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The Misunderstood Science of Metabolism

By Herman Pontzer
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Burn by Herman Pontzer

Burn (2021) shines a light on the science behind metabolism – the way our bodies burn energy. Packed with memorable insights and facts, it draws on the latest metabolic research and delves into the evolutionary history of the human body. 

Key idea 1 of 9

You are what you eat – literally.

In 1859, the French scientist Louis Pasteur made a revolutionary broth. What made it so special? Well, first, Pasteur realized that boiling the soup killed any germs that might be in the liquid. And second, he found that keeping it in an airtight flask prevented bugs and dirt from entering. This two-step system prevented the soup from spoiling – a revolutionary discovery.

This process came to be known as pasteurization, named after Pasteur himself. The experiment wasn’t just a practical triumph, though. It was also the final nail in the coffin of a theory as old as Aristotle – a theory known as spontaneous generation

Spontaneous generation tried to explain phenomena like the sudden emergence of maggots on rotting meat. Where did these maggots come from? Before the invention of powerful microscopes, it was hard to answer that question. From antiquity to the Middle Ages and well beyond, people argued they came from nowhere – that is, that they spontaneously generated from inanimate objects like meat. 

It’s easy to scoff at such an outdated notion, but a century of research into metabolism has shown that the truth is even stranger. 

The key message in this blink is: You are what you eat – literally. 

Today, we know that maggots don’t emerge from inert material. But look closely at a maggot-laying fly. What does it do? Essentially, it’s a small machine that transforms putrid protein into baby flies. Put differently, it spontaneously assembles its own and its offsprings’ bodies out of water, air, and the food it consumes. 

Like flies, humans are also spontaneous-generation machines. Every ounce of bone and pint of blood, every fingernail, eyelash, and strand of hair, is made out of the things we eat. Inanimate matter, it turns out, does generate life. 

How does this uncanny transformation occur? The answer is metabolism – the way our bodies burn energy. Let’s break that down. 

The body is made up of thousands of different, interacting molecules. These include enzymes, hormones, neurotransmitters, DNA, and more. Very few of them arrive in the body in usable form through our diets, however. Before they can be put to use, they have to be converted. 

This is the work of cells. A cell’s job is to pull useful molecules circulating in the bloodstream in through its membrane and then convert those molecules into something else. Take ovary cells. They pull cholesterol molecules inside, convert them, and then push the end-product – estrogen, a hormone that affects the whole body – back into the bloodstream.

The work of these cells is what keeps us alive. But it requires energy – lots of it. Metabolism is the body’s life-preserving furnace, “burning” the food we eat and unlocking its energy for this purpose. 

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