Humans’ exceptional ability to burn through calories fuels our evolution.
In an interview last September with Cyclist magazine, five-time winner of the Tour de France, Miguel Indurain, was asked about his extraordinarily low heart rate, which story after story had claimed was as low as 28 beats per minute. “Is it true?” the interviewer asked.
“One day we did a medical test and it read 28, so there is some truth in it,” Indurain said. “But normally it was a little bit higher.” By normally, the cyclist meant that it was usually 30 or 32 beats per minute. And although that have might have been normal for him, it is extraordinary compared with that of the average adult, whose heart bumps along at closer to 60–100 beats per minute.
Indurain is said to have near-super-human heart and lung capacity to go with his glacial pulse. He may also have an unusually low metabolism — a common way to estimate that particular physiological measure is simply to look at the heart rate. The more the heart pumps, the estimate assumes, the faster the body’s cells and tissues will be exhausting their reserves. If that is true, then having a slow metabolism would merely confirm that Indurain has a special physiological status. For as a species, humans tend to burn through calories as if they are about to go out of fashion.
We humans are a conundrum to physiologists when it comes to our energy use, because we seem to have evolved an ability to have our cake and eat it, too. Compared with our primate cousins, we breed more and have larger brains — both of which should sap our energy — and yet we live for longer.
This week, biologists offer an explanation. And it is similar to Indurain’s answer when he was asked to explain his success on the roads: we simply work harder.
In experiments described online on 4 May, scientists took direct measurements of daily energy use in more than a hundred people and in all other known species of great ape (H.Pontzeretal.Naturehttp://dx.doi.org/10.1038/nature17654;2016). Chimpanzees, bonobos, gorillas and orangutans all failed to keep up. Every human expended hundreds of kilocalories a day more than any other ape, and the difference is down to greater metabolic activity in our organs.
In other words, humans have evolved to use more energy. We are the original consumer society: our increased demand for physiological energy is driven by our more efficient way of walking, the energy-dense foods such as meat and tubers we have found, and the methods of cooking we have invented and adopted.
The unusually large energy budget of humans presents both an opportunity and a threat. For a start, it helps to power — and to explain the development of — our unusually large and concomitantly energy-hungry brains. We have always been proud of our large brains. Indeed a century or so ago, men of science (and they usually were all men) would routinely measure human heads and weigh their brainy contents to prove our dominance over the beasts. (They did this as well as making false claims on the primacy of certain human groups over others.) But how we found the fuel to maintain such an expensive cognitive prize, where other primates have not, has long been a puzzle.
Then there is the risk. To have a body that needs to be fed more just to exist is a dangerous strategy in lean times, just as use of gas-guzzling motor vehicles is considered antisocial in a resource-constrained world.
The human culture of food sharing helps us to keep the tank filled. So too does what seems to be a uniquely human trait among the primates: the ability to maintain significant fat reserves as a contingency. Even at his slimmest, Indurain would have struggled to match the body-fat content of the average chimpanzee. We may curse its effects today, but human fat tissue seems to have evolved to protect us from ourselves and our unquenchable thirst for energy. It’s true: those who struggle to keep those fat reserves under control really can blame their metabolism.
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Fat lot of good. Nature 533, 8 (2016). https://doi.org/10.1038/533008a