special report
Human aging: the enigma persists Howard J. Sanders The causes of human aging are almost as much of a mystery today as they were 10 years ago when C&EN published a two-part report on the subject, "The Enigma of Human Aging" (Feb. 12, 1962, page 138; Feb. 19, 1962, page 104). Then as now, numerous theories were under study—if only limited study —to explain the human aging process. The field abounded with hypotheses, often based on scant experimental evidence. Test results announced by one research worker were often dismissed by other scientists as inconclusive, erroneous, or based on false assumptions. Similar criticisms are heard today. For example, some scientists who have fed antioxidants to mice have claimed that the animals' longer life resulted from the action of these compounds in tying up harmful free radicals. Others claim that the mice lived longer because the animals ate less and thus gained less weight, and, as scientists have long known, reduced food intake tends to retard aging. In other experiments, research workers reported that DNA undergoes increased deleterious cross-linking with age. Subsequent studies showed that this finding was incorrect. In fact, some scientists doubt whether DNA changes at all with age. Writing last November in the New England Journal of Medicine, Dr. Samuel Goldstein, a physician at McMaster University, says, "Little is known about the origins of senescence, and little agreement exists regarding the true nature of the aging process.'' Because of the paucity of conclusive data, the field is "rife with speculation." Even though a cursory look at aging research may suggest that it is an almost hopeless morass, scientists in the field can point to evidences of progress in recent years. For example: • The growing research emphasis on some theories of aging that seem especially promising and the declining interest in theories that appear far less plausible. Scientists are spending less time on studies that, according to more and more people in the field, hold out little hope of being productive. For instance, a decade ago there was lively interest in the so-called "age pigments," which are a form of debris that accumulates in certain cells as a person grows older. This debris, which was the basis of the "clinker theory of aging," was
assumed to interfere with the functioning of the cells and thus to promote aging. Actually, no one has ever shown that age pigments are really harmful. Believing that these pigments are merely innocuous products of aging and are not a cause of aging, some scientists formerly working in this area have turned their attention to what they now regard as more fruitful lines of re- Washington Evening Star-Daily News search. • The increasing recognition that hu- past 10 years. Admittedly, this increase man aging is caused by a number of has not been spectacular. "Gerontoldifferent mechanisms operating simul- ogy," says NIH's Dr. Shock, "is not a taneously. A decade or more ago, the field in which a bright young scientist scientist who looked at aging was pre- can make an instant impact. Studies sented with a dozen or more theories to are long-term, and experimental reexplain human aging, one of which was sults and professional reputations are assumed to be correct. The problem was slow in coming. Hence, this is not a to find out which one. Now, scientists field to which large numbers of scientists are saying that more than one mecha- are irresistibly attracted." nism of aging may well be at work in the • Tlie growth in recent years of federal same individual. As Dr. Nathan W. support for aging research. The National Shock, chief of the National Institutes Institute of Child Health and Human of Health (NIH) Gerontology Research Development, which is the institute at Center in Baltimore, puts it, "Human NIH that is specifically concerned, aging may be a vastly more complex among other things, with supporting reprocess than can be explained by a search on aging, has increased its spendsingle underlying cause. Collagen, for ing in this area appreciably in the past example, probably ages for quite dif- few years. NICHD's total budget for agferent reasons than do the body's divid- ing research (research grants, training ing cells." Dr. Carl Eisdorfer, director grants, research contracts, intramural of Duke University's Center for the research, and administration) has risen Study of Aging and Human Develop- from $3 million in fiscal 1964 (not inment, says, "I think of aging funda- cluding $900,000 in intramural aging mentally as a process that results in the research funded by the National Heart destruction of cells. Since there are lots Institute) to $8 million in fiscal 1968 of ways to kill a cell, there may very well and to an estimated $11.2 million in be several mechanisms to account for fiscal 1972. Dr. Leroy E. Duncan, Jr., human aging." chief of NICHD's adult development • The improved quality of research in and aging branch, estimates that about the aging field. A decade or so ago, there 75% of these funds were devoted to was widespread criticism that many ex- studies of the biological processes of periments on aging were poorly designed aging. The remaining 25% were conand poorly carried out and thus were un- cerned with research on the behavioral likely to yield meaningful results. The aspects of aging. For some years, the caliber of research has risen noticeably Atomic Energy Commission and the since then, with greater use of more ad- Veterans Administration have likewise vanced techniques for studying aging on supported research on the biological processes of aging, although to a lesser both cellular and molecular levels. • The growing numbers of talented extent than does NICHD. • Mounting public awareness of the scientists working in the field. According to one estimate, the number of U.S. re- desirability of greater research on husearch workers studying the basic causes man aging. In its February 1972 issue, of aging (either in test animals or hu- Industrial Research disclosed the remans) has increased about 20% in the sults of a questionnaire on aging reJuly 24, 1972 C&EN
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Shock: vastly more complex process search answered by about 1850 scientists and engineers. Of these, 82% believed that aging research should be expanded. Some 81% agreed that research to retard the aging process and thus extend the human life span will provide significant benefits to society at large. • Rising concern for the aged. This is reflected not only in the widely publicized White House Conference on Aging last November but in the fact that the Subcommittee on Aging of the Senate's Labor and Public Welfare Committee held hearings in June 1971 and March 1972 to consider a bill to establish a National Institute of Aging at NIH. The subcommittee drafted and approved such a bill, which was later approved by the full committee in June and is expected to be voted on by the Senate before the end of July. In the House of Representatives, the Subcommittee on Public Health and Environment of the Interstate and Foreign Commerce Committee held hearings on a similar bill in March 1972. A House bill to establish a National Institute of Aging was approved by both the subcommittee and committee some weeks ago and was ratified by the House last week. In part, these efforts are evidence of the increasing public and governmental recognition that the aged are becoming a larger and larger percentage of the nation's growing population. Today, 10% of all Americans are 65 or older, compared to 4% in 1900. The needs of this important and increasingly vocal segment of the population clearly cannot be ignored. Over the years, numerous theories have been advanced to explain the causes of human aging. Although some theories have declined in favor, few have been totally discarded. Some scientists feel that, in view of the prevailing ignorance of what causes human aging, it would be premature to dismiss most theories that have been seriously proposed. Hardly anyone questions, for example, the idea that aging is governed in a major way by an organism's genetic inheritance or by what is sometimes called its "genetic timetable" or "genetic clock." Heredity helps to explain why fruit flies have a maximum life span of about 40 days, golden hamsters three years, blue jays four years, dogs 30 years, horses 50 years, and giant tortoises 180 years. Even though some people in remote parts of 14
C&EN July 24, 1972
the world where birth records are almost nonexistent have allegedly lived to ages of 165 years or more, scientists generally believe that the maximum life span of humans is about 120 years. In any event, there must be a reason why giant tortoises live so much longer than blue jays and why blue jays live so much longer than fruit flies. The answer is in their genetic makeup. Many research workers believe that every organism has "aging genes" that control its rate of aging and thus its maximum life span. Although no one has identified such genes, many scientists are convinced that they exist. The aging genes may explain, in part, the results disclosed in 1961 by Dr. Leonard Hayflick, then at Wistar Institute and now at Stanford University's school of medicine. In a classic experiment, he took fibroblast cells from the lung tissue of a four-month-old human embryo and found that, despite the fact that scientists had long assumed that such cells multiplied indefinitely, they were actually able to multiply only about 50 times. They then died. In contrast, such cells taken from the lung tissue of a 20-year-old man multiplied only about 20 times. Dr. Hayflick proposed that the cells eventually stopped subdividing and died not because of external forces but because of factors inherent in the cell. One inherent factor might be the genes that control longevity (aging genes). Another possibility is that a cell multiplies a finite number of times and then dies because its DNA deteriorates with time or because, after a given period, its DNA repair mechanisms no longer function properly. Although the concept of aging genes can help to explain how nature regulates the maximum life span of organisms, it does not explain the detailed processes of aging. It does not reveal the specific mechanisms by which the body loses cells, becomes more susceptible to disease, loses strength, shows reduced mental acuity, and develops all the other degenerative effects that accompany aging. A widely studied theory of aging is that, as a person grows older, his genetic material (DNA) gradually becomes impaired. This deterioration may in some cases be caused by accumulated errors in the replication of DNA. Such impairment would lead to the synthesis of defective proteins that, in turn, would not allow the body to carry out some of its normal functions. Result: the destructive processes of aging. Some research workers, such as Dr. Howard J. Curtis, a senior biologist at Brookhaven National Laboratory, have found that an increased number of abnormal chromosomes are formed in mice and dogs as they age. Other investigators, however, say that the deterioration of DNA with age has not been adequately demonstrated. Dr. Bernard L. Strehler, professor of biology at the University of Southern California, says,
"The main argument against the DNA deterioration theory of aging is that such agents as ionizing radiation, which damage DNA, do not shorten life spans in proportion to the DNA damage produced. . . . In my opinion, damage to DNA is not a primary cause of aging." Some scientists working in this general area have been investigating not DNA but the histones that are normally associated with DNA. In 1964 Dr. Holger P. von Hahn of the Institute of Experimental Gerontology in Basel, Switzerland, revealed that histones show increased and firmer binding to DNA in rat liver tissue and bovine thymus tissue from older animals than in the same type of tissue from young animals. Such binding could interfere with DNA's action as a template for RNA, which, in turn, acts as a template for the synthesis of proteins. An area of especially active research today involves the study of possible protein-synthesis errors that may result in aging. This study of the "error theory of aging" presumes that flaws in the production of proteins (including enzymes) are not necessarily the result of defects in DNA. Rather, the defects may be in any one or more of the many intermediary substances involved in making proteins. Age-dependent errors may occur, for example, in the synthetase enzymes involved in assembling amino acid chains of proteins on RNA templates. A defect in a synthetase could cause the enzyme to lose its specificity for a given amino acid. As Dr. Leslie E. Orgel, a chemist then at Cambridge and now at Salk Institute for Biological Studies, suggested in 1963, such defective enzymes could lead to an "error catastrophe" in the synthesis of proteins. The abnormal proteins formed could possibly be a basic cause of aging. One major problem here is that evidence that older people have larger amounts of abnormal proteins (including enzymes) than do young people is at present almost totally lacking. Says Dr. Shock, "There is very little proof of abnormal proteins among the aged— even though the error theory of aging looks splendid on paper. Although I think the error theory is probably valid, there is an obvious need for more data to support it." An intriguing but as yet unanswered question is: Assuming that a faulty synthetase can be produced in an older person, what causes it to form in the first place? In fact, what causes any defective protein to be produced? Some people argue that eventually, after a certain number of reactions or RNA replications occur, something is bound to go wrong in the synthesis of proteins. It's just a matter of chance, they contend. After all, no mechanism works flawlessly forever. Other scientists argue that the errors are not the result of blind chance but are specifically programed, probably by the aging genes. In other words, these
genes make sure that a certain number of errors in protein synthesis occur so that a person cannot live for more than, say, 120 years. All this, of course, is pure speculation. Evidence is growing that aging is caused, in part, by a progressive breakdown in the body's immunological system. When functioning normally, this system forms antibodies that are designed to protect the body from attack by bacteria, viruses, and other potentially damaging foreign substances. As Sir Macfarlane Burnet of the University of Melbourne and others have shown, the body can also produce antibodies that attack not foreign substances but substances that occur naturally and desirably in the body. These so-called autoantibodies have been suggested as a cause of rheumatoid arthritis, systemic lupus erythematosus, Hashimoto's thyroiditis, and other diseases. Some research workers believe that these damaging autoantibodies may also cause aging. They theorize that, as a person gets older, his immunological system becomes less able to distinguish normal proteins from foreign proteins and, as a result, proceeds to destroy normal, desirable proteins. The consequence, they say, is the deterioration of aging. No scientist, however, has ever proved that autoantibodies are a significant factor in aging. As an alternate theory, some investigators propose that aging is caused by the body's decreasing ability with age to produce enough protective antibodies to combat damaging foreign substances. Or, if a person is able to form the antibodies, he is unable to synthesize them fast enough. According to Dr. Shock, one of the main characteristics of aging is not the body's basic inability to adapt to stress but its inability to adapt rapidly enough. "Hence, the body," he says, "is quickly overwhelmed by attacking agents or other stresses before it can marshal its required defenses." Other research workers are looking into the possibility that aging is caused by those unstable, highly reactive molecular fragments known as free radicals (C&EN, June 7, 1971, page 34). Naturally produced in many living systems, free radicals, such as HO and HO2, can be involved in various oxidative reacEisdorfer: destruction of cells
tions. As Dr. Denham Harman, a pro- sidered fairly implausible and, therefessor of medicine and biochemistry at fore, are receiving only limited attenthe University of Nebraska, points out, tion. these oxidative reactions can lead to the Some scientists, for example, have deterioration of lipids, collagen, elastin, been examining the possibility that agand other body substances and thus ing is caused partly by harmful ionizing possibly cause at least some of the radiation from rocks, soil, and outer phenomena of aging. space. Other research workers are lookSome scientists have reported that the ing into the possibility that substances age pigments (also known as lipofuscin) in the diet, such as polyunsaturated are produced in cells partly by the reac- lipids, promote human aging. Some theories of human aging have by tion of free radicals with unsaturated lipids, to form peroxidation products. now been largely rejected. One of these As mentioned earlier, these pigments, is the "watch-spring theory" that claims which increase in concentration in cells that the human body, like a watch as a person grows older, have yet to be spring, gradually ceases to function beshown to be involved in the damaging cause its stored energy runs out. This analogy is dismissed by almost all sciprocesses of aging. Some investigators suggest that free entists as naive or absurd because, as a radicals may be especially harmful person ages, his body's energy does not when they attack unsaturated lipids in really "run out." Dr. Ewald W. Busse cell membranes. The resulting altered of Duke University describes this theory, lipids may seriously impede the dif- which was once taken fairly seriously by fusion of materials through the mem- some people, as "too simplistic." brane—the flow of nutrients into the Underlying much research on the cell and the flow of waste products out causes of aging is the hope that greater of the cell. This, they suggest, may lead knowledge of this process will lead to the cell death associated with aging. eventually—perhaps in 20 or 30 years— Also under study is the cross-linking to effective ways to inhibit the process theory of aging. This theory proposes and thus lengthen the human life span. that aging is caused by the progressive To date, however, no theory of human intermolecular cross-linking of proteins aging and no experimental evidence reor nucleic acids. Some investigators garding the possible factors that retard suggest that cross-linking may prevent aging have resulted in any practical and some protein molecules from being scientifically verifiable methods for metabolized by the body's enzymes. As slowing the aging process. a result, these proteins accumulate in Nevertheless, some scientists have the cells as "frozen metabolic pools," proposed various techniques to inhibit which clog the cells and ultimately de- aging. Unfortunately, many of these stroy them. suggested methods, based on experiA substance especially noted for form- ments in laboratory animals and simpler ing cross-linkages in the body is colla- organisms, may very well cause more gen, the major protein of connective harm than good in humans—at least in tissue. The cross-linking of collagen, the foreseeable future. Hence, many inmany scientists believe, may be re- vestigators view these proposed methods sponsible in part for the chemical and with skepticism. mechanical changes that occur with age Some people have suggested that in skin, tendon, bone, blood vessels, and aging might be retarded if the human other body components. Dr. Karl A. body temperature were lowered. The Piez and coworkers at NIH have found University of Southern California's Dr. that the cross-linking of collagen is a Strehler has predicted that, if the hunormal maturing process that takes man body temperature could be reduced place during the body's growth and de- by 2° to 3° C., the human life span velopment. However, says Dr. Piez, "No would "almost certainly" be extended evidence exists that this normal process by about 25%—or by about 20 years. can continue into later life and become Proposals to lengthen life spans by depathological." creasing body temperature have been Dr. Goldstein of McMaster University based entirely on research in test anisays, "At this juncture, there is no evi- mals, some of which are relatively simdence to incriminate collagen as the pri- ple organisms. In 1965 Dr. Charles H. mary agent in senescence." Charles G. Barrows, Jr., of NIH's Gerontology ReKormendy of Bristol Laboratories de- search Center showed that, when roticlares, "Collagen cross-linking is not a fers (cold-blooded aquatic animals about the size of a pinhead) are kept in water cause but simply a result of aging." Looking at the overall cross-linking at 35° C , their life span is only about 18 theory, Dr. Shock believes that this con- days. However, when they are kept in cept of aging is currently stuck on dead water at a normal temperature of 25° C , center. He points out, for example, that their life span is about 34 days. no one has demonstrated thus far that In 1917 Dr. Jacques Loeb and Dr. age-associated cross-linking occurs in- John H. Northrop at Rockefeller Instiside cells, where it could conceivably tute found that, when fruit flies (also cause significant cell damage and pro- cold-blooded organisms) are kept at mote aging. 19° C. instead of the usual 25° C , their Other theories of human aging are life span is doubled. Dr. Roy L. Walford, also being explored. Some possible a professor of pathology at the Univermechanisms, however, are now con- sity of California, Los Angeles, disclosed July 24, 1972 C&EN
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Volunteer in his 70's is tested for muscle strength at NIH center in 1970 that the life span of the tropical fish Cynolebias could be doubled by lowering its normal water temperature by 5° or 6° C. Formidable problems arise, however, when people attempt to extrapolate to humans the results reported in coldblooded animals. Man has a thermostat in his brain's hypothalamus that tends to maintain his body temperature at 37° C. Any attempt to reset or override this thermostat may have serious adverse effects. Dr. Clarence L. Hebert, chief of anesthesiology at NIH, believes that lowering the human body temperature by 2° or 3° C. in an unsedated or unanesthetized person would be impractical, if not dangerous. The person would suffer severe chills, violent shivering, and extreme discomfort. His metabolism would be sluggish, and his mental processes would probably be impaired. Over an extended period, such a lowering of the body temperature might very well shorten human life spans, not lengthen them. On the assumption that aging is caused or promoted by free radicals, some scientists, such as Nebraska's Dr. Harman, have proposed that human life span might be increased by using antioxidants, which tie up free radicals and inactivate them. He or others have proposed the possible use of such antioxidants as butylated hydroxytoluene (BHT), butylated hydroxy anisole (BHA), ethoxyquin, 2-mercaptoethylamine, or vitamin E. In 1966, Dr. Harman disclosed that adding 0.5% of BHT to the diets of laboratory mice increased their mean life span by 30 to 40%. Equally good results, he said, were obtained by feeding mice ethoxyquin. Worth noting is the fact that some antioxidants proposed as a means of increasing longevity, such as BHT and BHA, are already added to foods, but in concentrations carefully regulated by the Food and Drug Administration. BHT, for instance, can be added to animal fats and shortenings in maximum concentrations of 0.01%. The compound 16 C&EN July 24, 1972
can be added to dry breakfast cereals at levels of no more than 0.005%. It is also worth noting that ethoxyquin, which showed encouraging results in mice, is not even permitted by FDA for use in foods. The compound can, however, be used as an antioxidant in animal feeds and forage crops, as well as in various industrial products, such as rubber. Of course, the possibility always exists that, even if antioxidants currently available are not safe at the doses required, other antioxidants might eventually be developed that would be safe. All this assumes that an antioxidant of any type would really be useful in retarding human aging. Back in 1932, Dr. Clive M. McCay of Cornell University reported a landmark experiment in which he showed the amazing effects of curtailing food intake to increase longevity. When he took very young rats and fed them diets sharply reduced in calories, some of them lived as long as 1450 days. On the other hand, rats fed conventional diets seldom lived more than 965 days. Other research workers have shown that underfeeding likewise increases the longevity of rotifers, silkworms, fruit flies, bees, chickens, and other animals. The life span of a rotifer, for example, can be increased from a normal 34 days to as much as 55 days by reducing its food intake. The reasons for the greater longevity are not known. Might marked underfeeding also extend the lives of humans? Many scientists are skeptical. They point out that, in the case of virtually all of the animals studied to date, the low-calorie feeding must begin when the animal is quite young. As a result, the animal's growth is markedly stunted. If diets drastically reduced in calories were fed to children, they might never grow to heights of more than about 5% feet. Also, they might develop greatly increased susceptibility to disease. In any study to measure the effectiveness of a proposed method to increase longevity, the problems can be horrendous. One of the biggest hurdles is time. If a scientist starts testing a supposed life-extending compound in a group of volunteers, ages 20 to 30, the investigator might have to wait 60 years or more before all the volunteers die and provide the data the scientist is looking for. By that time, however, the investigator himself might be dead. Faced with this obstacle, scientists have been attempting to devise ways to determine the rate of human aging over relatively short periods. In this way, researchers would not have to wait until a person died to find out whether a specific treatment increased or decreased his rate of aging. Dr. Reubin Andres, chief of the clinical physiology branch at NIH's Gerontology Research Center, believes that it might well be possible to determine in as short a time as five years whether an antiaging drug is really effective.
In recent years, scientists at the Gerontology Research Center, beginning on a small scale with only about 100 volunteers in 1959, have been studying the effects of aging in a group of about 650 healthy men from the ages of about 20 to 95. These men come to the center once every 18 months if under 70 years of age or once every 12 months if 70 or over to receive a battery of about 80 clinical, physiological, biochemical, and psychological tests. These tests are generally given every visit, although some are given only once every four visits. The tests measure such variables as grip strength, arm strength, skin elasticity, heart and kidney function, glucose metabolism, serum cholesterol, serum proteins, hemoglobin, blood platelets, maximum breathing capacity, fluid pressure in the eye, hearing ability, problem-solving ability, memory ability, and so on. The results show how these variables change with age. Since the men receive no treatment specifically designed to retard their aging, the data can be used to calculate an average baseline, against which the results in treated volunteers might later be compared. Thus far, the findings show, as might be expected, that a tested characteristic, such as grip strength, differs considerably from person to person, even when all are of the same age. The results also indicate that, as an individual grows older, this characteristic has its ups and downs. Thus, although a person's grip strength, as viewed over the long term, clearly deteriorates with age, it may actually be greater when measured in 1972 than when measured in 1971 or possibly even in 1970. What with all the research going on in the aging field, when will significant, tangible progress be made in extending life spans with an antiaging drug or other treatment? Dr. Alex Comfort, director of gerontology research at London's University College and one of the most articulate and ebullient scientists in the field, predicts, "By 1990 we will know of an experimentally tested way of slowing down age changes in man that offers an increase of 20% in life span." The method, he believes, will involve the use of chemicals or special diets. Dr. Busse of Duke predicts that, in 10 years, a drug or other treatment will be available to retard the aging processes in the brain. Such treatment, he says, will be used initially in patients who are prematurely senile. Striking a realistic and, at the same time, optimistic note, Dr. Gunther L. Eichhorn, head of the molecular chemistry section at NIH's Gerontology Research Center, says, "At present, no one can honestly recommend, on the basis of valid experimental evidence, any treatment to retard human aging. Hopefully, however, as we gain a better understanding of the fundamental aging process, we will discover a genuinely rational approach to inhibiting aging in man."
