are used result in more discord. For example, should the ability to feel vibrations be included? What about clarity of eyesight? If these are included, is either of them more important than the resting heart rate? Or is maximum heart rate a better indicator of biological age? Then, too, are medically significant changes more important than those that affect a person’s chosen lifestyle (e.g., physical pursuits, artistic pursuits, intellectual pursuits)? Perhaps an overall biological age but only separate biological ages for the various parts of the body.

    Although this problem is far from resolved, attempts to find solutions are worthwhile for reasons similar to those that justify the study of biological aging. Once a biological age is determined for a person or a group of individuals, the factors that modify the aging processes can be discovered. This can lead to the formulation of improved care plans and can even lead to predictions of a person’s life expectancy. (Suggestion 18.01.04)

    Life Expectancy

    (These data are updated annually by the US Census Bureau and other agencies. Refer to the latest data and Updates for Chapter 1)

    Maximum Longevity

    How long can a person expect to live? The answer depends on many factors. The first factor to address is the one that establishes the longest life possible for humans. The longest life achieved by the members of a species is called the maximum longevity (XL) of that species. According to scientific records, the maximum longevity for humans is 122 years, the age attained by Jeanne Calment of France. In 1999, the oldest person was Sarah Knauss of Pennsylvania. Analysis of census and mortality data for the U.S. suggests that the human XL is probably 130 years. Human maximum longevity and the XL of other animals seem to be determined by genes. As described in Chapter 2, these genes may control activities such as the timing of life events, the time of death, the correcting of errors in other genes, and the repairing of molecules that carry out genetic instructions. (Suggestion 19.01.04) (Suggestion 19.01.05)

    While the maximum longevity in some animal species can be changed by selective breeding and genetic manipulations, some scientists believe that maximum longevity for humans probably cannot be altered. This is due largely to several limitations to altering human genes. First, ethical considerations make selective breeding of humans impossible. Second, the genes that determine maximum longevity have not been identified. Even if they were identified, the ways by which they control life span are not known and therefore are not subject to manipulation. Third, if the information and techniques needed to perform the required genetic engineering are discovered, the question of whether such interference should be carried out remains. Ethical, social, political, and economic problems will need to be addressed.

    Another reason militating against extending human XL derives from the techniques that might be required. The major discomfort or alteration in lifestyle caused by some procedures, such as the severe diet restrictions that lengthen the life span of some animals, might not be worth the possible gain in human life span. Some scientists believe that if these problems were solved, others, such as late life diseases not yet recognized, would become limiting factors. Finally, one would have to consider if having a longer life, with its many inevitable and unwanted age changes and increased likelihood of disease, is desirable. (Suggestion 18.02.03)

    Mean Longevity

    Though humans can live to an age of 122 years, this rarely happens. Even reaching the age of 100 is considered remarkable. One reason people live to different ages is the variation in genes controlling life span. Additionally, people are subjected to many other causes of death, such as accidents and disease. These other causes act before the genes determining life span have an opportunity to do so. Therefore, a statistic that is more useful for most people than maximum longevity is mean longevity (ML), the average age at which death occurs for the members of a population; this is also called the life expectancy of the population. The conditions that determine mean longevity provide the second part of the answer to the question of how long a person can expect to live. These conditions reduce life expectancy to a value less than maximum longevity.

    Statistically speaking, all people in a population have the same mean longevity at the time of birth. However, different populations have different MLs. One reason for this is the historical period in which birth occurred. For example, the mean longevity in America in 1776 was 35 years. By 1900 it had increased to about 47 years. It reached slightly over 68 by 1950 and climbed to almost 74 years of age by 1980.

    Between 1900 and 1970 the increase in mean longevity was due mostly to a decrease in the death rates of infants and children. Early in this period, poor provisions for public health (e.g., sanitation) and weak control of infectious diseases (e.g., vaccinations, antibiotics) were the main causes of high infant and child mortality (Figure 1.5). Harsh working conditions and limited education further shortened the lives not only of children but also of adults. The result was that few people lived long lives. As environmental and other external conditions improved, many more people survived the first few decades of life, and this led to a dramatic increase in mean longevity (Figure 1.6).

    Mean longevity in the United States has continued to rise since 1980. It reached 74.0 years for men and 79.4 years for women by 2000 and is expected to reach 76.5 for men and 80.8 for women by 2020. It will probably rise slowly but steadily well beyond the year 2030, reaching as high as 80.1 for men and 83.9 for women by the year 2060. Most of this increase is due more to decreased death rates for those above age 35 than to changes in death rates among younger people. The reason is that so much progress has been made in improving the extrinsic conditions that affect younger people that few advances in this area can be expected. Intrinsic factors and chronic diseases, which come into play in the later years of life, now have a more predominant influence on ML because they have become the main causes of death. This situation is expected to continue as long as human activity does not cause additional deterioration of the environment or become more self-destructive.

    Reasons other than historical periods cause differences among populations in mean longevity at birth. For example, gender affects mean longevity (ML). The population consisting of all women has a higher ML than does the population of all men. One factor contributing to this higher mean longevity seems to be that higher levels of certain hormones (estrogen and progesterone) help protect women from specific serious diseases (e.g., heart attacks). Another possible factor among women may be that female cells can use more of the genetic material (i.e., sex chromosomes) they contain. A third possible factor is that women have less iron before menopause due to periodic menstruation. With lower iron, women may sustain less damage to their molecules from free radicals (see Chapter 2). A fourth factor may be that over the past decades, lifestyles and careers traditionally involving primarily women provided less danger and stress than did those involving primarily men. Finally, men may be more willing to take serious physical risks.

    Another important factor affecting mean longevity is race. The white population has a higher mean longevity at birth than does the black population. Like the differences in mean longevity between women and men, these differences are probably due to differences in both genetics and lifestyle factors (e.g., nutrition, education, employment).

    The differences in mean longevities between sexes and among races and cultures have always existed in the United States, but the degrees of difference have not always been the same. Most recently the differences have been decreasing. It is uncertain whether these differences are more likely to decrease or increase in the next several decades.

    While all members of a population have the same mean longevity at birth, the mean longevities of individuals of different ages in that population are different (Figure 1.6). This is the case because as time passes, the death of some members of each birth cohort selects out those who do not survive well. This selection process spares those in the population who have better intrinsic characteristics for survival, better living conditions, or better mechanisms to adapt to life-shortening situations. These survivors thus have higher life expectancies. Thus, the life expectancy of those who were born in 2010 is age 76.2 years for men and 81.0 years for women, while the life expectancy of those who were 65 years of age is 82.7 years for men and 85.3 years for women. For those who were 75 years old in 2010, the corresponding figures are 86.0 and 87.9 respectively. (Table 15. Life expectancy at birth, at age 65, and at age 75, by sex, race, and Hispanic origin: United States, selected years 1900–2016 []).

    Thus far we have looked at life expectancy in broad terms. We will now point out a few examples of additional factors that help determine how long a person can expect to live.

    Some factors that influence life expectancy are fixed at birth. Here we must again mention genes. People who have parents who lived long lives tend also to live long lives. In fact, the more blood relatives with long lives a person has, the greater the chances that person has of living a long life. This is due only in part to the genes passed from one generation to the next, however. It is also due to the nurturing and culture that members of a family share.

    Two other influential characteristics that are not easily changed are intelligence and personality. Overall, people with higher intelligence and people with personalities that result in lower stress levels tend to live longer. By contrast, those whose personalities provide more stress, especially highly competitive perfectionists with a persistent sense of lacking sufficient time to accomplish their goals, tend to have lower life expectancies.

    While intelligence and personality are partly established by the time of birth, they can be modified by the environment to which an individual is exposed. Good education and positive social relationships can significantly shape and strengthen intelligence and personality, resulting in an increase in life expectancy.

    Though we have virtually no control over which genes people inherit, we can exert much influence over the environment in which people develop and live by providing conditions and opportunities known to increase life expectancy. People tend to live longer if they have proper nutrition, housing, and health care. Being employed, being married, having an adequate income, and receiving more education also increase life expectancy. Avoiding or reducing exposure to environmental insults (e.g., air pollution, smoking, excessive alcohol, toxic chemicals, radiation) are also positive influences. Living in areas where accidents are minimized also improves the chances for a longer life. Of course, preventing diseases makes a substantial contribution in this regard. Note that disease prevention is only one of many factors that increase life expectancy. It has been estimated that even if the diseases that are the top 10 causes of death were eliminated, mean longevity would increase only 11 years.

    There are, then, many conditions affecting life expectancy over which we have considerable control. Interestingly, these factors not only increase life expectancy, they also greatly affect the quality of life, including life in the later years. Modifying conditions to improve the quality of life is perhaps an even more important goal than modifying them simply to extend the length of life. Furthermore, just as with planning for our financial future and retirement, the sooner we get started and the more regular our contributions, the greater the chances for happy and successful aging.

    Status of an Individual

    Thus far, we have dealt with mean longevity, the average life expectancy for a group of people. Attempting to estimate the life expectancy of one individual would require considering all factors affecting the group. However, more information about the current biological status of the individual would also be very helpful. This is where a medical checkup or a determination of the person’s biological age becomes quite useful. An even better estimate of life expectancy can be formed if the individual is evaluated regularly to detect changes in the ability to maintain homeostasis. This can identify problems early in their development. Then steps can be taken to ward off or to compensate for the oncoming difficulty. An increase in life expectancy and in the quality of life in the years remaining could result.

    Just how long can a person expect to live? There are certain limits within which the answer lies. Although rough estimates can be made, finding the answer with accuracy is difficult. The answer depends on the unique combination of several factors that are present in a person’s life. Also, as the types and intensities of these factors change, the answer also changes. Perhaps we should be satisfied with the rough estimates and devote more time and energy to improving the quality of life we have left as we age.

    Quality of life

    Quality of life can be evaluated in several ways. When determining the quality of life of others, evaluators usually use quantitative observable parameters and use tests and interviews. A person's status in several areas may be evaluated including physical health, ability to perform activities of daily living (e.g., dressing, bathing, eating, mobility), psychological status, emotional status, economic status, social functioning, and involvement with life activities. When determining one's own quality of life, many elders use parameters different from those used by others who evaluate them. Elders often consider factors related to self-identity, sense of independence, sense of self-efficacy, sense of control of one's environment and life, and life satisfaction. (Suggestion 21.02.02)

    Evaluating quality of life and determining how to evaluate it for individuals is important in developing public policies (e.g., health care, retirement plans) and individual courses of action (e.g., purchases, finances, health care, family matters). Also, determining quality of life is needed when assessing outcomes. Through the interactions between the biology of the body and perceptions, quality of life affects health status, mean longevity, and how much contribution elders can make to society. (Suggestion 21.02.01)

    This page titled 1.5: What we know thus far is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Augustine G. DiGiovanna via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.