Burn summary

Cells do work, and this requires energy. But how are we defining these terms? 

In fact, the two concepts are interchangeable. In physics, work is a technical term. And, since work and energy are measured using the same units, we can use them interchangeably. In other words, work is energy. 

How do you measure energy expenditure? In theory, it’s simple: you follow the CO2.

Whether it’s coal or carbohydrates, burning fuel creates a byproduct – carbon dioxide. When the body burns energy, it emits CO2. That’s largely what you’re exhaling when you breathe. Figure out how much CO2 the body produces, and you’ve got a precise measurement of how much energy it’s burning. 

Why are so many Westerners overweight? One common theory goes something like this. 

The human body, including its metabolic system, evolved to cope with the environment in which the first Homo sapiens lived. Food was scarce, and these hunter-gatherers expended huge amounts of energy finding what little there was. 

Are the Hadza results a weird anomaly? 

Not really. Take a 2008 study by Amy Luke, a researcher at Loyola University Chicago. Luke used the Lifson method to compare the energy expenditure and physical activity of women living in rural Nigeria to African American women living in Chicago. Despite leading completely different lifestyles, it turned out that both groups expend the same amount of energy per day. 

Charles Darwin observed that natural history is shaped by the struggle for resources. Because there’s never enough food to go around, species evolve under conditions of scarcity. 

That’s why trade-offs are so important. Limited energy means you can’t have it all. Such limitations are clear to see in the case of evolutionary traits. For example, it could be that evolution provides a species with razor-sharp teeth – but also with tiny arms. That’s how you get the Tyrannosaurus rex. As Darwin put it in The Origin of Species, “to spend on one side, nature is forced to economize on the other side.” 

Apes and humans have a lot in common, including the fact that both are social animals. Of course, there are also lots of things that set us apart. Things like metabolism. 

What caused this divergence? And why did our metabolic system outstrip that of apes? The short answer is that humans share food – and apes don’t. 

Let’s recap. Metabolic research shows that modern urbanites with cars and comfy office chairs burn as many calories as hunter-gatherers. In other words, daily energy expenditure has likely remained unchanged since the Paleolithic era. We also know that daily energy expenditure is constrained, which means that increasing the amount we exercise has little effect on the number of calories we burn. 

What should we make of these findings? Well, they suggest it’s time to rethink the way we go about tackling obesity. Simply put, exercise doesn’t do much to push the dial – but controlling our diets does.