Researchers at the National Institute on Aging say they have preliminary evidence that biological markers that help rodents live much longer than normal may have the same effect on humans. Even if the evidence proves to be correct - which is not certain - it is unknown how much longer people might live.

The biological markers - lower temperature, lower insulin levels and a steady level of a steroid hormone called DHEAS - all occur in restricted-diet rodents that live about 40 percent longer than other rodents on a normal diet. The same biological markers have now been found in men who are living longest in a continuing study in Baltimore on human aging.

This means that the biological characteristics of animals that are on calorie-restricted diets seem to apply to longevity in people. The results should be considered "preliminary" and that nobody should start starving in hopes of living longer. Instead, he said, the study gives only tantalizing hints that are worthy of further investigation about helping people to extend life.

Other experts said the study offers new hope about science some day finding ways to slow aging and extend life. The study doesn't absolutely prove anything, but it suggests that the same mechanisms that operate in calorie-restricted animals can operate in humans.

The researchers drew their preliminary conclusions from the combination of studies on aging rodents, a 15-year study on aging monkeys, and the continuing project called the Baltimore Longitudinal Study of Aging, which follows the life span of people. The monkey study, conducted at the National Institute of Aging, is designed to test the longevity effects of calorie restriction on a subhuman primate that is closer biological relative to humans than are rodents.

The study started in 1987, but since Rhesus monkeys can live for 25 years, it may take four or five years more before the results are final. Statistically, at least half of the monkeys will have to complete their normal life span before the data is considered significant.

The same is true of the Baltimore study on aging people.

The researchers gathered preliminary conclusions by looking at early trends in the deaths of both the monkeys and the men. The researchers also divided men in the Baltimore study into two groups based on measurements of the key biomarkers - temperature, insulin and DHEAS levels - that were characteristic of the superaged lab rodents.

The men whose biomarkers were similar to those of the calorie-restricted, long-lived rodents were dying at a much slower rate than were men with other biomarker measurements. None of the Baltimore study's men is known to be on restricted diets, but clearly some are enjoying the same life span benefit that calorie restriction gave the laboratory animals. Researchers are not sure why.

In the monkeys on reduced feeding since the study started are dying at a rate that is about half that of the monkeys receiving a full food ration. He said all of the animals are fed the same nutritionally balanced food, but the longer-lived group gets 30 percent less. Although the findings suggested that a diet restriction of 30 percent or 40 percent could extend life, this is not practical for most people and could be unhealthy.

Science August 2, 2002;297(5582):811