Mice that stay skinny through a low-calorie diet, rather than exercise, live longer, researchers said.

The study at the University of Alabama-Birmingham was meant to find out why that is, lead author Derek M. Huffman said.

Previous studies showed that rats that exercise generally live longer than those that don't. However, the longest-lived exercising rats don't do better than the longest-lived sedentary rats.

But those that eat less and don't exercise lived the longest.

A news release from the American Physiological Society said that one theory is that exercise places stress on the body that can cause some damage.

In the new study, researchers found that the stress of exercise did not produce enough damage to explain why exercise does not lengthen life span. Instead, researchers said, they found that that caloric restriction creates beneficial changes in hormone levels, which exercise does not.

The researchers concluded that these metabolic changes play a role in extending life.

He said that some work has suggested similar results in humans, though Huffman pointed out there are differences between the kinds of animals. One is that people may be more prone to heart disease; exercise is known to lower that risk.