A new study from Harvard University shows that humans have a significantly higher metabolic rate than other mammals, including our closest relatives - apes and chimpanzees. Researchers believe humans have both a high resting and active metabolic rate, which allowed our hunter-gatherer ancestors to gather enough food while also supporting larger brains, longer lifespans and higher reproductive rates.
"Humans are very different from any creature we know so far in terms of how they use energy," said paleoanthropologist Daniel Lieberman, a co-author of the study.
The energy mystery: how we burn calories in different ways
The study, published in the Proceedings of the National Academy of Sciences, calls into question the long-held belief that humans and nonhuman primates have metabolic rates similar to or even lower than expected for their body size.
Using a refined comparative approach that takes into account body size, ambient temperature and body fat, the researchers found that humans, unlike most mammals, including other primates, evolved in a way that avoids the typical trade-off between resting and active metabolism.
Why chimpanzees are couch potatoes while humans stay active
Animals take in calories through food and spend them like a bank account, splitting metabolism into two main categories: rest and physical activity. Lieberman said there is a clear trade-off between resting and active metabolic rates in other primates, which helps explain why chimpanzees have high resting metabolic rates due to their large brains, costly reproductive strategies and long lifespans, but they are couch potatoes who spend most of their day eating.
Generally, the energy an animal spends on metabolism ends up as heat, which is difficult to dissipate in a warm environment. Because of this trade-off, animals such as chimpanzees that expend large amounts of energy in their resting metabolism while living in warm tropical environments have to reduce their activity levels.
Secret Weapon: Sweat for More Energy
"Not only have humans increased our resting metabolic rate beyond that of chimpanzees and monkeys, but due to our unique ability to shed heat through sweat, we are also able to increase our physical activity levels without reducing our resting metabolic rate," said Andrew Yegian, a senior researcher in Lieberman's lab. "The result is that we are energetically a unique species."
The team's analysis shows that monkeys and apes have evolved to have resting metabolic rates that are about 30 to 50 percent higher than other mammals of the same size, while humans have gone even further, burning 60 percent more calories than mammals of similar size.
"We initially questioned whether humans and other primates might have a relatively low overall metabolic rate, an idea that other researchers had raised," Yegian said. "We were trying to find a better way to analyze it using quotients. That's when we put our foot on the gas."
The next step is to study metabolism in the modern world
The research team, which includes collaborators at the Pennington Biomedical Research Center in Louisiana and the University of Kiel in Germany, plans to further study metabolic differences between different populations. For example, subsistence farmers, who grow all their food without the help of machines, have significantly higher levels of physical activity than hunter-gatherers and people like Americans who live in industrial environments. However, the resting metabolic rate of all people, regardless of their activity level, consumes similar amounts of energy to their body size.
"What we're really interested in is the difference in metabolic rate between people, especially today as technology becomes more advanced and exercise becomes less and less. Since our evolutionary process is active, how does clerical work change our metabolism and thus affect health?"
Compiled from /ScitechDaily