London, Dec 10 (IANS) Scientists seeking secrets of how calorie-restricted diets increase longevity, have discovered proteins in the fat cells of volunteers that change as pounds drop off.

The proteins could become markers for monitoring or boosting the effectiveness of calorie-restricted diets - the only scientifically proven way of extending life span in animals.

Edwin Mariman, biologist at Maastricht University in the Netherlands and colleagues note that scientists have long known that sharply restricting calorie intake while maintaining good nutrition makes animals live longer and stay healthier.

Recent studies suggest that people may gain similar benefits. But scientists know little about how these diets work in humans, particularly their effects on cells that store fat.

The new study focused on proteins in abdominal subcutaneous (below the skin) fat cells from a group of overweight people before and after they went on a five-week-long calorie-restricted diet.

The volunteers each lost an average of 21 pounds. Scientists identified changes in the levels of six proteins as the volunteers shed pounds, including proteins that tell the body to store fat.

These proteins could serve as important markers for improving or tracking the effectiveness of therapies involving calorie-restricted diets, they say.

Their study appeared online in the American Chemical Society's Journal of Proteome Research.