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Growing A Baby Uses More Energy Than Running A Marathon Every Day For 9 Months, Studies Reveal
Elite athletes push their bodies to extremes in pursuit of glory, but scientists have identified an endurance feat that surpasses even the most punishing races on Earth. And half the population can experience it.
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Athletes train for years to compete in events that test the outer limits of human capability. Cyclists pedal through mountain passes for three weeks straight. Ultrarunners cross entire continents on foot. Antarctic explorers trek through frozen wastelands where a single mistake means death.
Yet researchers studying the boundaries of human performance have discovered something unexpected. When they measured the metabolic demands of pregnancy against the world’s most extreme athletic competitions, they found that growing a baby for nine months ranks among the most taxing endurance events a human body can sustain.
Scientists didn’t arrive at that conclusion through guesswork or sentiment. Duke University researchers spent years tracking athletes through grueling competitions, measuring exactly how much energy their bodies burned each day. What they found challenged assumptions about the limits of human endurance and placed pregnancy in an entirely new category.
Scientists Finally Quantify What Women Already Knew
A landmark 2019 study published in Science Advances examined participants in some of humanity’s most demanding physical challenges. Researchers followed cyclists through the Tour de France, tracked runners completing 3,000-mile transcontinental races, and monitored explorers on Arctic expeditions.
Herman Pontzer, a Duke University professor who co-led the research, wanted to understand where human performance reaches its absolute ceiling. His team measured total energy expenditure in athletes competing in events lasting anywhere from half a day to 250 days. What emerged was a clear pattern that revealed fundamental limits built into human biology.
Pregnancy appeared in the data alongside these extreme athletic events. When researchers compared the metabolic demands of gestation to the energy burned by ultramarathon runners and Tour de France cyclists, the results were striking. Carrying a child for nine months pushes the body to nearly the same limit as running six marathons per week for five months straight.
Breaking Down the Body’s Energy Currency
Scientists measure human performance using Basal Metabolic Rate, or BMR. Your BMR represents the calories your body burns just to keep you alive while lying completely still. Heart beating, lungs breathing, brain thinking, all the invisible work that keeps you functioning burns a specific number of calories each day.
Any activity beyond rest increases that baseline. Walking to the store might push you to 1.5 times your BMR. Running a 5K could hit 2 times your BMR. Elite athletes competing in major events can spike much higher, but only for short periods.
Metabolic scope measures activity level as a multiple of BMR. Athletes performing at 5 times their BMR are burning five times more calories than they would at complete rest. Pontzer explained the pattern his team discovered: “You can do really intense amounts of work for a day or so, but if you have to last a week or so, you have to maintain less intensity.”
Marathon Runners Hit the Wall After 70 Days
Race Across the USA 2015 stands as one of the most documented extreme endurance events in scientific literature. Six athletes attempted to run roughly 3,000 miles from California to Washington, DC, covering approximately one marathon per day, six days per week, for 20 weeks.
Researchers used the doubly labeled water method to track exactly how much energy these runners expended. During week one, athletes operated at 3.76 times their BMR, burning an average of 6,202 calories daily. Bodies that normally needed around 1,650 calories at rest were suddenly processing nearly 10,000 calories per day to fuel the extreme workload.
Week 20 told a different story. Energy expenditure had crashed to 4,906 calories daily, with metabolic scope dropping to 2.81 times BMR. Despite maintaining similar running distances, their bodies had begun shutting down non-essential processes to conserve energy. Weight loss, reduced organ activity, and metabolic compensation all kicked in as survival mechanisms.
Scientists identified 2.5 times BMR as the hard ceiling for sustained human performance. Push beyond that limit for extended periods and your body starts consuming itself, burning through fat stores and eventually muscle tissue. Athletes discovered this boundary the hard way.
Tour de France Cyclists Burn Out in Three Weeks
Professional cyclists competing in grand tours operate at staggering energy outputs. During the 23 days of racing that comprise the Tour de France, riders maintain metabolic scopes around 4.9 times their BMR. Peak efforts on mountain stages can spike even higher.
But those cyclists get rest days. Between major tours, they recover for weeks or months. Elite marathon runners can hit 15.6 times their BMR for the two to three hours it takes to complete 26.2 miles. Antarctic trekkers manage 3.5 times their BMR while hauling sleds across frozen terrain.
All of these achievements push human capability to extreme heights. Yet none can be maintained for more than a few weeks. Past 20 to 30 days, even the fittest athletes on Earth hit the alimentary limit, the point where their digestive systems simply cannot process enough food to meet energy demands.
Pregnancy Operates at Near-Maximum for 280 Days Straight
Pregnant women run at 2.2 times their BMR for roughly 270 to 280 days consecutively. Day after day, month after month, their bodies operate just below the 2.5 times BMR ceiling that represents the absolute limit of sustained human endurance.
Unlike athletes who can stop running when exhaustion hits, pregnant women cannot take a rest day. Building a placenta, expanding blood volume, growing fetal tissue, and supporting two metabolic systems simultaneously demands continuous energy expenditure that rivals the most extreme athletic competitions ever documented.
Pontzer put the finding in stark terms: “To think about pregnancy in the same terms that we think about ultramarathon runners makes you realize how incredibly demanding pregnancy is on the body.”
While a marathon runner experiences higher peak metabolic rates for a few hours, pregnancy maintains near-maximum output for the better part of a year. Duration matters as much as intensity when measuring true endurance capability. By that metric, gestation stands alone.
50,000 Calories to Build a Human
A follow-up study published in Science in May 2024 calculated the total energy cost of a full-term pregnancy. Researchers arrived at a figure that shocked even veteran metabolic scientists: 49,753 dietary calories.
Putting that number in athletic terms makes the scale comprehensible. Running a marathon burns roughly 1,500 to 1,800 calories, depending on body weight and pace. Growing a baby from conception to birth requires the caloric equivalent of running more than 30 consecutive marathons.
Ninety-six percent of that energy goes toward running life support for the developing fetus. Building and maintaining the placenta alone demands enormous resources. Blood volume increases by 50 percent. Organs shift position. Metabolic waste products from two bodies must be processed and eliminated.
Pregnancy fatigue isn’t psychological or a matter of low fitness. Women experience profound exhaustion during gestation because their bodies are performing at near-maximum metabolic capacity day after day without relief. Science has now quantified what mothers have known intuitively for millennia.
Your Digestive System Sets the Limit
Pontzer and his team identified the alimentary system as the bottleneck constraining long-term endurance. Your digestive tract and liver have a maximum processing capacity. Beyond roughly 2.5 times your BMR, your body cannot extract enough energy from food fast enough to meet demand.
Athletes exceeding this limit during competitions burn through their own tissue. Fat stores provide fuel first, then muscle breaks down for energy. Weight loss during ultraendurance events isn’t from dehydration or poor nutrition. Bodies are literally consuming themselves because food intake cannot keep pace with expenditure.
Pregnancy operates differently from athletic competition but faces the same constraints. Women must maintain weight gain throughout gestation to ensure fetal health. Operating at 2.2 times BMR for nine months puts them right at the edge of sustainable metabolism while still allowing the weight gain necessary for a healthy pregnancy.
Other potential limits, cardiovascular capacity or thermoregulation, proved less relevant. Hearts and lungs can deliver oxygen at rates exceeding 10 times BMR for hours. Temperature regulation through sweating handles heat loads from intense exercise. But digestive capacity remains fixed at that 2.5 times BMR ceiling regardless of fitness level.
No Finish Line, No Medal, No Standing Ovation
Tour de France winners receive yellow jerseys, multimillion-dollar sponsorship deals, and global recognition. Ultramarathon finishers get medals, trophies, and cheering crowds at the finish line. Olympic athletes stand on podiums while national anthems play.
Pregnant women get none of that. Most work full-time jobs while operating at near-maximum metabolic output. Many receive minimal workplace accommodation. Society expects them to maintain normal schedules, professional responsibilities, and household duties while their bodies perform feats of endurance that would leave elite athletes bedridden.
No one lines the streets cheering for women in their third trimester. Strangers don’t high-five them for hitting month six of sustained metabolic performance. Maternity leave policies in many countries treat pregnancy as an inconvenience rather than a recognition of an extreme endurance event.
Athletes choose their challenges and train for years to compete. Women don’t get to opt out of pregnancy’s metabolic demands once conception occurs. Yet cultural attitudes still frame athletic achievement as heroic while treating pregnancy as a routine biological function.
What This Means for Understanding the Female Body
Reframing pregnancy as an extreme endurance event rather than a passive biological process changes the conversation around maternal health, workplace policy, and social support systems.
Science has provided objective measurements that validate what women have been saying for generations. Pregnancy exhaustion is real, quantifiable, and comparable to the fatigue experienced by athletes pushing the absolute limits of human capability. Dismissing it as a weakness or complaining ignores metabolic reality.
Perhaps most valuable is the simple validation these studies provide. When women describe pregnancy as exhausting, as pushing them to their limits, as one of the hardest things they’ve done physically, they’re not exaggerating. Science backs them up with hard data showing they’re performing an endurance feat that ranks alongside the most extreme athletic competitions on Earth.
Growing a human requires more sustained energy output than running across a continent. Biology has simply made that achievement so common that we have forgotten how extraordinary it really is.