Graphs We Care About: Fixed Human Energy Expenditure

Active people burn more calories than less active ones. This simple statement is compelling, important, and mostly wrong.

Graphs We Care About: Fixed Human Energy Expenditure

Active people burn more calories than less active ones. This simple statement is compelling, important, and may be mostly wrong. More physically active cultures or people don't seemingly have higher daily energy expenditures than their less active counterparts, as some provocative work from anthropologist Herman Pontzer and others implies.

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More physically active cultures don't have higher daily energy expenditures than less active ones

This is so counter-intuitive as to seem mad, so let's get into it in more detail.

Energy Expenditure: More Alike Than You Think

At the heart of this work lies an important research question: How do humans from various lifestyles and cultures differ in their energy expenditure? Surprisingly, after studying both urban dwellers and hunter-gatherer communities, researchers have found that energy expenditure rates are much more similar worldwide than previously believed. In other words, whether you're a commuting urbanite or a Hadza hunter-gatherer pursuing game, your body likely expends energy at comparable rates.

This is, or should be, shocking. What about ultra-marathoners? What about couch potatoes? What about hunter-gatherers? The striking thing about careful recent work is that it shows there seem to be hard relative limits on energy expenditures, and how the human body adapts in response to energy expenditures. Use more energy on one thing, and it cuts back in other areas. Does this mean exercise is pointless? Of course not, and we will return to that shortly.

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Does this mean exercise is pointless? Far from it

Scaling Energy Expenditure to Body Weight

While we will wait for more confirmatory work to be sure—all science is tentative, and anything in biological sciences doubly so—Pontzer's work shows how energy expenditure scales with body weight across different species, including humans. As with many biological processes, there’s a predictable scaling relationship. When you plot energy expenditure against body weight for various animals, including humans, a clear pattern emerges.

Source: Herman Pontzer, The Exercise Paradox (Scientific American, 2017)

Most humans burn calories at about the same rate per unit mass, regardless of where you live or what you do. Energy expenditure is, in short, very similar around the world, representing a kind of metabolic hard cap. Further, even the more active among us burn calories on par with the less active. This is, or should be, shocking.

The Evolutionary Advantage: Us vs. Great Apes

This makes a kind of evolutionary sense. Humans that burned prodigious amounts more energy would be selected against in the calorie-scarce and fluctuating calorie world in which we evolved. But that humans burn more calories than our closest relatives, the great apes (as the above graph shows, is an evolutionary advantage.

The difference lies in our brain – a power-hungry organ that consumes a significant fraction of our daily energy. The evolution of larger, more complex brains in Homo sapiens required a concurrent uptick in energy availability. This need might have driven our ancestors to develop strategies, both behavioral and physiological, to support such high energy demands.

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Our brains are energy hungry, driving higher caloric needs, but also giving an evolutionary advantage

Furthermore, our upright method of moving around, social structures, and aptitude for endurance running could also contribute to our elevated energy needs. These characteristics, which differentiate us from other primates, require additional caloric support.

The Implications

These findings have implications for various fields of study:

  1. Diet and Weight Loss: The relative consistency in energy expenditure, regardless of lifestyle, challenges some conventional weight loss wisdom. If most humans burn roughly the same calories regardless of activity levels, then dietary habits play an even more crucial role in weight management. What you eat matters even more than you thought.
  2. Fitness and health: That we expend similar amounts of energy, regardless of activity level, doesn't mean we shouldn't exercise. There are myriad benefits from exercise—heart, brain, metabolic system, mood, blood markers, etc.—that have nothing to do with weight loss.
  3. Evolutionary Biology: The fact that we expend more energy than great apes despite our comparable body size hints at underlying evolutionary pressures that shaped our species. It emphasizes the importance of diet, brain development, and unique human behaviors.
  4. Anthropology: The minimal difference in energy expenditure between urban dwellers and hunter-gatherers also underscores the adaptability of the human body. While our environments and ways of life have changed, our internal energy systems have remained stable, at least in evolutionary time.

Conclusion

Recent work on energy expenditure, while early and still subject to refutation, has the potential to reshape our understanding of human metabolism and evolution. By revealing the surprising consistencies in how humans use energy, and contrasting it with our closest primate relatives, we have new insights bridging anthropology, biology, and health. The human body is an evolved organism, one that responds to stressors—like exercise—in unexpected ways that aren't captured by fitness trackers.


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