The Modulation of Longevity

Chapter 9 discussed the mythical claims, ancient and modern, that humans can have very long lifespans. This one is devoted to the various ways in which ageing and longevity can be modulated, including either an increase or a decrease. We know that there are at least three major components that determine the expectation of life at birth. One is the species itself, because in mammals maximum lifespan varies by at least 30-fold, and if we accept the reports of extreme longevity in some whales, probably 60-fold. The second is the environment, which can obviously limit natural lifespan through predation, starvation and disease. Even in a protected environment of a zoo, the environment, including food, and possibly the opportunities for normal well-being and behaviour, are likely to influence longevity. The third, is an intrinsic variation of the determinants of longevity within the animal itself. We saw in Chapter 1 that identical twins do not have have identical lifespans, and even more telling are the inbred mice which have the same genes and live in the same environment. Their lifespans vary considerably, showing that chance or random events are very important determants. It would therefore be surprising if many ways and means were uncovered that had clear measurable effects on longevity.

One of the best examples is the effect of food deprivation on lifespan. It was discovered many decades ago that a low calorie diet significantly increases the lifespan of mice and rats. When animals receive only about 60% of what they would normally consume, their lifespan is increased by 40-50%. Moreover, the significant pathological changes that are normally seen during senescence are very significantly delayed. Another strong effect is on reproduction, because calorie-restricted rodents lose fertility, and may become sterile. This provides a strong biological clue as to why reduced food increases lifespan. In natural environments, small ground living animals are very likely to encounter a variable supply of food. A food glut allows continual reproduction, and populations may rapidly increase. In contrast, a shortage of food makes reproduction very difficult, and young animals may die from malnutrician. It is now widely agreed that the increased longevity of calorie-restricted animals is an evolutionary adaptation. The adaptation is cessation of breeding, and instead that resources available are chanelled into maintaining the adult in a healthy state. When the food supply increases, reproduction begins again. It has been established experimentally that calore-restricted females, when provided with a normal diet, can breed at later ages than animals kept throughout on a full diet.

An important question is whether calorie restriction increases lifespan in other mammalian species, and particularly larger species with much longer longevities than rodents, such as primates and humans. Experiment with monkeys are underway, but they take a very long time to complete, and the results so far cannot be regarded as conclusive. There are those who believe that large slow breeding mammals that have a fairly constant diet, such as tropical monkeys, will not respond to a low calorie one in the same way as rodents. However, we know that human females who are very physically active, such as ballet dancers and some athletes, may cease to menstruate. So in these cases the diversion of much available energy to physical activity, reduces fertility. It might be possible to obtain epidemio-logical data about human diets and longevities, but so far no definitive information has been obtained. There is a large literature that discusses the possibility of substantial increases in human lifespan by reducing calorie input. In the whole of this literature is is rarely mentioned that calorie-restriction has strong effects on fertility and presumably libido.

Another well documented example of increased lifespan was obtained by the Australian gerontologist Arthur Everitt. He removed the anteria pituitary of young rats (hypophysectomy) and discovered that this had an effect on lifespan comparable to calorie-resticted animals. There have been many studies of hormones on ageing, and claims that hormone treatment increase longevity. One is human growth hormone, because elderly males report increased muscular strength or other rejuvenating effects after a course of treatment. Another is melatonin, produced by the pineal gland in the brain, which is important for the control of sleep rythms. A number of extravagent claims have been made about about its beneficial effects in preventing age-related disease, or increasing the longevity of mice. The hormone DHEA (dihydroepiandrosterine) has also been credited with strong anti-ageing effects. There is no doubt that experiments with hormones will continue for a long time to come, and it is likely that modulating effects on longevity will be uncovered. However, the extreme effects discussed in Chapter 9 are very unlikely to be seen.

Antioxidants have become particularly popular, especially in the U.S.A., and these include Vitamin E and Vitamin C, and also carotene. Their efficacy is related to the strong view, which in some quarters is a belief, that the major cause of ageing is the damage produced by oxygen free radicals. If this is so, then antioxidants should be effective in reducing this damage, a conclusion that has been widely accepted by the public at large. The facts are not always easy to obtain. Lifespan experiments have been done with experimental animals provided with a diet containing one or more antioxidants in comparison to untreated control animals. Early reports suggested that life-extension was achieved, but those have not been repeated. A recent experiment carried our in Alec Morley's laboratory in Adelaide, demonstrated absolutely no effect of vitamin E on the longevity of mice. It is a reasonable presumption that other experiments that gave negative results have never been published, and remain unknown. One can, in fact, be fairly sure that any experiment that gave a positive result would not only be published, but would also be given extensive coverage in magazines and newspaper articles.

A detailed long-term study was carried out in Finland with physicians who supplemented their diet with Vitamin E, or Vitamin C, or both, or neither. Their health was carefully monitored over a number of years, and the incidence of age-associated maladies or disease was properly monitored. There was no evidence that either anti-oxidant had any rejuvenating effects, nor significantly altered the normal incidence of age-related pathologies. (There was one possible exception among the twenty or more studied, but note that routine statistical tests of significance give, by chance, a significant result once in 20 tests). Not surprisingly, the result of this important study has not received much publicity.

An interesting suggestion has been made by the gerontologist Roy Walford. In cold-blooded animals it is well known that lifespan is related to body temperature, that is, the lower the temperature the longer the lifespan. Walford pointed out that this might apply to warm blooded animals as well. If body temperature was reduced to 34° C or 35° C, lifespan might be very significantly longer. Although there are means to change body temperature within certain limits, obviously the proposal cannot be tested directly. However, an indirect test exists. Human cells in culture have a well defined lifespan, so in my laboratory years ago we decided to look at the effect of incubation temperature. It was found that cells grown at 34° C had exactly the same lifespan as those grown at the normal temperature of 37° C. On the other hand cells at 40° C had a sharply reduced lifespan. This experimental system has been exploited in other ways to try to uncover the causes of cellular senescence. Many years ago it was reported that the addition of Vitamin E to the culture medium doubled the lifespan of these cells. Unfortunately this claim had to be retracted, because the result could never be repeated.

Another more recent study in my laboratory was carried out with a small naturally occurring peptide known as carnosine. This is present at high concentration in tissues such as brain and muscle. Carnosine consists of an unusual amino acid not found in proteins called (Beta alanine, linked to one of the normal amino acids, histidine). It was found that human cells grown in the presence of a physiological concentration of carnosine had a significantly increased lifespan. Moreover, the cells retained a juvenile appearance throughout growth, whereas normal cells develop a number of obvious abnormalities when they become senescent.

Senescent cells switched to a medium containng carnosine were rejuvenated. Such rejuvenated cells when deprived of carnosine quickly become senescent. The cells used are called fibroblasts, which secrete collagen and are an important component of skin tissue. Carnosine has been marketed under the trade name (Beta-alistine), for the prevention of skin ageing. It is possible that the peptide has an important role in cell maintenance, and it is highly significant that the amount in human muscle tissue is about twenty times higher than that in mouse tissue. Another treatment has also been discovered which has the effect of preventing the usual features of senescence in long-term cultures of human cells. This is a well known plant hormone kinetin, and it is effective at very low concentrations. There is no information about the effects of carnosine or kinetin on the longevities of experimental animals.

If any new treatment or chemical is reported to increase longevity, it receives wide publicity. Even ageing experiments in yeast (where the finite number of new cells budded off a mother cells is counted), are extrapolated to extravagent claims that the same treatmnent would increase human longevity. This was the case for resveritol, which is present in red wine and some other plants or their products. It was reported to increase life spans in several different experimental systems. However, other laboratories have not seen the same effects.

Another approach is known hormesis, which is the response to repeated treatments that induce mild stress. Such stress can be induced by heat shock, pro-oxidants, heavy metals and irradiation, and it is reported that the overall regime of treatments, has measurable beneficial effects in several experimental systems. The interpretation is that cells that are induced to protect themselves from stress (which is a well known phenomenon), also turn on cell maintenance mechanisms that are beneficial. Extrapolating to humans is a difficult question, because repeated mild doses of irradiation, or low levels of heavy metals, would hardly receive medical approval, especially as any beneficial outcomes might take years to be seen.

One of the documented effects of irradiation is in fact a shortening of the lifespan. There were many studies done after the second world war, and the earlier senescence of irradiated animals was very well documented. There is now little discussion of these results, and some claim that the induced premature ageing was different from normal ageing. However, in the most thorough studies, post-mortem examinations showed that there was little, if any, difference in causes of death in the treated and control animals. A major target for irradiation in cells is DNA, and it is likely that DNA damage is responsible for the shortened life span. The important damage may be of a type that evades normal repair, but to this day it has not been identified.

There is now huge interest in life style effects on disease, general well-being, and that part of the whole lifespan that is sometimes called the healthspan. In addition to the expectation of life at birth, there is now a recognised statistic known as the expectation of a healthy life. Obviously it is the aim of gerontologists and geriatricians to bring the expectation of a healthy life as close as possible to the total expectation. With early diagnosis and increasingly successful treatments this aim is to some extent being achieved. Much more needs to be learned, for example, about the long term effects of excercise, or of particular diets on the healthspan. Unfortunately, much is also being learned about the long terms effects of unhealthy diets. Obesity has become something of an epidemic in some developed countries, and particularly in some communities in the USA. This is associated with the appearance of late onset diabetes, which has many deleterious side effects that together bring about a reduction in lifespan. This is a major cause of the shorter expectation of life of aboriginal people in Australia, nearly 20 years in comparison to the rest of the population. The primary cause is probability the availability of cheap carbohydrate foods, coupled with a scarcity of money. One would have thought that this problem would be solvable.

There is a great deal to be learned about lifesyle and longevity. One way to obtain information over a long time scale is known as a longitudinal study. The most important of these was started nearly fifty years on the initiative of Nathan Shock, and it is known the Baltimore Longitudinal Study of Ageing. Healthy volunteers from all age groups are enrolled, and then examined and tested every two years, using over 100 procedures. The study documents life-style parameters, "biomarkers" of ageing, health and illness for a substantial number of individuals, from the time they volunteer to the time of their death. Not as much information has come out of the study as one might have hoped, but presumably there will be definitive reports and publications.

Western developed coutries have grown used to a gradual increase in the expectation of life at birth, and with the increasing success in the treatment of age-associated disease, this trend will continue. Yet the total elimination of carcinomas as a cause of death, the expectation of life would not be greatly increased. The main reason for this is that there are multiple causes of ageing and the elimination of one does not affect the other. So if you do not die from cancer, you may die from a heart attack, from a stroke, from kidney failure, and so on. It has been calculated that if all major diseases were eliminated the expectation of life would increase by about 15 years. There are still remain less important age-associated diseases, and a multiplicity of cellular and molecular defects accumulating all the time. Apart from everything else, the treatment of every new pathology as it arises becomes prohibitively expensive. It is yet another example of the law of diminishing returns - the more expensive the treatment becomes the smaller the gain in terms of healthy life is gained.

How To Add Ten Years To Your Life

How To Add Ten Years To Your Life

When over eighty years of age, the poet Bryant said that he had added more than ten years to his life by taking a simple exercise while dressing in the morning. Those who knew Bryant and the facts of his life never doubted the truth of this statement.

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