Review and Summary: Burn

Burn Book Cover

When I first stumbled upon the title of this book, I couldn’t resist the allure. Drawing from his groundbreaking study on the Hadza tribe’s daily energy expenditure, which Herman Pontzer published in 2012, he posits a provocative thesis: to effectively combat the obesity epidemic, our focus should be on the energy we consume, not necessarily on how much energy we burn. As an evolutionary anthropologist known for his research on human metabolism and energy expenditure, Pontzer suggests, the energy we burn has inherent limits and remains relatively constant which is called constrained daily energy expenditure.

Regardless of our daily routines, Pontzer contends that our energy expenditure remains steady. Astoundingly, he asserts that the variations in energy output among sedentary individuals are more significant than the difference between an average sedentary person and someone who is highly active. Not surprisingly, this claim has sparked fiery debates in the scientific community. A notable counter-argument comes from Shaun Ward, as detailed in his piece, “Exercise and Weight Loss: Responding to Herman Pontzer.”

While I don’t find myself wholly on board with Pontzer’s central argument, there’s much in the book to appreciate. For instance, Pontzer demystifies complex metabolic pathways associated with carbohydrates, fats, and proteins, pathways that might have perplexed many of us during our high school biology classes.

In essence, this book offers a riveting exploration of the intricate dance between diet, energy intake, and energy output, challenging us to think critically about our dietary choices and the science behind them.

Summary

Our bodies don’t work like simple fuel-burning machines because they aren’t products of engineering, they’re products of evolution.

  • Our bodies able to respond to changes in exercise and diet in ways that make evolutionary sense even if they frustrate our attempts to stay trim and healthy. Consequently, more exercise doesn’t necessarily mean more energy burned per day, and burning more energy doesn’t protect against getting fat.

Over the course of evolutionary history, metabolic rate had been tuned up or down.

  • In the human lineage, our cells evolved to work harder → these metabolic adaptations brought on other major changes on the way our bodies work and how we behave. Energy expenditure evolved hand in hand with massive changes in diet, and in the ways we acquire, prepare, and share our food. A faster metabolism favored an increased capacity to store fat. Our metabolism also responds to changes in exercise and diet in ways that thwart our attempts to lose weight.

Daily expenditure rises steeply with body size in children

  • their cells burn much more energy each day than larger, older people. Each pound of a typical 3 yo burns ~35 kcal per day. That number steadily declines through childhood and adolescence, flattening out around ~15 kcal per pound each day in our early 20s

Speed affects energy

Speed affects energy in two ways:

  1. The rate at which we burn energy (kcal per minute) will increase directly with speed:
  • The faster we move → the faster our muscles have to do the work of moving our bodies → the faster we burn calories.

2. Regardless of how fast you run, you’ll burn the same number of calories per km.

  • You burn the same number of calories to run 3 km any fastest pace as you do to jog it casually. It feels harder to run fast because fatigue is related to how hard we work.
  • The same isn’t true for swimming, walking, and cycling: at certain speed, it’s actually cheaper to run at at that speed that it is to walk.

Walking costs (kcal/km) increase with speed

It is because of the inherent mechanics of a walking gait.

  • The up-and-down movement with each step gets harder to do as we move faster. When we switch to run → our legs transform from rigid struts to springy pogo sticks, and we bounce from step to step. We still up and down with each step, but the springlike mechanics of running result in a flat vs speed relationship.

Cycling and swimming costs increase with speed

  • The causes are different than for walking
  • Move body through fluid (water or air) → lose energy fighting drag. The faster you move → the more drag fights to slow you down.
  • In swimming, increasing speed from 2 mph to 3 mph → increase energy burned per mil by 40%
  • In cycling, the costs of fighting drag aren’t too noticeable below about 10 mph (which is one reason that air drag isn’t a factor in running). But >10 mph, the effect of drag really grows.

The cost of physical activity are disappointingly small

For an adult with 68 kg:

  • 10,000 steps per day burns 250 kcal
  • Climbing one flight of stairs burns 3.5 kcal
  • Run ~5.6 km burns 340 kcal
  • Run ~12.8 km burns 840 kcal
  • Running marathon burns ~2,690 kcal
  • Doing ironman triathlon burns ~8,000 kcal

Constrained daily energy expenditure

  • Around the globe, despite our fantastic cultural diversity and superficial differences in the way we look, our bodies all work the same way. The body seemed to be maintaining daily energy expenditure within some narrow window, regardless of lifestyle, called constrained daily energy expenditure.
  • Our metabolic engines shift and change to make room for increased activity costs, ultimately keeping daily energy expenditure within a narrow window.

Trying to outrun obesity

Constrained daily energy expenditure

  • It means that increasing daily activity through exercise or other programs will ultimately have little effect on the calories burned per day. So, change the way we tackle obesity by focusing on the amount of energy we eat, instead. Yet, exercise is still essential for health.

Hunger levels at brain

Calories in, calories out, and the metabolic magician

How pointless it can seem to count calories:

  • Our bodies do such a good job of adjusting to the calories we eat and expend that it can feel like calories aren’t real at all.
  • Problem: we’re atrocious at keeping track of the food we consume, and our evolved metabolic trickery makes it nearly impossible to keep track of energy we expend.

How does anyone avoid the obesity trap?

  • Genetic reasons
  • Build our diet around foods that are filling and nutrient rich without packing in a lot of calories
  • Lower stress

A different way of thinking about exercise energetics

  • Our bodies work on a fixed energy budget (constrained daily energy expenditure) → everything is a trade off.
  • As you increase the amount of energy burned on physical activity → the energy available for other tasks is diminished → exercise has wide-ranging effects on how our metabolism is managed and where our calories are spent, which has enormous effects on our health.


Author: Herman Pontzer

Publication date: 2 March 2021

Number of pages: 374 pages


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