Key to Longevity; Humans and Other Primates Burn 50 percent Fewer Calories Than Mammals

Scientists have finally uncovered the secret of why humans generally outlive other mammals. A team of international scientists for the first time reveal that primates including humans burn less than 50 percent of calories each day compared to other mammals their size.

The study reported in the Proceedings of the National Academy of Sciences credits the slow metabolism in humans and other comparable primates to their slow pace of life. Also this study highlights that the primates living in zoos expend the same amount of energy as their wild counterparts, indicating physical activity does not impact the daily energy expenditure.

For years scientists were puzzled by how most mammals live a fast-paced life, reach adulthood in months and reproduce prodigiously whereas primates including humans reproduce infrequently and live longer lives. Through this study, researchers found that primates grow up burning less than 50 percent of the calories.

The researchers uncovered the mystery after calculating the daily energy expenditure in 17 primate species living in zoos, sanctuaries and the wild. They wanted to confirm whether the slow pace of life in primates such as gorillas and mouse lemurs results from slow metabolism.

The daily energy expenditure was calculated using a non-invasive technique called the 'double labeled water'. This technique helps in tracking the body's production of carbon dioxide, using this, the study team measured the amount of calories the primates burned in 10 days. The measurements made were later combined with similar data taken from other studies. The researchers compared the daily energy expenditure of primates to that of other mammals.

"The results were a real surprise," lead author Herman Pontzer, an anthropologist at Hunter College in New York, said 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."

The researchers say that the remarkable slow rate of metabolism is responsible for the unhurried  pace of life in humans. They also state that evolution has played a role in the metabolic rate to shape the primates' slow pace of life. The dramatically low energy expenditure in primates including humans is due to the environmental conditions that favor them to burn few calories only.

They also noticed no difference in the amount of calories burned every day between zoo primates and those living in the wild indicating physical activity contributes less to the total amount of calories burnt.

"The completion of this non-invasive study of primate metabolism in zoos and sanctuaries demonstrates the depth of research potential for these settings. It also sheds light on the fact that zoo-housed primates are relatively active, with the same daily energy expenditures as wild primates," said coauthor Steve Ross

This study may help in understanding about human aging and how to keep humans healthier.