April 27, 2011 -- Research has suggested that very low-calorie diets may increase life expectancy in animals, and now a new study in humans provides some important clues as to why this may occur.
In the new study, individuals who had higher metabolic rates -- the amount of energy the body uses for normal body functions -- were more likely to die early from natural causes than those who had lower metabolic rates.
The new findings appear in the Journal of Clinical Endocrinology & Metabolism.
Higher metabolic rates may accelerate the natural aging process via the production of damaging free radicals. These substances are linked to many diseases of aging and may promote organ damage.
Researchers measured the 24-hour energy expenditure of 508 Pima Indian volunteers in metabolic respiratory chambers and the resting metabolic rate of 384 volunteers. Two hundred forty participants underwent both measurements on different days. Pima Indians are known to have high rates of type 2 diabetes, but none of the participants had diabetes when the study began.
During the 15-year follow-up period, 27 people died of natural causes. The risk of dying increased with the amount of metabolic energy expenditure. Those volunteers with higher energy turnover -- as measured by increased metabolic rate -- were among the most likely to die early, the study showed. The new study does not apply to energy expenditure from exercise.
“The results of this study may help us understand some of the underlying mechanisms of human aging and indicate why reductions in metabolic rate, for instance via low-calorie diets, appear to be beneficial for human health,” says study researcher Reiner Jumpertz, MD, of the National Institute of Diabetes and Digestive and Kidney Diseases, in a news release.
“Studies in animals show reduced metabolic rate after caloric restriction increases longevity -- that is what we know,” says Spyros Mezitis, MD, an endocrinologist at Lenox Hill Hospital in New York City.
“The new findings need to be confirmed in larger studies with similar methods,” he says.
But “if you eat less than 1,500 calories a day, you have less energy expenditure so you have a lower metabolic rate, and, in animals, this results in an increased life span,” he says.
“You have a higher energy metabolism if you have a lot of body fat,” says endocrinologist Loren Wissner Greene, MD, a clinical associate professor of medicine at New York University Langone Medical Center in New York City. “Obesity increases risk of early death and the mechanism might have something to do with increased metabolic rate and free radical production.”
“It’s interesting association that they found between energy expenditure and what appears to be longevity, but it’s hard to interpret what we should do and if it is generalizable to other populations,” says Jonathan Waitman, MD, Comprehensive Weight Control Program at NY-Presbyterian/Weill Cornell.