American Scientist

To the Editors:

I was frustrated by Robert Arking's article ("Aging: A Biological Perspective," November–December) because, as they say, the devil is in the details. Without a natural-history context to provide the details, the generalities presented seem meaningless. What does it mean to "reduce caloric intake" by 40 percent? Reduce from what? I have no idea.

Animals don't normally sit in front of a pile of food with nothing to do but chow down. They have to forage and exercise to get their food. Such exercise increases appetite and food consumption. Is the reduced longevity of exercising animals (according to the model presented) because of their increased food consumption or their increased exercise?

Isn't it at least plausible that the explanation of the proximate correlations of increased aging with increased caloric ingestion, plus the decreased aging effects with increased exercise, might instead involve an optimum energy balance?

Bernd Heinrich
University of Vermont

To the Editors:

Professor Arking's interesting article used photos of athletes in two of its illustrations, raising the question: To what extent does physical exercise influence the aging process? For example, would a person who reduced caloric intake and walked a mile a day live longer than the person who likewise reduced their calories but spent their time snoozing? Are current experiments taking into account the activity factor?

Frank R. Tangherlini
San Diego, California

To the Editors:

"By the year 2000, the average person could expect to live roughly 80 years . . . these longer lives resulted from . . . sanitary sewers and clean drinking water," states the caption of Figure 2 in Robert Arking's fascinating article on aging.

About 4 billion people in the world would be interested to read that caption—because, of course, living in developing countries, their life expectancy is usually nowhere near 80 years, and many have never seen a sewer nor experienced clean drinking water in their (short) lives.

The average person in the rich West might expect to live roughly 80 years but not the average Indian, African or Asian. In several developing countries, the average is closer to 50 years than to 80 years.

Alexander Harcourt
University of California, Davis

Dr. Arking replies:

The need to write a concise article foreclosed examination of many interesting questions, including those raised by Drs. Heinrich and Tangherlini. Caged animals that eat ad libitum (at will) clearly represent an unusual physiological context in which to measure longevity. It is true that some experiments used bored animals with 24-hour access to food as the control. But other experiments were better constructed (for example Weindruch et al., 1986, Journal of Nutrition 116:641–654). My point was that over many studies, caloric intake and longevity are inversely correlated, and this point is surely the important observation.

The interaction of caloric restriction and exercise is complex. It may have positive, neutral or negative effects depending on the level of exertion and genotype. However, the beneficial effects of exercise are well documented. Even with moderate exercise, mortality in humans decreases by 20 percent relative to sedentary controls. One of the primary effects is lowering of blood insulin.

Insulin now appears to be part of the ancient mechanism of caloric-restriction biology, as even yeast cells activate an insulin-like signaling pathway during these conditions (see review articles in the February 28, 2003, issue of Science). Among its other functions, this signaling pathway seems to dictate just such an energy balance as Dr. Heinrich suggests—"deciding" whether to expend energy to grow fast and reproduce quickly, or to spend energy on somatic maintenance and thus reproduce later.

In a different vein, Dr. Harcourt's objection is well founded. Obviously an 80-year life expectancy is found only in the developed countries. And despite the difficulties of making changes necessary to develop long-lived societies, it is morally (and economically) necessary to do so.