Scientists wanted to find the upper limit of human endurance. So they tracked ultramarathon runners who covered 3,000 miles across America, measuring every calorie burned during their 140-day journey. Athletes ran six marathons per week for nearly five months while researchers collected data on their bodies’ energy expenditure.
Results revealed something unexpected. After compiling measurements from the transcontinental race and comparing them with data from other extreme endurance events, researchers discovered a hard biological ceiling that no human can sustain indefinitely. But the most surprising finding came when they looked at pregnancy data.
One group matched the ultrarunners. One group pushed human metabolism to nearly the same extreme limits as athletes crossing continents. Expectant mothers.
Your Body Burns Calories Even While Sleeping
Every human body requires a baseline amount of energy just to stay alive. Hearts pump blood. Lungs draw breath. Cells divide and repair themselves. Brains process information. All of these processes happen whether you move or not. Scientists call this baseline energy requirement your Basal Metabolic Rate, or BMR.
Any activity beyond lying still increases energy expenditure above BMR. Walking to the kitchen burns more than sitting. Running burns more than walking. Scientists measure activity intensity as “metabolic scope,” expressed as a multiple of BMR. Light daily activities might put you at 1.5 times BMR. Moderate exercise could reach 2 times BMR.
But what happens at the extreme upper range? How high can humans push their metabolic rate, and more importantly, for how long can they sustain it?
A Race Across an Entire Country
Duke University researchers found their testing ground in 2015. Race Across the USA sent runners from Huntington Beach, California, to Washington, D.C., covering 4,957 kilometers over 20 weeks. Participants ran approximately 42 kilometers per day, six days per week. Unlike shorter races where athletes can push hard and collapse at the finish line, these runners had to maintain effort day after day for months.
Six athletes agreed to participate in the metabolic study. Researchers used the doubly labeled water method, a gold standard technique that tracks isotopes in urine to calculate exact energy expenditure. Measurements happened before the race started, during the first week, and again during the final week.
Week one data showed bodies working at extreme levels. Athletes burned around 6,200 calories per day, operating at 3.76 times their resting metabolic rate. Water intake averaged 8 liters daily to keep up with fluid loss. Energy expenditure matched predictions based purely on running workload. Bodies were handling the stress exactly as expected.
Bodies Slowed Down Without Permission
By week 20, the three runners who completed the planned course on schedule showed a dramatic shift. Daily energy expenditure dropped by 1,224 calories compared to week one, a 20% reduction. Metabolic scope fell to just 2.81 times BMR. Resting metabolic rate stayed essentially unchanged, so the reduction came from somewhere else.
Runners covered slightly less distance per day by the end. Body mass decreased somewhat. But even after accounting for these changes, measured energy expenditure still came in 596 calories per day lower than predictions. Bodies had adapted by reducing energy spent on activities beyond running. Non-exercise movement decreased. Organ systems dialed back their activity. Running efficiency may have improved slightly.
Whatever the mechanisms, one pattern became clear: humans cannot maintain extreme energy expenditure indefinitely. Duration matters.
A Curve That Captures Every Endurance Event
Researchers compiled metabolic data from endurance activities spanning half a day to 95 days. Tour de France cyclists. Arctic trekkers. Ironman triathletes. Each study measured total energy expenditure and basal metabolic rate, allowing calculation of metabolic scope.
Plotting metabolic scope against event duration revealed a strong pattern. Short events allowed massive energy expenditure. Marathon runners hit 15.6 times BMR for a few hours. Triathletes reached 9.4 times BMR for 11 hours. A 25-hour ultramarathon showed 8.5 times BMR.
But longer events told a different story. A 10-day Arctic trek measured 6.6 times BMR. By 23 days, Tour de France cyclists operated at roughly 4 to 5 times BMR. Events lasting several months showed even lower sustained rates. Every data point fit a smooth logarithmic curve that decreased with duration and eventually plateaued.
Maximum sustained metabolic scope leveled off below 3 times BMR. No measured human performance exceeded this curve.
Herman Pontzer, a Duke University professor who co-led the study, explained the pattern: “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.”
Digestive Systems Cannot Keep Up
Why does this ceiling exist? Athletes’ hearts and lungs can deliver oxygen and nutrients at more than 10 times BMR for hours. Muscles can handle intense workloads. The cardiovascular systems function well under stress. Yet something constrains long-term performance.
Researchers tested whether the digestive system imposed the limit. They examined overfeeding studies where participants consumed far more calories than they burned while keeping physical activity very low. If energy intake has a maximum rate regardless of expenditure, it would point to an alimentary constraint.
Results from six overfeeding studies showed maximum energy intake averaged 2.36 times BMR. Whether exercising or sedentary, humans cannot process and absorb energy faster than this rate. Combined with endurance event data, evidence pointed to an alimentary limit around 2.5 times BMR.
Energy expenditure above this threshold requires drawing down the body’s fat and muscle stores. Athletes burning 4 times BMR during week one of the transcontinental race were consuming their own tissue to supplement dietary intake. Weight loss became inevitable. Bodies cannot sustain this indefinitely because energy reserves eventually run out.
Pregnancy Sits Right at the Limit
Researchers examined metabolic data from pregnancy studies. Measurements showed metabolic rate increasing as pregnancy progressed, peaking at 2.2 times pre-pregnancy BMR. Lactation added further energy demands.
Expectant mothers sustained this elevated metabolism for approximately 280 days. Nine full months. When researchers plotted pregnancy on the same graph as ultramarathons and Arctic expeditions, it fit perfectly on the curve. Gestation length and energy expenditure matched the mathematical relationship seen in every other endurance activity.
Pregnancy operates just below the 2.5 times BMR ceiling. Metabolic demands cannot increase beyond this point without compromising fetal growth or shortening gestation. Bodies hit the same fundamental limit whether running across continents or growing another human.
Pontzer put it directly: “If you think about pregnancy, it is the ultimate endurance activity.”
Nearly 50,000 Calories to Build a Human
A 2024 study calculated the total energy cost of pregnancy. Researchers arrived at a number that vastly exceeded previous estimates: approximately 49,753 dietary calories for a full-term pregnancy.
Breaking down this enormous figure, 96% of energy goes toward maternal life support systems. Blood volume increases dramatically. The placenta grows and functions as a temporary organ. Metabolism runs at elevated rates continuously. Only a small fraction goes directly to building fetal tissue.
Comparing this to marathon running puts the demand in perspective. An average marathon burns 1,500 to 1,800 calories. Dividing the total pregnancy cost by the per-marathon cost yields more than 30 marathons’ worth of energy.
But marathons take 3 to 5 hours, then stop. Runners rest, recover, and eat at normal levels until the next race. Pregnancy demands near-maximal metabolic output 24 hours per day for nine straight months. No rest days. No tapering period. No off-season.
Duration Beats Intensity for Extreme Feats
A marathon runner operates at a higher metabolic rate than a pregnant woman during the race. Peak performance can hit 15 times BMR compared to pregnancy’s 2.2 times BMR. But the runner cannot maintain that intensity.
After several hours, energy expenditure must drop. After several days of repeated efforts, it drops further. By the time an event reaches several months, sustainable metabolic scope falls below 3 times BMR for everyone.
Pregnancy wins on duration. Operating at 2.2 times BMR for 270 consecutive days represents a feat that surpasses every athletic event ever measured. No runner, cyclist, or swimmer has sustained elevated metabolism for that long at that intensity.
Why Pregnancy Fatigue Feels So Deep
Women describing pregnancy exhaustion are not exaggerating. Bodies run at a metabolic rate comparable to elite athlete training loads, sustained continuously for months. Fatigue is not a sign of weakness but a physiological reality of operating near the human performance ceiling.
Pregnant women usually continue working, caring for other children, and managing daily responsibilities while their bodies function at near-maximal sustainable capacity. No training camp. No support staff. No cheering crowds.
Athletes train for years to prepare for extreme endurance events. Bodies adapt gradually to increasing workloads. Even then, transcontinental runners showed metabolic adaptation and reduced energy expenditure after several weeks.
Pregnant women start from normal daily activity levels and ramp up to extreme metabolic demands over nine months, often while experiencing nausea, discomfort, and significant physical changes.
One Biological Ceiling Governs All Activities
Running, cycling, swimming, trekking, and pregnancy all follow the same curve relating metabolic scope to duration. Different muscle groups. Different organ systems. Different purposes. Yet the same constraint applies.
Evidence points away from limitations in specific tissues or organs. If peripheral muscles set the limit, different activities would show different ceilings. If thermoregulation constrained performance, cold-weather events would allow higher metabolic rates than warm-weather events. But Arctic trekking data matches Tour de France data.
Research suggests the digestive tract and liver impose the constraint. Food must be broken down, nutrients absorbed, and energy extracted. Overfeeding studies show this process has a maximum rate around 2.5 times BMR. Above this threshold, the body cannot extract calories fast enough from food, regardless of how much someone eats.
Humans evolved a lower baseline metabolism than other mammals. We burn about 50% fewer calories at rest compared to non-primate mammals of similar size. Lower baseline metabolism reduces heat production and may have selected for reduced digestive capacity as a trade-off.
Global Populations Cluster Below the Ceiling
Researchers examined habitual physical activity levels in 130 populations around the world. People with sedentary lifestyles. Manual laborers. Hunter-gatherers. Athletes in training.
Despite enormous differences in daily activity, everyone clustered below 2.5 times BMR as a long-term average. Habitual metabolic scope across diverse populations rarely exceeded 2 times BMR. Spending more energy than the alimentary system can supply requires consuming body tissue. Over time, this becomes unsustainable.
Only during specific, limited periods do humans exceed this ceiling: extreme athletic events, pregnancy, and lactation. Each pushes close to the biological maximum but cannot continue indefinitely at that level.
Recognition Lags Behind Reality
Athletes completing ultramarathons receive medals, prize money, and public recognition. Finish lines await them with cheering crowds. The media covers their achievements. Society acknowledges the physical accomplishment.
Pregnant women complete a longer, more demanding endurance event while receiving little recognition for the physical feat. Most continue normal daily activities throughout pregnancy. Many work full-time jobs until delivery. Society expects them to handle this extreme metabolic demand as routine.
Scientific research now provides objective measurements confirming what mothers knew from experience. Growing a baby pushes human metabolism to its sustainable limit. Pregnancy belongs in the same category as crossing continents on foot or cycling through mountain ranges for weeks. Perhaps it deserves the same respect.
