Stone Age bodies, Space Age abundance
Why a Tanzanian hunter-gatherer and a Manchester office worker burn exactly the same number of calories—and what this reveals about the real cause of the obesity epidemic
Herman Pontzer places a small vial of water into the hands of a Hadza hunter-gatherer in northern Tanzania. This water looks ordinary but contains isotopes that will track every calorie burned over the next week. When the results arrive months later in his Durham laboratory, they overturn everything we thought we knew about getting fat.
The impossible data: this forager, walking fifteen thousand steps daily across the savanna, burns exactly the same number of calories as a Manchester office worker shuffling three thousand steps between car and desk. Physics says this cannot happen. Human intuition screams it makes no sense. But the isotopes don't lie.
Pontzer's groundbreaking study, published this week in the Proceedings of the National Academy of Sciences, examined more than 4,200 adults across 34 countries—from African tribes to Norwegian executives. Using doubly labelled water, the gold standard for measuring energy expenditure, his team reached a conclusion that challenges everything we thought we knew about obesity: the modern epidemic has almost nothing to do with how much energy we burn, and everything to do with what we eat.
The findings suggest diet drives roughly 90% of obesity differences between populations. Physical activity? A mere 10%. We're not getting fat because we've become lazy. We're getting fat because we've solved hunger so completely that our bodies—still operating as if famine lurks around every corner—cannot cope with endless plenty.
The impossible equation
Evolution equipped all humans with identical metabolic engines, regardless of how those engines get used. The Hadza forager burns calories through movement but rations energy spent on immune responses and tissue repair. The office worker reverses this allocation, saving physical energy whilst increasing expenditure on internal processes.
Pontzer calls this "constrained energy expenditure"—perhaps the most democratic system biology ever devised. Despite vast lifestyle differences, all humans receive roughly equal energy budgets: 2,500 to 3,000 calories daily. A subsistence farmer in Bangladesh, a banker in Zurich, a retiree in Brisbane—nature allocates similar energy allowances across extraordinary variations in work, wealth, and environment.
This represents evolution's masterpiece of resource allocation. But a system perfected for scarcity now confronts limitless abundance—and the mismatch is making us fat.
Bodies built for shortage
Walk through any pre-1900 portrait gallery and obesity appears mainly in paintings of merchants, nobility, and clergy—those wealthy enough to eat consistently. Everyone else looks startlingly lean by modern standards.
West Point records from the 1800s tell the story precisely: 19-year-old military cadets averaged BMI values of 20.5. By today's measures, 90% would be classified as underweight. These weren't malnourished weaklings but the physical elite of their generation.
Our ancestors survived because they possessed bodies of extraordinary efficiency—metabolic systems capable of extracting maximum nutrition from minimal intake whilst carefully rationing every calorie burned. Those who lived through famines, failed harvests, and seasonal shortages passed along genes for energy conservation.
The industrial revolution initially improved this situation perfectly. Food production increased whilst physical demands remained high, creating larger, healthier populations. Governments actively encouraged this transition, recognising that moving citizens from undernourishment toward normal weight provided military and economic advantages.
But the process continued past its optimal stopping point. What began as solving malnutrition evolved into creating abundance that our Stone Age metabolism simply cannot manage.
The abundance trap
Ultra-processed foods solved the greatest problem in human history: feeding everyone. These industrial creations—products requiring five or more manufactured ingredients—eliminated famine, prevented food poisoning, and made nutrition available regardless of season, geography, or income.
But solving hunger created an unexpected problem. The same technologies that preserve foods and prevent malnutrition also optimise for taste, convenience, and shelf stability. Corporations discovered that products combining precise ratios of sugar, salt, and fat triggered consumption patterns impossible to achieve with traditional foods.
Pontzer's data reveals the result: countries consuming more ultra-processed foods show consistently higher obesity rates. The correlation appears across continents and cultures. Industrial food processing, humanity's solution to starvation, became the primary driver of overconsumption.
This wasn't malicious design but market evolution. Food companies compete in environments where products must taste better than alternatives to survive. The corporation producing bland but nutritious foods loses market share to competitors creating more appealing options. Over decades, this competition drives innovation toward hyperpalatability—foods engineered to override natural satiety signals.
Consider breakfast cereal: fortified with vitamins to prevent nutritional deficiencies, engineered to remain fresh for months, designed to taste irresistible to children. The same product that ended rickets in industrial populations also teaches young palates to expect sweetness with every meal.
Democracy's caloric cost
The most profound revelation in Pontzer's research isn't about diet or exercise—it's about equality. Evolution has created the most democratic system imaginable: metabolic fairness. Rich or poor, active or sedentary, ancient or modern, all humans receive nearly identical energy allocations from nature.
This creates an unprecedented challenge in post-scarcity societies. When everyone burns similar energy but food becomes limitlessly available, weight differences emerge almost entirely from what people consume rather than how much they move.
The implications overturn decades of public health strategy. Campaigns exhorting people to "move more" miss the fundamental biology. Exercise provides enormous benefits for cardiovascular health, mental wellbeing, and functional capacity. But its role in weight management appears far more limited than anyone assumed.
Some countries have begun targeting food environments instead. Chile requires warning labels on ultra-processed products. Mexico taxes sugary drinks. Brazil's dietary guidelines explicitly recommend limiting industrial foods whilst celebrating traditional cooking.
Yet these policies confront powerful economic realities. Ultra-processed food manufacturing represents one of the world's most profitable industries, employing millions whilst generating substantial tax revenues. The companies that solved hunger have become economic titans whose continued growth depends on consumption patterns incompatible with human metabolism.
The path through abundance
Those isotopes in the Tanzanian laboratory tell a story bigger than obesity. They reveal humans as the only species that has transcended the basic biological constraint of energy scarcity—and discovered that solving ancient problems creates entirely new ones.
The Hadza forager and Manchester office worker burn identical calories because evolution equipped them with the same magnificent engine. But one operates in an environment of natural scarcity, the other in artificial abundance. Their bodies cannot tell the difference.
Creating food systems worthy of our metabolic democracy requires acknowledging this mismatch. We need technologies that satisfy human cravings whilst supporting rather than subverting metabolic health. We need economic systems that profit from wellbeing rather than overconsumption. We need abundance that works with human nature rather than against it.
The hunt for wild tubers shaped our metabolism for millions of years. The meal deal has been available for twenty. Understanding which force proves stronger may determine whether our species thrives in the abundance we've created—or becomes its most prominent casualty.