Scientists have known for some time that laboratory mice on a low calorie diet live longer than mice that eat as much as they want. Now a researcher from Washington University in St Louis, Missouri, says a low calorie diet might be key to slowing aging in people, too.

In a new study, Dr John Holloszy compared three groups of people: those who ate fewer calories than in the typical western diet; regular eaters who also did strenuous exercise; and another group of regular eaters who didn't exercise.

He found those who were on a calorie-restricted diet had less of the thyroid hormone T-3, which regulates their metabolism. "This is not an abnormal decrease," he says, "and it's just a decline into the lower end of normal."

In effect, metabolic hormones regulate the rate at which cells function. Scientists think this might be related to how fast we age. "So if you have high thyroid hormone levels," Holloszy explains, "you use a lot of oxygen, you have a fast heart rate and you need large amounts of food in order to maintain weight. If you have low thyroid hormone levels, your metabolic rate is slowed." He says studies on rodents have shown that skinny, long-lived laboratory mice had low thyroid hormone levels.

Another group in his study ate what they wanted but exercised intensively. The exercise didn't seem to affect their thyroid hormone levels, which remained high. Holloszy says they are continuing research to see if eating less can slow aging in people as well as mice.

"Determining whether humans on caloric restriction live longer is not something we're going to find out, at least not in my lifetime," he says with a laugh. "You know, that's a 50 year study."