Humans, primates expend fewer calories than other mammals

Humans and other primates burn 50 percent fewer calories each day than other mammals, and these remarkably slow metabolisms may explain why they grow up so slowly and live such long lives, U.S. researchers said Monday. The study, published in the U.S. journal Proceedings of the National Academy of Sciences, also found that primates in zoos expend as much energy as those in the wild, suggesting that physical activity may have less of an impact on daily energy expenditure than is often thought. Researchers examined daily energy expenditure in 17 primate species, from gorillas to mouse lemurs, to test whether primates' slow pace of life results from a slow metabolism. Using a safe and non-invasive technique known as "doubly labeled water", which tracks the body's production of carbon dioxide, the researchers measured the number of calories that primates burned over a 10-day period. Combining these measurements with similar data from other studies, the team compared daily energy expenditure among primates to that of other mammals. "The results were a real surprise," said lead author Herman Pontzer, an anthropologist at Hunter College in New York, in a statement. "Humans, chimpanzees, baboons, and other primates expend only half the calories we'd expect for a mammal. To put that in perspective, a human -- even someone with a very physically active lifestyle -- would need to run a marathon each day just to approach the average daily energy expenditure of a mammal their size." Perhaps just as surprising, the measurements show that primates in captivity expend as many calories each day as their wild counterparts, the researchers said. These results speak to the health and well-being of primates in world-class zoos and sanctuaries, and they also suggest that physical activity may contribute less to total energy expenditure than is often thought. The study may hold intriguing implications for understanding health and longevity in humans. According to the researchers, linking the rate of growth, reproduction, and aging to daily energy expenditure may shed light on the processes by which our bodies develop and age. And unraveling the surprisingly complex relationship between physical activity and daily energy expenditure may improve our understanding of obesity and other metabolic diseases. "Humans live longer than other apes, and tend to carry more body fat," said Pontzer. "Understanding how human metabolism compares to our closest relatives will help us understand how our bodies evolved, and how to keep them healthy."