This scientist busts myths about how humans burn calories—and why

By borrowing a method developed by physiologists studying obesity,
Pontzer and colleagues systematically measure the total energy used
per day by animals and people in various walks of life. The answers
coming from their data are often surprising: Exercise doesn’t help you
burn more energy on average; active hunter-gatherers in Africa don’t
expend more energy daily than sedentary office workers in Illinois;
pregnant women don’t burn more calories per day than other adults,
after adjusting for body mass.  Metabolism over the life span

Adjusted for body mass, toddlers burn the most calories per day. Total
energy expenditure (TEE) declines after age 60, although individuals
show some variation (gray dots).  TEE was close to 150% of what
scientists expected in toddlers, leveled out around age 25, and slowly
declined after age 60.  (Graphic) C. Bickel and N. Desai/Science;
(Data) H. Pontzer et al., Science, 373, 6556 (2021)

He realized he had to go back to basics, measuring the calories
expended by humans and animals walking and running on
treadmills. Mammals use oxygen to convert sugars from food into
energy, with CO2 as a byproduct. The more CO2 a mammal exhales, the
more oxygen—and calories—it has burned.

For his Ph.D. thesis, Pontzer measured how much CO2 dogs and goats
exhaled while running and walking. He found, for example, that dogs
with long legs used less energy to run than corgis, as he reported in
2007, soon after he got his first job at Washington University in
St. Louis. Over time, he says, “What started as an innocent project
measuring the cost of walking and running in humans, dogs, and goats
grew into a sort of professional obsession with measuring energy
expenditures.”

Pontzer still measures exhaled CO2 to get at calories burned in a
particular activity, as he did with Christina’s stress test. But he
found that physiologists had developed a better way to measure TEE
over a day: the doubly labeled water method, which measures TEE
without asking a subject to breathe into a hood all day.

Physiologist Dale Schoeller, now at the University of Wisconsin,
Madison, had adapted the method, first used in mice, to humans. People
drink a harmless cocktail of labeled water, in which distinct isotopes
of hydrogen and oxygen replace the common forms. Then researchers
sample their urine several times over 1 week. The labeled hydrogen
passes through the body into urine, sweat, and other fluids, but as a
person burns calories, some of the labeled oxygen is exhaled as
CO2. The ratio of labeled oxygen to labeled hydrogen in the urine thus
serves as a measure of how much oxygen a person’s cells used on
average in a day and therefore how many calories were burned. The
method is the gold standard for total energy use, but it costs $600
per test and was out of reach for most evolutionary biologists.

Pontzer’s first of many breakthroughs with the method came in 2008
when, with $20,000 from the Wenner-Gren Foundation, he got the chance
to collect urine samples at what was then the Great Ape Trust, a
sanctuary and research center in Iowa. There, primatologist Rob
Shumaker poured isotope-laced sugar-free iced tea into the mouths of
four orangutans. Pontzer worried about collecting the urine from a
full-grown ape, but Shumaker reassured him the orangs were trained to
pee in a cup.

Later that fall, when Pontzer got the urine results, he didn’t believe
them: The orangutans burned one-third of the energy expected for a
mammal their size. A retest returned the same results: Azy, a
113-kilogram adult male, for example, burned 2050 kilocalories per
day, much less than the 3300 a 113-kilogram man typically burns. “I
was in total disbelief,” Pontzer says. Orangs were perhaps the “sloths
in the ape family tree,” he thought, because they suffered prolonged
food scarcity in their past and had evolved to survive on fewer
calories per day.

Subsequent doubly labeled water studies of apes in captivity and in
sanctuaries shattered the consensus view that mammals all have similar
metabolic rates when adjusted for body mass. Among great apes, humans
are the outlier. When adjusted for body mass, we burn 20% more energy
per day than chimps and bonobos, 40% more than gorillas, and 60% more
than orangutans, Pontzer and colleagues reported in Nature in 2016.
The high-energy ape

Humans burn far more energy daily—and also store much more energy as
fat—than other apes. Our total energy expenditure (TEE) includes our
basal metabolic rate (BMR) plus other activities including exercise.
Human TEE is a few hundred kilocalories per day higher than other
apes.  (Graphic) C. Bickel and N. Desai/Science; (Data) H. Pontzer et
al., Nature, 533, 390 (2016)

Pontzer says the difference in body fat is just as shocking: Male
humans pack on twice as much fat as other male apes and women three
times as much as other female apes. He thinks our hefty body fat
evolved in tandem with our faster metabolic rate: Fat burns less
energy than lean tissue and provides a fuel reserve. “Our metabolic
engines were not crafted by millions of years of evolution to
guarantee a beach-ready bikini body,” Pontzer writes in Burn.

Our ability to convert food and fat stores into energy faster than
other apes has important payoffs, however: It gives us more energy
every day so we can fuel our big brains as well as feed and protect
offspring with long, energetically costly childhoods.

Pontzer thinks characteristically human traits in behavior and anatomy
help us maintain amped-up metabolisms. For example, humans routinely
share more food with other adults than do other apes. Sharing food is
more efficient for the group, and would have given early humans an
energy safety net. And our big brains created a positive feedback
loop. They demanded more energy but also gave early humans the smarts
to invent better tools, control fire, cook, and adapt in other ways to
get or save more energy.

Pontzer got a lesson in the value of food sharing in 2010, when he
traveled to Tanzania to study the energy budgets of the Hadza
hunter-gatherers. One of the first things he noticed was how often the
Hadza used the word “za,” which means “to give.” It’s the magic word
all Hadza learn as children to get someone to share berries, honey, or
other foods with them. Such sharing helps all the Hadza be active: As
they hunt and forage, Hadza women walk about 8 kilometers daily; men,
14 kilometers—more than a typical American walks in 1 week.

To learn about their energy expenditure, Pontzer asked the Hadza
whether they’d drink his tasteless water cocktail and give urine
samples. They agreed. He almost couldn’t get funding for the study,
because other researchers assumed the answer was obvious. “Everyone
knew the Hadza had exceptionally high energy expenditures because they
were so physically active,” he recalls. “Except they didn’t.”

Individual Hadza had days of more and less activity, and some burned
10% more or less calories than average. But when adjusted for nonfat
body mass, Hadza men and women burned the same amount of energy per
day on average as men and women in the United States, as well as those
in Europe, Russia, and Japan, he reported in PLOS ONE in 2012. “It’s
surprising when you consider the differences in physical activity,”
Schoeller says.

One person who wasn’t surprised was epidemiologist Amy Luke at Loyola
University Chicago. She’d already gotten a similar result with doubly
labeled water studies, showing female farmers in western Africa used
the same amount of energy daily when adjusted for fat-free body mass
as women in Chicago —about 2400 kilocalories for a 75-kilogram
woman. Luke says her work was not well known—until Pontzer’s paper
made a splash. The two have collaborated ever since.

Studies of other hunter-gatherer and forager groups have confirmed the
Hadza are not an anomaly. Pontzer thinks hunter-gatherers’ bodies
adjust for more activity by spending fewer calories on other unseen
tasks, such as inflammation and stress responses. “Instead of
increasing the calories burned per day, the Hadza’s physical activity
was changing the way they spend their calories,” he says.

He backed this up with a new analysis of data from another team’s
study of sedentary women trained to run half marathons: After weeks of
training, they barely burned more energy per day when they were
running 40 kilometers per week than before they started to train. In
another study of marathoners who ran 42.6 kilometers daily 6 days per
week for 140 days in the Race Across the USA, Pontzer and his
colleagues found the runners burned gradually less energy over
time—4900 calories per day at the end of the race compared with 6200
calories at the start.

As the athletes’ ran more and more over weeks or months, their
metabolic engines cut back elsewhere to make room for the extra
exercise costs, Pontzer says. Conversely, if you’re a couch potato,
you might still spend almost as many calories daily, leaving more
energy for your body to spend on internal processes such as a stress
response.

Our metabolic engines were not crafted by millions of years of
evolution to guarantee a beach-ready bikini body.

This is Pontzer’s “most controversial and interesting idea,” says
Harvard paleoanthropologist Daniel Lieberman, who was Pontzer’s thesis
adviser. “This morning I ran about 5 miles; I spent about 500 calories
running. In a very simplistic model that would mean my TEE would be
500 calories higher. … According to Herman, humans who are more active
don’t have that much higher TEE as you’d predict … but we still don’t
know why or how that occurs.”

Pontzer’s findings have a discouraging implication for people wanting
to lose weight. “You can’t exercise your way out of obesity,” says
evolutionary physiologist John Speakman of the Chinese Academy of
Sciences. “It’s one of those zombie ideas that refuses to die.”
Already the research is influencing dietary guidelines for nutrition
and weight loss. The U.K. National Food Strategy, for example, notes
that “you can’t outrun a bad diet.”

But Thyfault warns that message may do more harm than good. People who
exercise are less likely to gain weight in the first place, and those
who exercise while they diet tend to keep weight off better, he
says. Exercise also can impact where fat is stored on the body and the
risk of diabetes and heart disease, he says.

Pontzer agrees that exercise is essential for good health: The Hadza,
who are active and fit into their 70s and 80s, don’t get diabetes and
heart disease. And, he adds, “If exercise is tamping down the stress
response, that compensation is a good thing.” But he says it’s not
fair to mislead dieters: “Exercise prevents you from getting sick, but
diet is your best tool for weight management.”

Meanwhile, Pontzer was laying the groundwork for other surprises. Last
year, he and Speakman co-led an effort to assemble a remarkable new
resource, the International Atomic Energy Agency Doubly Labelled Water
Database. This includes existing doubly labeled water studies of
almost 6800 people between the ages of 8 days and 95 years.

They used the database to do the first comprehensive study of human
energy use over the life span. Again a popular assumption was at
stake: that teenagers and pregnant women have higher metabolisms. But
Pontzer found it was toddlers who are the dynamos. Newborns have the
same metabolic rate as their pregnant mothers, which is no different
from other women when adjusted for body size. But between the ages of
9 and 15 months, babies expend 50% more energy in a day than do
adults, when adjusted for body size and fat (see graphic,
above). That’s likely to fuel their growing brain and, perhaps,
developing immune systems. The findings, reported in Science, help
explain why malnourished infants may show stunted growth.

Children’s metabolisms stay high, when adjusted for body size, until
about age 5, when they begin a slow decline until age 20, and
stabilize in adulthood. Humans begin to use less energy at age 60, and
by age 90, elders use 26% less than middle-aged adults.

Pontzer is now probing a mystery that emerged from his studies of
athletes: There seems to be a hard limit on how many calories our
bodies can burn per day, set by how fast we can digest food and turn
it into energy. He calculates that the ceiling for an 85-kilogram man
would be about 4650 calories per day.

Speakman thinks that limit is too low, noting that cyclists in the
Tour de France in the 1980s and ’90s exceeded it. But they were
injecting fat and glucose directly into their bloodstreams, a practice
Pontzer thinks might have helped them bypass the physiological limits
on converting food into energy. Elite athletes can push the limits for
several months, as the study of marathoners showed, but can’t sustain
it indefinitely, Pontzer says.

To understand how the body can fuel intense exercise or fight off
disease without busting energy limits, Pontzer and his students are
exploring how the body tamps down other activities. “I think we’re
going to find these adjustments lower inflammation, lower our stress
reaction. We do it to make the energy books balance.”

That’s why he wanted to know how much energy Christina burned while he
grilled her in the lab. After the test, Christina said she “definitely
was stressed.” As it went on her heart rate rose from 75 to 80 beats
per minute to 115. And her energy use rose from 1.2 kilocalories per
minute to as much as 1.7 kilocalories per minute.

“She burned 40% more energy per minute in the math test and 30% in the
interview,” Pontzer says. “Think about any other process that boosts
your energy by about 40%.”

He hopes data points like hers will help reveal the hidden cost of
mental stress. Measuring how stress and immune reactions amp up energy
use could help reveal how these invisible activities add up and are
traded off in our daily energy budgets. Pontzer knows he’s got his
work cut out for him. “Until we can show how the levers get pulled to
make these adjustments in energy use, people will always be
skeptical. It’s on us to do the next generation of experiments.”