Eat Less to Live Longer? - Food for Thought - Harvard Commentaries

Is it possible to live a good deal longer by eating a good deal less? Proponents of a strict dietary regimen called calorie restriction (CR) claim that it can extend your life and prevent diseases associated with aging. The diet consists of eating a very low-calorie but nutritionally balanced diet that meets 100% of vitamin, mineral, protein and essential fat needs. But can such a lifestyle, which means a dramatic change in eating habits for most Americans, be the "fountain of youth" its followers claim?

CR gained momentum in the 1930s after studies on mice showed that reducing their normal food intake by about 40% increased their maximum life span by 30% to 40%. Since then, research on worms and monkeys has shown that CR works in other species.

In humans, CR has been shown to improve markers of cardiovascular aging in overweight people. According to a 2006 study reported in the Journal of the American Medical Association, calorie restriction reduces certain biomarkers of aging and disease, such as lower body temperature and fasting insulin level. The subjects who participated in the six-month study were all overweight. They cut their daily calories by 25% either through diet alone or through diet and exercise. Their core body temperatures and fasting insulin levels fell. These findings suggest that calorie restriction may help prolong life by preventing diseases and aging in overweight people. Whether people who have normal body weight can show similar improvements has not been proven.

The mechanisms behind CR are still theoretical. New research is casting doubt on the theory that CR slows aging by reducing free radicals and oxidative damage. Instead, CR may redirect energy away from growth and reproduction toward cell maintenance, repair and protection. Another theory is that CR may shift the body into preservation mode where damage is more efficiently corrected. Based on this theory, CR is not recommended for those younger than 18 or for pregnant women.

Americans in particular have made little headway in the battle to control their culturally large appetites and obesity-prone environment. The levels of calorie restriction needed to achieve the benefits may be too low for most people, especially with the potential for uncomfortable signs of physical starvation experienced by research animals. Researchers are testing ways to mimic CR through pills or other methods to achieve its benefits without the hunger — an area known as CR "mimetics."

Humans might not get as many benefits from CR as animals because of different energy needs. Mice, for example, expend exponential amounts of energy on reproducing and nursing compared to humans. If CR works by slowing or halting reproduction and thereby conserving energy, animals indeed have more to gain.

The human maximum lifespan may have little room to expand compared to that of animals. The maximum human lifespan recorded so far is of a 122-year old French woman. (Longevity experts expected an individual from Okinawa to hold the record since they take in 40% fewer calories than Americans and 20% fewer calories that other Japanese.)

Weight can drop to dangerously low levels, causing menstruation to stop in women. CR may also increase the risk of osteoporosis, especially if the diet is too low in calcium and vitamin D. According to The Calorie Restriction Society's website, other side effects of CR can include food obsessions, hording, obsession with conserving energy, and seeking vicarious food experiences through TV or cooking for others.

Even though the CR regimen stresses the importance of eating the right foods to avoid malnutrition, it can produce side effects that resemble famine-ridden populations or anorexia nervosa including cold intolerance and loss of strength. Decreased sex drive is also a common side effect, which may support the theory that CR redirects energy away from reproduction.

As a lifestyle, CR requires considerable meal planning, food restriction, and enduring some hunger. Some CR followers only allow themselves 1,000 to 1,200 calories a day. Others suggest a more modest 25% reduction in calorie intake. Given that on average men consume 2,618 calories a day and women 1,877, trimming your food intake to even 1,200 calories a day means cutting out 54% and 36% respectively of what you normally consume.

Low-calorie diets are difficult to adhere to, especially given the need to insure adequate intake of essential vitamins and minerals. The potential physical and psychological side effects may outweigh the still unproven promises of longevity. However, CR supports age-old advice to eat nutritious foods in modest amounts and maintain a healthy weight as the best protection against premature death and disease.

Julie Redfern, R.D., L.D.N., is a registered dietitian and manager of the Nutrition Consultation Services at Brigham and Women's Hospital. She specializes in nutrition counseling for the obstetrics and gynecology department. She is a graduate of the University of Vermont and completed her dietetic internship at the University of Cincinnati Medical Center.

Jaimie Winkler is a dietetic intern at the Brigham and Women's Hospital. She completed her BS in nutrition at West Chester University and a BA in History/journalism at the University of Michigan.