Researchers note differences between people and animals on calorie r...(e who ate a standard Western diet. ...)

Because calorie restriction is linked to extraordinary increases in maximal lifespan in rats and mice, Fontana and colleagues at Washington University, including principal investigator John O. Holloszy, M.D., professor of medicine, have been involved in a scientific study that compares calorie restriction to exercise and measures many biological factors linked to longevity and health. Called the CALERIE study (Comprehensive Assessment of the Long term Effects of Reducing Intake of Energy), the project randomly 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. Again they 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," Fontana explains. "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."

Again, Fontana had a ready-made study group. His team has 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 says. "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."

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