In 1935 scientists at Cornell University made an extraordinary discovery. By placing rats on a very low calorie diet, Clive M. McCay and his colleagues extended the outer limit of the animals' life span by 33 percent, from three years to four. They subsequently found that rats on low-calorie diets stayed youthful longer and suffered fewer late-life diseases than did their normally fed counterparts.Since the 1930s, calorie restriction has been the only intervention shown convincingly to slow aging in rodents (which are mammals, like us) and in creatures ranging from single-celled protozoans to roundworms, fruit flies and fish.
Naturally, the great power of the method raises the question of whether it can extend survival and good health in people. That issue is very much open, but the fact that the approach works in an array of organisms suggests the answer could well be yes. Some intriguing clues from monkeys and humans support the idea, too.
Of course, even if calorie austerity turns out to be a fountain of youth for humans, it might never catch on. After all, our track record for adhering to severe diets is poor. But scientists may one day develop drugs that will safely control our appetite over the long term or will mimic the beneficial influences of calorie control on the bodys tissues. This last approach could enable people to consume fairly regular diets while still reaping the healthful effects of limiting their food intake. Many laboratories, including mine at the University of Wisconsin-Madison, are working to understand the cellular and molecular basis of how calorie restriction retards aging in animals. Our efforts may yield useful alternatives to strict dieting, although at the moment most of us are focused primarily on understanding the aging process (or processes) itself.
Less Is More for Rodents
RESEARCH into calorie restriction has uncovered an astonishing range of benefits in animals--provided that the nutrient needs of the dieters are guarded carefully. In most studies the test animals, usually mice or rats, consume 30 to 50 percent fewer calories than are ingested by control subjects, and they weigh 30 to 50 percent less as well. At the same time, they receive enough protein, fat, vitamins and minerals to maintain efficient operation of their tissues. In other words, the animals follow an exaggerated form of a prudent diet, in which they consume minimal calories without becoming malnourished.
If the nutrient needs of the animals are protected, calorie restriction will consistently increase not only the average life span of a population but also the maximum life span--that is, the lifetime of the longest-surviving members of the group. This last outcome means that calorie restriction tinkers with some basic aging process. Anything that forestalls premature death, such as is caused by a preventable or treatable disease or by an accident, will increase the average life span of a population. But one must truly slow the rate of aging in order for the hardiest individuals to surpass the existing maximum.
Beyond altering survival, low-calorie diets in rodents have postponed most major diseases that are common late in life [see box on page 57], including cancers of the breast, prostate, immune system and gastrointestinal tract. Moreover, of the 300 or so measures of aging that have been studied, some 90 percent stay "younger" longer in calorie-restricted rodents than in well-fed ones. For example, certain immune responses decrease in normal mice at one year of age (middle age) but do not decline in slimmer but genetically identical mice until age two. Similarly, as rodents grow older they generally clear glucose, a simple sugar, from their blood less efficiently than they did in youth (a change that can progress to diabetes); they also synthesize needed proteins more slowly, undergo increased cross-linking (and thus stiffening) of long-lived proteins in tissues, lose muscle mass and learn less rapidly. In calorie-restricted animals, all these changes are delayed.
Not surprisingly, investigators have wondered whether calorie (energy) restriction per se is responsible for the advantages reaped from low-calorie diets or whether limiting fat or some other component of the diet accounts for the success. It turns out the first possibility is correct. Restriction of fat, protein or carbohydrate without calorie reduction does not increase the maximum life span of rodents. Supplementation alone with multivitamins or high doses of antioxidants does not work, and neither does variation in the type of dietary fat, carbohydrate or protein.
The studies also suggest, hearteningly, that calorie restriction can be useful even if it is not started until middle age. Indeed, the most exciting discovery of my career has been that calorie restriction initiated in mice at early middle age can extend the maximum life span by 10 to 20 percent and can oppose the development of cancer. Further, although limiting the calorie intake to about half of that consumed by free-feeding animals increases the maximum life span the most, less severe restriction, whether begun early in life or later, also provides some benefit.
Naturally, scientists would be more confident that diet restriction could routinely postpone aging in men and women if the results in rodents could be confirmed in studies of monkeys (which more closely resemble people) or in members of our own species. To be most informative, such investigations would have to follow subjects for many years--an expensive and logistically difficult undertaking. Nevertheless, two major trials of monkeys are in progress.
Lean, but Striking, Primate Data
IT IS TOO EARLY to tell whether low-calorie diets will prolong life or youthfulness in the monkeys over time. The projects have, however, been able to measure the effects of calorie restriction on so-called biomarkers of aging: attributes that generally change with age and may help predict the future span of health or life. For example, as primates grow older, their blood pressure and their blood levels of both insulin and glucose rise; at the same time, insulin sensitivity (the ability of cells to take up glucose in response to signals from insulin) declines. Postponement of these changes would imply that the experimental diet was probably slowing at least some aspects of aging.
One of the monkey studies, led by George S. Roth of the National Institute on Aging, began in 1987. It is examining rhesus monkeys, which typically live to about 27 years and sometimes reach 40 years, and squirrel monkeys, which rarely survive beyond 20 years. Some animals began diet restriction in youth (at one to two years), others after reaching puberty. The second project, involving only rhesus monkeys, was initiated in 1989 by William B. Ershler, Joseph W. Kemnitz and Ellen B. Roecker of the University of WisconsinMadison; I joined the team a year later. Our monkeys began calorie restriction as young adults, at eight to 14 years old. Both studies enforce a level of calorie restriction that is about 30 percent below the intake of normally fed control subjects.
So far the preliminary results are encouraging. The dieting animals in both projects seem healthy and happy, albeit eager for their meals, and their bodies seem to be responding to the regimen much as those of rodents do. Blood pressure and glucose levels are lower than in control animals, and insulin sensitivity is greater. The levels of insulin in the blood are lower as well.
No one has yet performed carefully controlled studies of long-term calorie restriction in average-weight humans over time. And data from populations forced by poverty to live on relatively few calories are uninformative, because such groups generally cannot attain adequate amounts of essential nutrients. Still, some human studies offer indirect evidence that calorie restriction could be of value. Consider the people of Okinawa, many of whom consume diets that are low in calories but provide needed nutrients. The incidence of centenarians there is high--up to 40 times greater than that of any other Japanese island. In addition, epidemiological surveys in the U.S. and elsewhere indicate that certain cancers, notably those of the breast, colon and stomach, occur less frequently in people reporting low calorie intakes.
Intriguing results were also obtained after eight people living in a self-contained environment--Biosphere 2, near Tucson, Ariz.--were forced to curtail their food intake sharply for two years because of poorer than expected yields from their food-producing efforts. The scientific merits of the overall project have been questioned, but those of us interested in the effects of low-calorie diets were fortunate that the late Roy L. Walford of the University of California, Los Angeles, who was an expert on calorie restriction and aging (and was my scientific mentor), was the teams physician. Walford helped his colleagues avoid malnutrition and monitored various aspects of the groups physiology. His analyses reveal that calorie restriction led to lowered blood pressure and glucose levels--just as it does in rodents and monkeys. Total serum cholesterol declined as well.
The results in monkeys and humans may be preliminary, but the rodent data show unequivocally that calorie restriction can exert a variety of beneficial effects. This variety raises something of a problem for researchers: Which of the many documented changes (if any) contributes most to increased longevity and youthfulness? Scientists have not yet reached a consensus, but they have ruled out a few once viable proposals. For instance, it is known that a low intake of energy retards growth and also shrinks the amount of fat in the body. Both these effects were once prime contenders as the main changes that lead to longevity but have now been discounted.
Several other hypotheses remain under consideration, however, and all of them have at least some experimental support. One such hypothesis holds that calorie restriction slows the rate of cell division in many tissues. Because the uncontrolled proliferation of cells is a hallmark of cancer, that change could potentially explain why the incidence of several late-life cancers is reduced in animals fed low-calorie diets. Another proposal is based on the finding that calorie restriction tends to lower glucose levels. Less glucose circulating in the blood would slow the accumulation of sugar on long-lived proteins and would thus moderate the disruptive effects of this buildup.
A Radical Explanation
THE VIEW that has so far garnered the most convincing support, though, holds that calorie restriction extends survival and vitality primarily by limiting injury of mitochondria by free radicals. Mitochondria are the tiny intracellular structures that serve as the power plants of cells. Free radicals are highly reactive molecules (usually derived from oxygen) that carry an unpaired electron at their surface. Molecules in this state are prone to destructively oxidizing, or snatching electrons from, any compound they encounter. Free radicals have been suspected of contributing to aging since the 1950s, when Denham Harman of the University of Nebraska Medical School suggested that their generation in the course of normal metabolism gradually disrupts cells. But it was not until the 1980s that scientists began to realize that mitochondria were probably the targets hit hardest.
The mitochondrial free-radical hypothesis of aging derives in part from an understanding of how mitochondria produce ATP (adenosine triphosphate)--the molecule that provides the energy for most cellular processes, such as pumping ions across cell membranes, contracting muscle fibers and constructing proteins. ATP synthesis occurs by a very complicated sequence of reactions, but essentially it involves activity by a series of molecular complexes embedded in an internal membrane--the inner membrane--of mitochondria. With help from oxygen, the complexes extract energy from nutrients and use that energy to manufacture ATP.
Unfortunately, the mitochondrial machinery that draws energy from nutrients also produces free radicals as a by-product. Indeed, mitochondria are thought to be responsible for creating most of the free radicals in cells. One such by-product is the superoxide radical (O2.–). (The dot in the formula represents the unpaired electron.) This renegade is destructive in its own right but can also be converted into hydrogen peroxide (H2O2), which technically is not a free radical but can readily form the extremely aggressive hydroxyl free radical (OH.–).
Once formed, free radicals can damage proteins, lipids (fats) and DNA anywhere in the cell. But the components of mitochondria--including the ATP-synthesizing machinery and the mitochondrial DNA that gives rise to some of that machinery--are believed to be most vulnerable. Presumably they are at risk in part because they reside at or near the "ground zero" site of free-radical generation and so are constantly bombarded by the oxidizing agents. Moreover, mitochondrial DNA lacks the protein shield that helps to protect nuclear DNA from destructive agents. Consistent with this view is that mitochondrial DNA suffers much more oxidative damage than does nuclear DNA drawn from the same tissue.
Proponents of the mitochondrial free-radical hypothesis of aging suggest that damage to mitochondria by free radicals eventually interferes with the efficiency of ATP production and increases the output of free radicals. The rise in free radicals, in turn, accelerates the oxidative injury of mitochondrial components, which inhibits ATP production even more. At the same time, free radicals attack cellular components outside the mitochondria, further impairing cell functioning. As cells become less efficient, so do the tissues and organs they compose, and the body itself becomes less able to cope with challenges to its stability. The body does try to counteract the noxious effects of the oxidizing agents. Cells possess antioxidant enzymes that detoxify free radicals, and they make other enzymes that repair oxidative damage. Neither of these systems is 100 percent effective, though, and so such injury is likely to accumulate over time.
Experimental Support
THE PROPOSAL that aging stems to a great extent from free-radical-induced damage to mitochondria and other cellular components has been buttressed by a number of findings. In one striking example, Rajindar S. Sohal of the University of Southern California, William C. Orr of Southern Methodist University and their colleagues investigated rodents and several other organisms, including fruit flies, houseflies, pigs and cows. They noted increases with age in free-radical generation by mitochondria and in oxidative changes to the inner mitochondrial membrane (where ATP is synthesized) and to mitochondrial proteins and DNA. They also observed that greater rates of free-radical production correlate with shortened average and maximum life spans in several of the species.
It turns out, too, that ATP manufacture decreases with age in the brain, heart and skeletal muscle, as would be expected if mitochondrial proteins and DNA in those tissues were irreparably impaired by free radicals. Similar decreases also occur in human tissues and may help explain why degenerative diseases of the nervous system and heart are common late in life and why muscles lose mass and weaken.
Some of the strongest support for the proposition that calorie restriction retards aging by slowing oxidative injury of mitochondria comes from Sohals group. When the workers looked at mitochondria harvested from the brain, heart and kidney of mice, they discovered that the levels of the superoxide radical and of hydrogen peroxide were markedly lower in animals subjected to long-term calorie restriction than in normally fed controls. In addition, a significant increase of free-radical production with age seen in the control groups was blunted by calorie restriction in the experimental group. This blunted increase was, moreover, accompanied by lessened amounts of oxidative insult to mitochondrial proteins and DNA. Other work indicates that calorie restriction helps to prevent age-related changes in the activities of some antioxidant enzymes--although many investigators, including me, suspect that strict dieting ameliorates oxidative damage mainly through slowing free-radical production.
Applications to Humans?
BY WHAT MECHANISM might calorie restriction reduce the generation of free radicals? No one yet knows. One proposal holds that a lowered intake of calories may somehow lead to slower consumption of oxygen by mitochondria--either overall or in selected cell types. Alternatively, low-calorie diets may increase the efficiency with which mitochondria use oxygen, so that fewer free radicals are made per unit of oxygen consumed. Less use of oxygen or more efficient use would presumably result in the formation of fewer free radicals. Studies also intimate that calorie control may minimize free-radical generation in mitochondria by reducing levels of a circulating thyroid hormone known as triiodothyronine, or T3, through unknown mechanisms.
Until research into primates has progressed further, few scientists would be prepared to recommend that large numbers of people embark on a severe calorie-restriction regimen. Nevertheless, the accumulated findings do offer some concrete lessons for those who wonder how such programs might be implemented in humans.
One implication is that sharp curtailment of food intake would probably be detrimental to children, considering that it retards growth in young rodents. Also, because children cannot tolerate starvation as well as adults can, they would presumably be more susceptible to any as yet unrecognized negative effects of a low-calorie diet (even though calorie restriction is not equivalent to starvation). An onset at about 20 years of age in humans should avoid such drawbacks and would probably provide the greatest extension of life.
The speed with which calories are reduced needs to be considered, too. Early researchers were unable to prolong survival of rats when diet control was instituted in adulthood. I suspect the failure arose because the animals were put on the regimen too suddenly or were given too few calories, or both. Working with year-old mice, my colleagues and I have found that a gradual tapering of calories to about 65 percent of normal did increase survival.
How might one determine the appropriate calorie intake for a human being? Extrapolating from rodents is difficult, but some findings imply that many people would do best by consuming an amount that enabled them to weigh 10 to 25 percent less than their personal set point. The set point is essentially the weight the body is "programmed" to maintain, if one does not eat in response to external cues, such as television commercials. The problem with this guideline is that determining an individuals set point is tricky. Instead of trying to identify their set point, dieters (with assistance from their health advisers) might engage in some trial and error to find the calorie level that reduces the blood glucose or cholesterol level, or some other measures of health, by a predetermined amount.
The research in animals further implies that a reasonable calorie-restriction regimen for humans might involve a daily intake of roughly one gram (0.04 ounce) of protein and no more than about half a gram of fat for each kilogram (2.2 pounds) of current body weight. The diet would also include enough complex carbohydrate (the long chains of sugars abundant in fruits and vegetables) to reach the desired level of calories. To attain the standard recommended daily allowances for all essential nutrients, an individual would have to select foods with extreme care and probably take vitamins or other supplements.
Anyone who contemplated following a calorie-restriction regimen would also have to consider potential disadvantages beyond hunger pangs and would certainly want to undertake the program with the guidance of a physician. Depending on the severity of the diet, the weight loss that inevitably results might impede fertility in females. Also, a prolonged anovulatory state, if accompanied by a diminution of estrogen production, might increase the risk of osteoporosis (bone loss) and loss of muscle mass later in life. It is also possible that calorie restriction would compromise a persons ability to withstand stress, such as injury, infection or exposure to extreme temperatures. Oddly enough, stress resistance has been little studied in rodents on low-calorie diets, and so they have little to teach about this issue.
It may take another 10 or 20 years before scientists have a firm idea of whether calorie restriction can be as beneficial for humans as it clearly is for rats, mice and a variety of other creatures. Meanwhile investigators studying this intervention are sure to learn much about the nature of aging and to gain ideas about how to slow it--whether through calorie restriction, through drugs that reproduce the effects of dieting or by methods awaiting discovery.
Postscript
SINCE PUBLICATION of this article a decade ago, the calorie-restriction (CR) field has become "hot," with great progress made on several fronts. One striking example is the use of short-lived "model organisms," such as yeasts, flies and worms, to rapidly obtain mechanistic insights into CRs effect on life span. The ease of genetic manipulation of these models has enabled the identification of key pathways and regulators of the response to CR. It is not surprising that most of these pathways involve aspects of energy metabolism. The core feature of CR is, after all, energy intake restriction.
New technology has also fueled rapid advances in the understanding of CR. The human genome comprises some 30,000 genes. Before 1998 one could evaluate the activity of only one gene at a time, by measuring the level of the messenger RNA molecule that it encodes. With the development of microarray technology, in a single experiment one can now evaluate the activities of thousands of genes. My colleagues and I were the first to implement the use of this technology in the context of aging and CR, by providing a global view of the activities of more than 6,000 genes in mouse muscle. Subsequently, this approach has been widely used in aging research.
The ultimate goal of the field is to understand the potential of CR in humans. Along the way, we hope to determine whether CR can slow the aging process in nonhuman primates, including species that share much of their genetic makeup with us. We have been comparing the effects of CR and a control diet on rhesus monkeys since 1989 and 1994 (two sets of studies were begun, with animals that were between eight and 14 years old at the outset). The monkeys on CR display signs of improved health and an emerging survival advantage compared with their age-matched controls. But the rhesus monkeys at our primate center have an average life span of about 27 years and a maximum life span of about 40 years, so it may be another 25 years before we obtain full survival data.
Progress has also been made on understanding the effects of long-term CR in humans. Direct evidence comes from studies of long-term practitioners of CR who display markedly improved risk-factor profiles for cardiovascular disease, including reductions in circulating insulin and glucose levels. These individuals also display fewer signs of deterioration in diastolic heart function. Additional progress in human CR is expected; the National Institute on Aging has funded three sites to conduct long-term CR investigations in humans.
The impressive accrual of knowledge on multiple aspects of CR can be expected to continue, and its pace to hasten, as increasing numbers of investigators focus on this fascinating intervention. The mechanistic understanding of calorie restriction will increase the likelihood of the development of drugs or nutrients that mimic the effects of CR in people consuming a normal diet. And if researchers can find a safe way to curb appetite, widespread practice of CR may become possible. Either way, calorie restriction appears well situated to contribute to aging retardation in humans.
THE AUTHOR
RICHARD WEINDRUCH, who earned his Ph.D. in experimental pathology at the University of California, Los Angeles, is professor of medicine at the University of WisconsinMadison and a researcher at the Veterans Administration Geriatric Research, Education and Clinical Center in Madison. He has devoted his career to the study of calorie restriction and its effects on the body and practices mild restriction himself. He has not, however, attempted to put his family, his dog or his two cats on the regimen.
