Previous studies have shown that laboratory animals given 30 percent to 50 percent less food can live up to 50 percent longer, however, a new study suggests the diet may not have the desired effect in people unless they focus on their protein intake.
The team led by John O. Holloszy have found discrepancy between humans and animals on calorie restriction.
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.
"We looked at IGF-1 in humans doing calorie restriction," said first author Luigi Fontana, M.D., Ph.D., assistant professor of medicine at Washington University and an investigator at the Istituto Superiore di Sanitā in Rome, Italy.
"For years, we have been following a cohort of people from the CR Society who have been on long-term calorie restriction. We found no difference in IGF-1 levels between people on calorie restriction and those who are not."
Fontana and colleagues at Washington University have been involved in a scientific study that compares calorie restriction to exercise and measures many biological factors linked to longevity and health.
For the study, the researchers divided 48 people into three groups: Eighteen cut their caloric intake by 25 percent for one year. Another 18 started exercising to increase their energy expenditure by 25 percent for a year. A third group of 10 people didn't change anything.
At the end of that year, the investigators measured IGF-1 levels in all three groups and found no reductions in the group on calorie restriction.
"That was puzzling because it was the first time we hadn't seen agreement between mice and rats on calorie restriction and humans on calorie restriction," said Fontana.
"But we know there are two major influences on IGF-1 levels: calorie intake and protein intake. So we decided to look at the influence of protein," she added.
Fontana had been following a population of strict vegans for several years. They tend to eat less protein than the CRONies from the CR Society, so he compared IGF-1 levels between the two groups.
"The vegans had significantly less circulating IGF-1, even if they were heavier and had more body fat than CRONies," he said.
"Protein in the diet seemed to correlate with the lower levels of IGF-1. The strict vegans took in about 10 percent of their total calories from protein, whereas those on calorie restriction tended to get about 23 or 24 percent of calories from protein," he added.