New Human Metabolism Research Upends Conventional Wisdom about How We Burn Calories

New Human Metabolism Research Upends Conventional Wisdom about How We Burn Calories

It was my daughter Clara’s seventh birthday party, a scene at once familiar and bizarre. The celebration was an American take on a classic script: a shared meal of pizza and picnic food, a few close COVID-compliant friends and family, a beaming kid blowing out candles on a heavily iced cake. With roughly 380,000 boys and girls around the world turning seven each day, it was a ritual no doubt repeated by many, the world’s most prolific primate singing “Happy Birthday” in an unbroken global chorus.

Such a wholesome setting seems an unlikely place for rampant rule breaking. But as an evolutionary anthropologist, I can’t help but notice the blatant disregard our species shows for the natural order. Nearly every aspect of our modern lives marks a cheerfully outrageous departure from the laws that govern every other species on the planet, and this birthday party was no exception. Aside from the fresh veggies left wilting in the sun, none of the food was recognizable as a product of nature. The cake was a heat-treated amalgam of pulverized grass seed, chicken eggs, cow milk and extracted beet sugar. The raw materials for the snacks and drinks would take a forensic chemist years to reconstruct. It was a calorie bonanza that animals foraging in the wild could only dream about, and we were giving it away to people who didn’t even share our genes. All this to celebrate some obscure astronomical alignment, the moment our planet swept through the same position relative to its star as on the day my daughter was born. At seven years old, most mammals are grandparents if they’re lucky enough to be alive. Clara was still a kid, dependent on us for food and shelter and years away from independence.

Humans weren’t always such scofflaws. We come from a good Family. The living apes, our closest relatives, are well-behaved primates, eating fruit and leaves straight from the tree and nibbling on the occasional meal of insects or small game. Like every other mammal, apes learn early to fend for themselves, foraging on their own as soon as they’re weaned, and they know better than to give their hard-earned food away. Fossils from deep in the human lineage, the first four million years after we broke from the other apes, indicate our early ancestors played by the same ecological rules.

Around 2.5 million years ago things took an unlikely turn. Early populations of the genus Homo stumbled onto a new way of making a living, something unprecedented in the history of life. Instead of pursuing a career as a plant eater, carnivore or generalist, they tried a strange, dual strategy: some would hunt, others would gather, and they’d share whatever they acquired. This cooperative approach placed a premium on intelligence, and over millennia brain size began to increase. Our Paleolithic ancestors learned to knap delicate blades from round stone cobbles, hunt large game and cook their food. They built hearths and homes and began changing the landscape, developing an ecological mastery that led eventually to farming.

These evolutionary shifts reverberate today. The cooperative foraging that pushed our hunting, gathering and farming ancestors to flout long-established ecological rules didn’t just change the foods we eat. It altered fundamental aspects of our biology, including our metabolism. The same unlikely series of events that gave us birthday cake has also shaped the way we eat it—and how we use the calories.

For all the talk about metabolism in the exercise and dieting worlds, you would think the science was settled. In reality, we’ve been embarrassingly short on hard data about the calories we burn each day and how we evolved to obtain them. But recently my colleagues and I have made important strides in understanding how our bodies use energy. Our findings have overturned much of the received wisdom about the ways human energy requirements change over the course of a lifetime. And, as we’ve discovered in a parallel effort, our energy needs are deeply intertwined with the evolution of our food-production strategies: foraging and farming. Together these studies provide the clearest picture yet of the inner workings of the human engine—and how our strategy for earning, burning and sharing calories underpins our extraordinary success as a species.

Energy Budgets

Our bodies are wonders of coordinated chaos. Every second of every day, each of your 37 trillion cells is hard at work, pulling in nutrients, building new proteins and doing the myriad other tasks that keep you alive. All of this work takes energy. Our metabolism is the energy we expend (or the calories we burn) each day. That energy comes from the food we eat, and so our metabolism also sets our energy requirements. Calories in, calories out.

Evolutionary biologists often think about metabolism as an organism’s energy budget. Life’s essential tasks, including growth, reproduction and bodily maintenance, require energy. And every organism must balance its books.

Humans are a striking example of this evolutionary book-keeping in action. The traits that distinguish us from the other apes, including our huge brains, big babies and long lives, all require a lot of energy. We pay for some of these costs by spending less on our digestive system, having evolved a shorter intestinal tract and smaller liver. But we have also increased our metabolic rate and the size of our energy budget. For our body size, humans consume and burn more calories each day than any of the other apes. Our cells have evolved to work harder.

The work our bodies do changes as we age, the activities of our cells waxing and waning in a choreographed dance from growth to adulthood to senescence. Tracking those changes through our metabolism could provide a better understanding of the work our cells do at each age as well as our changing calorie needs. But a clear audit of our metabolism over the human life span has been hard to obtain.

It’s obvious that adults need more calories than infants—bigger people have more cells doing more work, so they burn more energy. We also know that elderly people tend to eat less, although that’s often accompanied by a loss of body weight, particularly muscle mass. But if we want to know how active our cells are and whether metabolism gets faster or slower as we grow up and grow old, we need to separate the effects of age and size, which is not easy. You need a large sample with people of all ages, measured with the same methods. Ideally, you’d want measures of total daily energy expenditure, a full tally of the calories used each day.

Researchers have been measuring metabolic rates at rest for more than a century, with some evidence for faster metabolism in children and slower metabolism among the elderly. Yet resting metabolism accounts for only 60 percent or so of the calories we burn over 24 hours and doesn’t include the energy we spend on exercise and other physical activity. Online calorie calculators purport to include activity costs, but they’re really just a guess based on your self-reported weight and physical activity. In the absence of solid evidence, a kind of folk wisdom has developed, cheered on and cultivated by charismatic hucksters selling metabolic boosters and other snake oil. We’re often told our metabolism speeds up at puberty and slows down in middle age, particularly with menopause, and that men have faster metabolisms than women. None of these claims is based on real science.

One chart plots total daily energy use against fat-free body mass; another plots relative daily energy use against age.
Credit: Herman Pontzer (restyled by Jen Christiansen); Source: “Daily Energy Expenditure through the Human Life Course,” by Herman Pontzer et al., in Science, Vol. 373; August 2021

A Metabolic Database

My colleagues and I have begun to fill that gap in scientific understanding. In 2014 John Speakman, a researcher in metabolism with laboratories at the University of Aberdeen in Scotland and the Chinese Academy of Sciences in Shenzhen, organized an international effort to develop a large metabolic database. Crucially, this database would focus on total daily energy expenditure measured using the doubly labeled water method, an isotope-tracking technique that measures the carbon dioxide produced by the body (and thus the calories burned) over one to two weeks. Doubly labeled water is the gold standard for measuring daily energy expenditures, but it’s expensive, and you need a specialized lab for the isotope analyses. So even though this technique has been around for decades, studies are typically small. Led by Speakman, my lab joined a dozen others around the world in pooling decades of data. We ended up with more than 6,400 measurements of people ranging from babies just eight days old to men and women in their 90s.

In 2021, after years of collaborative effort, we published the first comprehensive study investigating the effects of age and body size on daily energy expenditure. As expected, we found that metabolic rates increase with body size: bigger people burn more calories. In particular, fat-free mass (the muscles and other organs) is the single strongest predictor of daily energy expenditure. This makes good sense. Fat cells aren’t as active as those in the liver, brain, or other tissues, and they don’t contribute much to your daily expenditure. More important, with the relation between mass and metabolic rate clearly established from thousands of measurements, we could finally test whether metabolism at each age was faster or slower than we’d expect from size.

The results were a revelation, the first clear road map of metabolism over the human life span. We found that, metabolically, babies are born like tiny adults, reflecting their development as part of their mom’s energy budget. But metabolism skyrockets over the first year of life, so that by their first birthday children are burning 50 percent more energy than we’d expect for their size. Their cells are far busier than adults’ cells, hard at work on growth and development. Earlier studies measuring glucose uptake in the brain during childhood suggest some of this work is neuronal growth and synapse development. Maturation in other systems no doubt contributes as well. Metabolism stays elevated through childhood, slowly decelerating through adolescence to land at adult levels around age 20. Boys decline more slowly than girls, consistent with boys’ slower development, but there’s no bump at puberty in males or females.

Perhaps the biggest surprise was the stability of our metabolism through middle age. Daily energy expenditures hold remarkably steady from age 20 to 60. No middle age slowdown, no change with menopause. The weight gain so many of us experience in adulthood cannot be blamed on a declining metabolism. As a man in my 40s, I had sort of believed the folk wisdom that metabolism slowed as we aged. My body definitely feels different than it did 10 or 20 years ago. But like hunting some metabolic Sasquatch, when you actually look there’s nothing there. Same for the much touted metabolic differences between men and women. Women have lower daily energy expenditures on average, but that is only because women tend to be smaller and carry more of their weight as fat. Compare men and women with the same body weight and body fat percentage, and the metabolic difference disappears.

We did find a decline in metabolism with age, but it doesn’t kick in until we hit 60. After 60, metabolism slows by around 7 percent per decade. By the time men and women are in their 90s, their daily expenditures are 20 to 25 percent lower, on average, than those of adults in their 50s. That’s after we account for body size and composition. Weight loss with old age, especially diminished muscle mass, compounds the decline in expenditure. As with all age groups, there’s a good amount of individual variability. Maintaining a younger, faster metabolism into old age might be a sign of aging well, or perhaps it is even protective against heart disease, dementia and other age-related disease. We can now start to investigate these connections. Guided by our metabolic road map, we have a new world of research ahead of us.

What is already apparent, however, is that a bite of birthday cake does different things for a seven-year-old girl, her middle-aged dad and her elderly grandmother. Clara’s bite is likely to be gobbled up by busy cells, fueling development. Mine might go to maintenance, repairing all the little bits of damage accrued through the course of the day. As for Grandma, her aging cells might be slow to use the calories at all, storing them instead as glycogen or fat. Indeed, for any of us, the cake will end up as fat if we eat more calories than we burn.

The road map also highlights a major conundrum of the human condition. Whether they’re born into a hunter-gatherer camp, a farming village or an industrial megacity, human youngsters need a lot of help getting food. Other apes learn to forage for themselves by the time they stop nursing, around the age of three or four. Our children are wholly dependent on others for food for years and aren’t self-sufficient until their teens. And those least able to fend for themselves have the greatest energy needs. Not only has our species evolved a faster metabolic rate and greater energy demands than other apes, but we must also provision each costly offspring for more than a decade. Where do we get all those calories? Recently my colleagues and I worked out this part of the human energy equation, too.

Costly Kids

The question of calories looms largest in hunter-gatherer and farming communities, where daily life revolves around food production. For most of our species’ history, as for most species, there was no other line of work. Every kid knew what they were going to be when they grew up. As late as the mid-1800s, more than half of the American workforce was made up of farmers.

For the past decade I’ve been working with colleagues to understand the calorie economy in the Hadza community of northern Tanzania. The Hadza are a small population of 1,000 or so, and about half of them maintain a traditional hunting-and-gathering way of life, foraging on the savanna landscape they call home. No population alive today is a perfect model of the past, but groups like the Hadza, who continue these traditions, provide a living example of how these systems work. Men spend most days hunting with bow and arrow or chopping into hollow tree limbs to pillage honey from beehives. Women gather berries and other plant foods or dig for wild tubers in the rocky soil. Hadza camps, small collections of grass houses tucked among the acacia trees, are alive all day with kids being kids, running around, laughing, playing—and waiting for adults to bring them food.

We’ve measured Hadza energy budgets using doubly labeled water, giving us a clear idea of the calories men and women consume and expend each day. We’ve also lugged portable respirometry equipment into the bush, a metabolic lab in a briefcase, to measure the energy costs of foraging activities such as walking, climbing, digging tubers and chopping trees. And we’ve got years of careful observation recording the hours spent each day on different foraging tasks and the amount of food acquired. After more than a decade of work, we’ve got a complete accounting of the Hadza energy economy: the calories spent to get food, the calories acquired, the proportions shared and consumed.

Tom Kraft of the University of Utah led our team’s effort to compare the energy budgets of the Hadza population with similar data from other human groups and from other species of apes. It was a massive project, poring over old ethnographic accounts of hunter-gatherer and farming groups and combing through ecological studies and doubly labeled water measurements in apes to reconstruct their foraging economies. But when we were finished, what emerged was a new understanding of the energetic foundation for our species’ success. We could finally see where all those calories come from, the energy needed to fuel expensive human metabolisms and provision helpless kids.

Clever Cooperators

It turns out humans’ unique, cooperative foraging strategy, combined with our clever brains and tools, makes hunting and gathering extremely productive. Even in the harsh, dry savanna of northern Tanzania, Hadza men and women acquire 500 to 1,000 kilocalories of food an hour, on average. Ethnographic records from other groups around the world suggest these rates are typical for hunter-gatherers. Five hours of hunting and gathering can reliably bring in 3,000 to 5,000 kilocalories of food, enough to meet a forager’s daily needs and provision the camps’ children.

It’s the positive feedback engine that propelled the human species to new heights. Hunting and gathering is so productive that it creates an energy surplus. Those extra calories are channeled to offspring, meaning they can take longer to develop, learning skills that make them effective foragers. Reaching adulthood, they’ll do just as their parents did, acquiring extra food and plowing those calories into the next generation. Over evolutionary time childhood grows longer as foraging strategies grow more complex. Life spans get extended, too, with natural selection favoring additional years of productive foraging to support children and grandchildren. Grandparents, once rare, become a fixture of the social network.

Apes in the wild are not nearly as productive in gathering food. A forensic accounting of the energy budgets for chimpanzees, gorillas and orangutans shows that males and females get around 200 to 300 kilocalories an hour. It takes them seven hours of foraging just to meet their own needs each day. No wonder they don’t share.

Our hyperproductive foraging isn’t cheap. People in hunter-gatherer communities expend more than twice as much energy to acquire food as apes in the wild. Surprisingly, human technology and smarts don’t make us very energy-efficient. Hadza men and women achieve the same paltry ratio of energy acquired to energy expended that we find in wild apes. Cooperation and culture enable human foragers to be incredibly time-efficient, acquiring lots of calories an hour, but our unique foraging strategies are still energetically demanding. Hunting and gathering is hard work.

Farming isn’t any easier, but our analyses found it can be even more productive. When we compared the energy budgets for the Hadza and other hunter-gatherer populations with those of traditional farming groups, we found that farmers typically produce far more calories an hour. The Tsimane community, a population in the Amazonian rain forest of Bolivia, provides a useful point of comparison. The Tsimane get most of their calories from farming, but they also hunt, fish and collect wild plants. With farmed foods as their energy staple, they produce nearly twice as many calories an hour as the Hadza. They’re more energy-efficient as well, getting more food from every calorie they spend foraging and farming.

Those extra calories are embodied in the children running around Tsimane villages. More food and faster production mean a lighter workload for mothers because others in the community can more easily share the time and energy costs of caring for kids. As with many subsistence farming communities, Tsimane families tend to be large. Women have an average of nine children over the course of their lives. Compare that with the average fertility rate of six children per mother in the Hadza community, and the impact of that extra energy is inescapable. And it’s not just the Tsimane. Farming communities tend to have higher fertility rates than hunter-gatherer communities. Increased fertility is an important reason farming overtook hunting and gathering in the Neolithic age, the time spanning roughly 12,000 to 6,500 years ago. Archaeological sites across Eurasia and the Americas document a rising tide of children and adolescents following the development of agriculture.

Having Our Cake

From this perspective, a kid’s birthday party is more than a personal milestone. It’s a celebration of our improbable evolutionary story. There’s the food, of course. We get the flour and sugar for the cake from our farming ancestors, the fire to bake it from the Paleolithic era. The milk and eggs come from animals that we’ve completely transformed from species we once hunted, shaped to our will over generations of careful husbandry. And there’s the calendar we use to mark our days and measure our years, an invention of agriculturalists who needed to know precisely when to reap and sow. Hunter-gatherers track the seasons and lunar cycles but have little use for accurate annual calendars. There are no birthdays in a Hadza camp.

But the key element of any celebration is the community of friends and relatives, multiple generations gathering to eat and laugh and sing. Our evolved social contract—to hunt, gather and farm collectively—tied us together, gave us our childhood and extended our golden years. Cooperative foraging also helped to fuel the cultural complexity and innovation that make birthdays and other rituals so fantastical and diverse. And at the center of it all is the universal commitment to share.

With eight billion humans on the planet today, one might begin to worry that we’ve taken things a bit too far. We’ve learned to turbocharge our energy budgets by tapping into climate-changing fossil fuels and flooding our world with cheap food. Calories are so easy to produce that very few of us spend our days foraging, a first in the history of life. This massive shift has been a boon to our collective creativity, enabling many to spend their lives as artists, doctors, teachers, scientists—a range of careers outside of food production. Having carved out our own strange niche, far removed from the laws that govern the rest of the natural world, we have only ourselves to look to for guidance. With a little luck and a lot of cooperation, we just might secure the human lineage another couple million birthdays. Make a wish.

This article was originally published with the title “The Human Engine” in Scientific American 328, 1, 24-29 (January 2023)




    Herman Pontzer is a professor of evolutionary anthropology at Duke University. He studies how evolution has shaped human physiology and health. He is author of Burn: New Research Blows the Lid Off How We Really Burn Calories, Stay Healthy, and Lose Weight (Avery, 2021). Credit: Nick Higgins

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