In a 23-year study, cutting calories by one-third improved monkeys' health, but didn't extend their lives. Those results, published August 29, contrast with another recent study that did show longer life for monkeys on very-low-calorie diets. Other studies have found that earthworms, mice and other animals live longer with fewer calories. The new study included 120 rhesus macaques. They were randomly divided into two groups. One group ate a normal controlled diet. The other group ate about 30% fewer calories. The restricted diet included vitamin and mineral supplements. When the study began, 86 monkeys were young, up to age 14. The others were 16 to 23 years old. The average macaque lives about 27 years. After 23 years, the calorie-restricted monkeys weighed less and had lower rates of diabetes than the other group. The younger monkeys also had lower rates of cancer. Older monkeys on the calorie-restricted diet had lower levels of triglycerides, a blood fat, than those on standard diets. The males also had lower cholesterol and blood sugar. But the monkeys on restricted diets are not living longer. The journal Nature published the study online. HealthDay News and USA Today wrote about it.

By Howard LeWine, M.D.
Harvard Medical School

What Is the Doctor's Reaction?

Mice live longer when they are fed less. The effect is a large one. A very low-calorie diet can extend a rodent's life span by 33%.

But mice are not men. Monkeys aren't either, but the resemblance is closer. Researchers at the National Institute on Aging have been studying the effects of calorie restriction on monkeys since 1987. Animals of the same ages were divided into 2 groups. One group was fed a normal diet. The other group received a balanced diet that contained 30% fewer calories.

Early on, the thin monkeys appeared to be aging more slowly. But now we learn that these calorie-restricted monkeys didn't live any longer than those fed a normal diet.

So does that mean calorie restriction in humans won't affect their health or life span? The jury is still out. A different study did find that monkeys lived longer with calorie restriction.

In humans, calorie restriction improves biomarkers of aging in overweight people.

A biomarker of aging is something that can help predict how well a person or animal will function at an older age. Scientists have not found anything close to perfect biomarkers of aging. Examples of ones that are used in research include:

• The levels of certain types of white blood cells that reflect immune strength
• Insulin blood levels, as a way to look at efficiency of energy use

But we don't know whether improvement in biomarkers translates to longer life in overweight humans. And we know even less about people with normal body weight.

Adding to the confusion, scientists are uncertain as to why calorie restriction works to slow down aging in some animals. Theories include:

• A reduction in how fast body metabolism operates (metabolic rate), resulting in less production of harmful byproducts
• A more efficient response of cells to insulin
• Decreased nerve and hormone stimulation of the body's "fight or flight response," adding to chronic stress

But some studies have suggested that none of these is relevant to aging. For example, a study done on yeast showed some surprising results. As predicted, yeast that were sugar-deprived (and exposed to fewer calories) lived longer than those that were bathed in high-sugar solutions. But instead of slowing their metabolism as a response to no sugar, the yeast actually speeded it up. The low-calorie environment "turned on" a gene that made this happen.

What Changes Can I Make Now?

Even people who believe in near starvation to prolong life have a difficult time sticking to it. And without clear evidence of benefit, I recommend you don't even try it.

But you should do the important things that will keep you lean and healthy.

• Keep your calorie intake modest.
• Eat lots of vitamin-rich vegetables and fruits and fiber-rich whole grains.
• Eat only small amounts of high-fat foods from animal sources.
• Exercise daily to burn away excess calories and fat and to improve fitness.

To maintain body weight, we need many fewer calories than most of us eat. A general guide is:

• 11 to 12 calories per pound of body weight if you get no exercise
• 13 to 14 calories per pound of body weight if you are somewhat active
• 15 to 17 calories per pound of body weight if you exercise 5 or more times per week

If you need to lose weight, cut the number of calories by 200 to 300 a day. Start by eliminating all simple sugars from your diet.

What Can I Expect Looking to the Future?

My prediction is that severe calorie restriction in people with normal weights will not lead to longer lives. However, for people who are obese and perhaps for those who are approaching obesity, calorie reduction is a must. It clearly decreases the risk of long-term diseases, such as diabetes and heart disease. And it increases the chance you will live longer, especially if combined with regular exercise.