How Low Can You Go? — Cutting Calories to Extend Life
Today’s Dietitian
By Kate Jackson
Vol. 6 No. 7 p. 30

Some sing the praises of calorie restriction. Animal research supports the claim. Does the science hold up in the human equation?

It may seem paradoxical to those who associate sustenance with abundance, but research has demonstrated that members of a number of species—including mice, worms, dogs, monkeys, and fruit flies—live longer when fed less. For these and other animals, reducing calories by one-third increases the lifespan by as much as 50% while it decreases disease and minimizes the signs of aging.

A number of theories have been proposed to explain this reaction in animals. Digestion of food and metabolism results in the production of free radicals, which are harmful to the genetic material of cells. With less fuel for the metabolic process, fewer free radicals are produced, so less damage is done to the cells. Another possibility is that calorie restriction slows cell division in some tissues, while still another is that it essentially conserves energy.

What Science Says
Scientists at the National Institutes of Health and other research centers are eager to find out if slashing calories will allow humans, like animals, to live significantly longer, but the results of their studies may be some time in coming. “Scientists don’t have time to follow their subjects for a lifetime,” says Katherine Tallmadge, MA, RD, LD, president of Personalized Nutrition, author of Diet Simple (LifeLine Press, 2004), and national spokesperson for the American Dietetic Association. “So to shorten the process, they’re looking for things like biomarkers, which research has told them are important biological signs that aging may be slowing.” It’s known, for example, that people who live longer usually have lower sex hormone levels, lower body temperatures, and lower plasma insulin levels, she explains. Still, researchers have not amassed evidence that will tempt great numbers of Americans to eat like monks.

Nevertheless, intrigued by the promise of this research, a calorie restriction movement has arisen, its ranks swelled by those who aren’t waiting for the scientific verdict. The Calorie Restriction (CR) Society, in Gardena, Calif. (www.calorierestriction.org), for example, boasts nearly 1,200 members, all of whom believe reducing normal caloric intake by as much as 40% will greatly slow the aging process and lengthen life. “The purpose of the CR Society,” says its public relations representative, Warren Taylor, “is to retard aging and disease by advancing the science, instruction, and application of calorie restriction principles. The society, which grew from an Internet-based Usenet discussion group created by Brian M. Delaney, helps people to live longer, with those extra years in superior health.” The calorie restriction diet, says Taylor, “increases average lifespan, maximum lifespan, and total health span.” Its members, known as CRONies (Calorie Restriction with Optimal Nutrition), he adds, “eat to live rather than live to eat.”

While these believers may sound overzealous, their beliefs stem from a considerable amount of solid scientific evidence that calorie restriction produces these effects in animals. For more than six decades, researchers have extended the lives of laboratory rats and other creatures by reducing their calorie intake. Roy L. Walford, MD, a University of California, Los Angeles School of Medicine professor of pathology, is credited with publicizing the research and promise of the low-calorie path to longevity. His books, including Maximum Life Span, Beyond the 120 Year Diet, and The Anti-Aging Plan, and his dietary trials in the Biosphere experiment of the 1990s garnered enormous attention and are often credited with catalyzing the calorie restriction movement.

Numerous studies bolster the movement’s suppositions. In early 2003, for example, a study by investigators at the National Institute on Aging (NIA) Intramural Research program found that reducing caloric intake “protects nerve cells from genetically induced damage, delays the onset of Huntington’s disease-like symptoms in mice, and prolongs the lives of affected rodents.” Studies further have indicated that calorie-restricted diets have retarded age-related disease and changes—including diabetes, certain cancers, kidney disease, and immune system dysfunction—in a variety of animals.

The Human Equation
The question at the core of the controversy surrounding calorie restriction is: Will it work in humans? The CR Society, of course, believes it will—and its members live by that belief. “We place our bets on the laboratory research that shows the robustness and universality of the calorie restriction effect in all animal species. And human lab measurements show the same remarkable physiological benefits as seen in animals,” explains Taylor. And many researchers tentatively suggest this belief may not be far off the mark. In a press release announcing the NIA’s study of calorie restriction and Huntington’s disease-like symptoms in mice, Mark P. Mattson, PhD, chief of the NIA’s laboratory of neurosciences, said, “If reducing food intake has the same effects in humans as it does in mice, then it may be theoretically possible to delay the onset of the disease and extend the lives of Huntington’s patients by prescribing low-caloric diets or diets with reduced meal frequency.”

But even some who devote a considerable amount of their professional undertakings to its study not only discourage individuals from participating in a severely calorie-restricted lifestyle but also doubt the application of animal research to humans. Without question, says Charles V. Mobbs, PhD, associate professor in neurobiology and geriatrics, Mt. Sinai School of Medicine, “calorie restriction is robustly effective across the species under standard laboratory conditions.” While this type of dietetic restriction is not necessarily healthy even for animals, he observes, it effectively increases their lifespan in what he describes as the relatively benign conditions under which they live, but he speculates that if you put calorie-restricted animals out in the wild, they probably wouldn’t live longer.

Mobbs isn’t convinced that the lifestyle can be practiced in a healthy way by humans or, more to the point, that it will extend human life. His reasoning is simple: “Throughout history, there have been so many kinds of people that have basically functioned under calorie restriction, and yet there’s no evidence that anyone ever lived beyond maximum lifespan.” One would think, he speculates, that some monks or nuns, for example, who have lived the caloric restriction lifestyle for whatever reason, would have been documented to have lived longer than 120 years, but no such documentation exists. “This is how millions of Buddhist monks lived their lives,” he notes, “not to mention peasants throughout history—99% of the human population—who never had more than 1,200 calories a day.” Such an ample diet, he says, is an absolutely modern invention, yet those earlier populations never lived beyond 120 years, although many did live to a ripe old age.

If he’s so convinced that calorie restriction won’t lengthen human life, why is he committed to researching the proposition? Animal research indicates that there’s a mechanism for extending lifespan, and in experimental models with mice, calorie restriction appears to trigger that mechanism. Although he doesn’t think calorie restriction will work similarly in humans, Mobbs does believe a molecular mechanism to extend life does exist in humans and can likely be activated by some means other than calorie restriction.

How Low Should You Go?
Experts agree that those who are overweight or consume an excess of calories should significantly cut back on calories while maintaining adequate nutrition. “Obviously people would be better off generally if they ate fewer calories,” says Mobbs. “If they’re overweight, absolutely they should cut back from 2,400 calories a day to 1,800 calories,” he suggests, adding, “it’s not going to increase their maximum lifespan, and it’s not even going to increase their average lifespan by much, but they’ll certainly be healthier.” In such cases, cutting back by 20% or 10% of calories will reverse obesity or overweightness and decrease the risk of weight-related diseases, but going to extremes of reducing calories by 30% merely puts people at risk. Moreover, he observes, “Most of us don’t have the discipline or motivation to do this very difficult thing.”

Despite his belief in the potential of research leading to longer-lasting life, Mobbs is opposed to severe caloric restriction in humans not only because there’s little evidence that it will extend life but also because it’s tough on the body and a rather unpleasant way to live. “People claim that they get used to it—maybe it’s true, I don’t know.” Beyond the difficulty, among the more serious risks of calorie restriction, he observes, is the likelihood of developing osteoporosis. Additional negative side effects of severe calorie restriction, as described by the CR Society, include hunger, anemia, negative appearance changes, cold sensitivity, physical discomfort due to reduced padding that protects body tissues, depression, reduced energy reserves, loss of libido, menstrual irregularities, infertility, risks in pregnancy, loss of stamina, slower wound healing, and an inability to combat long-term illness.

Mobbs is concerned as well about the potential for developing eating disorders as a result of severe calorie restrictions. “It can be a fine line between calorie restriction and anorexia, and obviously anorexia can kill you.” Tallmadge is equally concerned with the potential for the development of eating disorders. “I wouldn’t feel comfortable advocating calorie restriction with clients,” she explains. “I’ve seen the negative consequences of rigid dieting: eating disorders and weight fluctuation.” Furthermore, she says, “It basically advocates lowering metabolism to create less oxidative stress. But you counter this oxidative stress by eating more fruits and vegetables and foods with antioxidants.”

Those who desire longevity beyond current reasonable expectation may be drawn to a calorie restriction lifestyle, despite the lack of evidence of a life-extending effect in humans and despite the risks and challenges. While many experts might suggest that it’s too soon to say whether or not there’s a truly healthful way to severely curtail calories, they’d advise those who wish to attempt this lifestyle to do so only under the care of a physician and dietitian. “They must be very careful about having an otherwise balanced diet and possibly changing the composition of their diet,” says Mobbs, who adds that he’s most concerned that those who restrict calories will not get adequate protein.

Older adults are particularly vulnerable to both the lure and potential negative consequences of caloric restriction. “During aging, muscles begin to atrophy,” says Mobbs, a condition that he speculates could worsen with inadequate calories. “It’s very possible that calorie restriction will worsen many age-related changes—glucocorticoids, for example, become elevated during aging and are also elevated by caloric restriction.” Older adults, agrees Tallmadge, have increased nutrient requirements and are at high risk for not getting enough protein. “They’re not the population that should be exploring this,” she insists. Not only can calorie restriction be more dangerous for older people, Mobbs explains, it’s even less likely to extend life in that population. Even if he were inclined to believe it will work in humans, “certainly the older you are when you start caloric restriction, the less effective it would be. So it’s very likely that if you start it after midlife, it probably wouldn’t do much good.”

The Real Message
Like Mobbs, Tallmadge comes down against calorie restriction because the benefits are uncertain and some of the disadvantages are clear. “I’d feel comfortable telling people to avoid severe calorie restriction at this time,” she says. Instead, she advises clients to achieve an ideal weight through healthful eating and activity. “It sounds simple, but those are the things that science has proven will improve your quality of life and longevity.”

The take-home message of calorie restriction studies, suggests Tallmadge, is not that we have to abnormally restrict calories to 25% or 30% below normal but rather that we should not overeat. “These studies underline the importance of eating the right amount of calories from a high-quality diet full of antioxidants and adding exercise to give you some of the same benefits of calorie restriction.” She suggests focusing on the most promising aspects of the research: “There are certain aspects of aging that we can change. Exercising and weight loss bring many of the benefits you would receive by restricting calories. It reduces plasma insulin levels, keeps you from eating too many calories, and you can eat foods high in antioxidants to prevent oxidative damage.”

More active people, she adds, expend more calories than those who are inactive, which would be physiologically similar to being on a restricted energy intake diet, and, she’s quick to point out, they tend tend to live longer.

While the benefits of calorie restriction may be demonstrated by science in the future and its promise of longevity may one day be realized, few scientists or healthcare providers will go out on a limb to support it at this time. “There’s a danger in jumping on the bandwagon too soon,” concludes Tallmadge. “There’s just too much that we don’t know.”

— Kate Jackson is a staff writer for Today’s Dietitian.

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