Researchers at the Washington University School of Medicine in St. Louis said that previous research had shown that laboratory animals given 30 percent to 50 percent less food can live up to 50 percent longer.

First author Dr. Luigi Fontana said that because of these findings, some people have adopted calorie restriction in the hope that they can lengthen their lives, however, new research suggests the diet may not have the desired effect unless people on calorie restriction also pay attention to their protein intake.

In the majority of the animal models of longevity, extended lifespan involves pathways related to a growth factor called IGF-1 -- insulin-like growth factor-1 -- which is produced primarily in the liver. Production is stimulated by growth hormone and can be reduced by fasting or by insensitivity to growth hormone. In calorie-restricted animals, levels of circulating IGF-1 decline between 30 percent and 40 percent.

"For years, we have been following a cohort of people who have been on long-term calorie restriction," Fontana said in a statement. "We found no difference in IGF-1 levels between people on calorie restriction and those who are not."

The findings are published in the journal Aging Cell.