Metabolism is a broad term that covers all of the work your cells do. The vast majority of this work involves pumping molecules in or out of cell membranes (their walls) and converting one kind of molecule into another. Your body is a walking, sloshing bucket of thousands of molecules interacting — enzymes, hormones, neurotransmitters, DNA, and more — and hardly any of it comes in its usable form directly from your diet.
Instead, cells are constantly bringing nutrients and other useful molecules circulating in the bloodstream in through their walls for use as fuel or building blocks, converting those molecules to something else, and then pushing the stuff they’ve built out of their walls to be used elsewhere in the body. Cells in the ovaries pull cholesterol molecules inside, build estrogen out of them, and then push the estrogen — a hormone with effects all over the body — out into the bloodstream.
Nerves and neurons are constantly pumping ions (positively or negatively charged molecules) in and out to maintain a negative internal charge. Pancreas cells, guided by DNA, assemble insulin and a long list of digestive enzymes from amino acids. The list goes on and on. The amount of metabolic work happening right now in your body is staggering.
All of this work requires energy. In fact, work is energy. We measure work and energy using the same units and can talk about them interchangeably. Throw a baseball, and its kinetic energy as it leaves your hand is, by definition, exactly equal to the amount of work you did to accelerate it.
Heat is another common form of energy. Microwave a cup of milk to warm it for your kid, and the increase in temperature tells you how much electromagnetic energy was captured by the milk. The energy released from burning gasoline is equal to the work done to move the car along the road plus the heat generated by the engine.
Energy consumed is always equal to the combination of work done and heat gained, whether we’re talking about your body, your car, or your smartphone. We all play by the same laws of physics.
Energy can also be stored in things that have the potential to do work or create heat, like the gasoline in a fuel tank. A stretched rubber band or the spring of a mousetrap set to go off has strain energy. A bowling ball set precariously on a high shelf, one that could crash to the floor, has potential energy. The bonds that hold molecules together can store chemical energy, which gets released when the molecules break apart. When the molecules in a pound of nitroglycerin (chemical formula: C3H5N3O9) are broken into nitrogen (N2), water (H2O), carbon monoxide (CO), and oxygen (O2) during detonation, it violently releases enough energy (730 kilocalories) to launch a 165-pound man two and a half miles straight up into the sky (which would be work) or vaporize him (which would be heat), or some combination of the two.
This brings us to our last point about energy: It can be converted among its many forms — kinetic energy, heat, work, chemical energy, and so on — but it can never be lost.
Calories and joules are the two standard units used to measure energy, whether it’s the chemical energy stored in food, the heat from a fire, or the work done by a machine. Calories are most common in the United States when discussing food, but we’ve managed to muck up the standard usage. One calorie is defined as the energy needed to raise the temperature of one milliliter of water (one-fifth of a teaspoon) by one degree Celsius (1.8 degrees Fahrenheit). It’s a tiny amount of energy — too small to be a useful unit of measure when we talk about food (like road signs giving driving distances in inches).
Instead, when we talk about “calories” in food, we’re actually talking about kilocalories, or 1,000 calories. A cup of dry Cheerios has 100 calories according to the nutrition label on the box, but they actually mean 100 kilocalories, or 100,000 calories.
So why don’t we just say kilocalories or kcal instead of abusing the term calorie? Bizarrely, in the late 1800s, when scientists were deciding to adopt calories as the preferred unit of measure for food energy, the influential and pioneering American nutritionist Wilbur Atwater decided to stick with an early, arcane convention and simply capitalize Calories when referring to kilocalories. That’s about as sensible as capitalizing Yards to refer to miles. We’ve been stuck with the confusing use of calories (or Calories) on our food labels ever since.
Since work and energy are two sides of the same coin, we can think about all the work that our cells do and all the energy they consume as two ways of measuring the same thing. We can use metabolism and energy expenditure interchangeably. That’s why evolutionary biologists like me, as well as doctors and people in public health, are so fixated on energy expenditure, which is how we measure metabolism: It is the fundamental measure of the body’s activity. The speed with which a cell does its work determines metabolic rate, the energy used per minute. Add up the work of all the cells in your body and you’ve got your body’s metabolic rate, the energy you expend each minute. Your metabolic rate is the full force of your cellular orchestra, 37 trillion microscopic musicians blending together in an intricate symphony.
The sophisticated metabolic system that sustains us, and that we all take for granted, is a marvel of evolution. It took nearly a billion years — untold trillions of generations, quadrillions of false starts and dead ends — for the basic framework of today’s simplest single-cell metabolic systems to evolve on this planet, an eternity of trial and (mostly) error. It took another two billion years for the simplest multi-celled organisms, with their integrated metabolic systems and divisions of labor, to evolve. Along the way, life had to confront some major challenges in basic chemistry. Oils had to mix with water. Oxygen, a chemical that burns and kills, had to be harnessed for life. Fats and sugars, holding more energy per gram than nitroglycerin, had to be burned carefully for fuel without blowing organisms up or boiling them alive.
That’s not even the strangest part. All the work our bodies do is powered by microscopic alien life forms called mitochondria living within your cells. Mitochondria have their own DNA and their own two-billion-year evolutionary history, including saving all life on Earth from certain doom. And much of the work done to digest your food into usable bits is done by a vast ecosystem that lives in your gut. This microbiome is made of trillions of bacteria that make their home all along your digestive tract, the long and serpentine passageway that connects your mouth to your butt.
It’s a story you’ve probably heard before, but likely with the magic boiled out of it and served cold from a textbook. It’s well worth another listen. If nothing else, it’s the essential foundation of how diet affects your health and how your body burns energy — in short, how life actually works.
From BURN: New Research Blows the Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy by Herman Pontzer, Ph.D., to be published in April, with permission from Avery, an imprint of the Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2021 by Herman Pontzer.
This article is featured in the January/February 2022 issue of The Saturday Evening Post. Subscribe to the magazine for more art, inspiring stories, fiction, humor, and features from our archives.
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