Why We Eat (Too Much) summary

Snakes, mushrooms, birds, roses, and humans – all living beings are descendants of the same single-celled ancestor. That means that all these organisms generate energy in the same way. 

How? Well, the mitochondria inside cells convert food into energy, which is then stored inside battery-like molecules called adenosine triphosphate, or ATP. These molecules are the currency of life. Even viruses use them, although they hijack the ATP of the cells they invade. 

Prehumans evolved larger brains than other primates long before Homo sapiens appeared on the scene some 250,000 years ago. 

Homo erectus, for example, outsmarted chimps a million years ago. Archaeological evidence tells us what they did with that extra brainpower: they crafted rudimentary tools and perfected the art of hunting. Slowly but surely, they abandoned the virtually vegetarian diet of their primate cousins. 

So far, we’ve followed the evolutionary path that led from single-celled bacteria to big-brained, meat-eating Homo sapiens. Our guiding thread was energy. 

Before we can continue our exploration into how we generate and use energy, however, we need to take a detour and talk about negative feedback systems.

Let’s recap. Negative feedback regulates our water levels by correcting two potentially fatal deviations: overhydration and dehydration. 

Of course, water isn’t the only thing we need to survive – our bodies also require energy to keep cells and organs working. 

The first rule of metabolism takes the form of an equation: energy in minus energy out equals energy stored. Let’s rephrase that rule to make it a little less abstract. 

Food contains energy, which the body converts into heat, movement, and thought. These activities in turn burn off energy. If you eat more calories than you expend, you’ll store unused energy in fat cells. 

The body can’t tell we’re consciously dieting. From its perspective, calorie restriction looks like a famine and a threat to our survival. That’s why it reduces its metabolic rate: it’s trying to conserve energy. 

Our metabolism doesn’t return to its previous level once this period of calorie restriction is over, either. We continue burning fuel more slowly and storing energy in fat cells – the body’s safety buffer to get us through the next famine. All this explains why diets don’t work. Our biology rebels against starvation. 

Our ancestors were hunter-gatherers. Meat and starchy tubers like yams and sweet potatoes made up the bulk of their diet. They prized fatty offal like liver and bone marrow. Natural vegetables, fruits, seeds, nuts, and herbs rounded out their meals.

Our bodies still resemble those of early Homo sapiens, but the energy we use to fuel them couldn’t be more different. Take a 2016 survey analyzing the eating habits of over 9,000 Americans.