Burn: New Research Blows the Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy

by Herman Pontzer

Your body is made up of roughly 37 trillion cells, each humming along like microscopic factories, every second of every day. Together, they burn enough energy every twenty-four hours to bring eight gallons (about thirty liters) of ice water to a raging boil. Our cells outshine the stars: each ounce of living human tissue burns ten thousand times more energy each day than an ounce of the Sun. A small portion of this activity is under our conscious control—namely the muscle activity we use to move. Some of it we’re dimly aware of, like our heartbeat and breathing. But most of this teeming activity goes on completely beneath the surface, in a vast and unseen ocean of cellular processes that keep us alive.

Nine-year-olds burn 2,000 calories; for adults, it’s closer to 3,000, depending on how much you weigh and how much fat you carry (and for the record, the correct term when we’re talking about our daily energy needs is kilocalories, not calories).

Our bodies have gotten very crafty, 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.

And yet the human pace of life is anything but common. We are off-the-charts freaks among the animal kingdom when it comes to our life history, the rate at which we grow up, procreate, grow old, and die. We live life in slow motion. If humans lived like a typical mammal our size, we’d hit puberty before age two and be dead by twenty-five. Women would give birth every year, to five-pound babies. The average six-year-old would already be a grandparent. Daily life would be unrecognizable.

BMR is measured with the subject at rest (nearly asleep), so it doesn’t represent all the calories the organism burns each day, just a fraction. Also, BMR can be tricky to measure. If the subject is agitated or cold or sick or young and growing, the measurement can be elevated—and

Our analyses showed that a typical placental mammal our size burns well over 5,000 kilocalories per day. That’s the daily energy expenditure of Olympic athletes at the peak of training!

There’s a great quote of uncertain provenance, often attributed to Einstein, that “if you can’t explain something simply, you don’t really understand it.”

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.

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.

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.

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.

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.

In fact, despite the recent popularity of low-carb diets, humans across cultures and around the globe, including hunter-gatherers like the Hadza, typically get more calories from carbohydrates than from fats or proteins (Chapter 6). We’re primates, after all, and primates eat plants—especially ripe, sweet fruits. Carbohydrates are our main source of fuel, and we have a 65-million-year history of relying on them.

Carbohydrates come in three basic forms: sugars, starches, and fiber. Sugars and starches are digested and either used to build glycogen stores or burned for energy

Fiber is a different beast, with an important role in the gut regulating the digestion and absorption of sugars and starches, and feeding the trillions of bacteria and other critters in our intestinal microbiome. In fact, the microbiome plays an essential role in digesting fiber, and without it we’re in trouble.

The monosaccharides are glucose, fructose, and galactose. The other sugars—sucrose, lactose, and maltose—consist of two monosaccharides stuck together and are called disaccharides (“two sugars”). Sucrose (table sugar) is just a glucose and fructose bound together. Lactose (milk sugar) is glucose and galactose. Maltose is two glucoses.

Starches are simply a bunch of sugar molecules strung together in a long chain. Because there are so many sugar molecules stuck together, starches are also called polysaccharides (“poly” meaning many) or complex carbohydrates.

Starch and sugar digestion continues until all the polysaccharides and disaccharides are broken down into monosaccharides. Since much of the carbohydrate in your diet comes from starch, and starch is made entirely from glucose, about 80 percent of the starches and sugars that you eat end up as glucose. The rest is broken down to fructose (about 15 percent) or galactose (about 5 percent).

These sugars are absorbed through the intestinal wall and into the bloodstream. The walls of our intestines are chock-full of blood vessels, and blood flow to our guts more than doubles after a meal to carry away nutrients. The result is the familiar rise in blood sugar (almost all glucose) after a meal, particularly one high in carbs.

The unsung heroes in all this digestive work are dietary fiber and your microbiome. Fiber is a class of carbohydrate (there are many varieties of fiber) that our bodies can’t digest—at least, not on their own. These tough, stringy molecules are what give plant parts their strength and structure. Fiber from our food covers the intestinal walls like a wet knit blanket, forming a lattice-like filter that slows the absorption of sugars and other nutrients into the bloodstream. That’s why the glycemic index—the rush of sugar into the blood—is about 25 percent higher for orange juice, which doesn’t have much fiber, compared to a piece of orange, which does.

With trillions of bacteria, each with their own thousands of genes, the microbiome is like a four-pound superorganism living inside of you. These bacteria digest much of the fiber we eat, using enzymes our own cells can’t make and producing short-chain fatty acids that our cells absorb and use for energy. Our microbiome also digests other stuff that escapes the small intestine, aids in immune system activity, helps produce vitamins and other essential nutrients, and keeps the digestive tract running properly.

Blood sugar that isn’t burned immediately is packed away into glycogen stores in your muscles and liver. Glycogen is a complex carbohydrate similar to plant starch. It’s easy to tap into when energy is needed, but relatively heavy because it holds an equal proportion of carbon and water (hence the term “carbohydrate”).

When your body’s energy needs are met and your glycogen stores are full, the excess sugar in your blood is converted to fat, as we’ll discuss below. Fat stores are a bit more difficult to use for fuel—there are more intermediate steps to convert them to a burnable form. But fat is a much more efficient way to store energy than glycogen, because it’s energy dense and doesn’t hold water. And as we know all too well, there’s virtually no limit to how much fat the human body is able to store.

Fats have a fairly simple itinerary—they are digested down into fatty acids and glycerides and then built back up into fat in your body, which is eventually burned for energy. The challenge, though, is that fats are hard to digest. It comes down to basic, familiar chemistry: oil and water don’t mix.

Bile is a green juice produced by your liver and stored in your gall bladder, which is a small, thumb-sized pouch that sits between the liver and small intestine, connected to both with short ducts. When fats enter the small intestine from the stomach, the gall bladder squirts a bit of bile into the mush of food. Bile acids (also called bile salts) act like detergents, breaking up the globs of fat and oil into tiny emulsion droplets. Once the fat is emulsified, enzymes called “lipases,” produced by the pancreas, are added to the mix and break these emulsion droplets down to an even smaller size, to microscopic droplets called micelles, just a hundredth the diameter of human hair.

Fatty acids and glycerides are absorbed into the intestinal wall and re-formed into triglycerides (three fatty acids attached like streamers to a glycerol molecule), the standard form of fats in the body. Here the body confronts the next challenge of digesting fats: because they don’t mix well with water, they tend to clump together in water-based solutions like blood. Lumpy blood would kill you, clogging up the small vessels in your brain, lungs, and other organs. The evolved solution is to pack triglycerides into spherical containers called chylomicrons.

Instead, the fat molecules, packed in chylomicrons, are dumped into the lymphatic vessels. Part surveillance system, part garbage collection, the lymphatic vessels have their own network throughout your body, picking up debris, bacteria, and other detritus and bringing it to the lymph nodes, spleen, and other immune system organs to be dealt with. It’s well suited to pick up big particles like chylomicrons stuffed with fat. The lymphatic vessels also collect all the plasma that leaks out of your blood vessels (about three quarts a day) and returns it to your circulatory system, so it offers a port of entry into the bloodstream.

Lipoprotein lipase enzymes in the blood vessel walls first break the triglycerides into fatty acids and glycerol, which are pulled into waiting cells by aptly named fatty acid transporter molecules before being reassembled into triglycerides. Most fat is stored in fat cells (adipocytes) and muscles, forming a reserve fuel tank. These stored triglycerides are the fat that we feel in our belly and thighs, or see marbled into a nice cut of steak.

A small proportion of the fats we eat are used to build structures like cell membranes, the myelin sheaths that coat our nerves, and parts of our brain. Some of the fatty acids needed to build these tissues can’t be made by reformulating others, and so are considered essential fatty acids—you need to get them from the food you eat.

Unlike fats and carbohydrates, proteins aren’t a primary source of energy (unless you’re a carnivore). The main role of protein is to build and rebuild your muscles and other tissues as they break down each day. Your body does burn protein for energy, but it’s a small contributor to your daily energy budget.

All proteins get digested down to their basic building blocks: amino acids. Amino acids are a class of molecules shaped a bit like a kite—a head attached to a tail. They all have the same head: a nitrogen-containing amine group connected to a carboxyl acid. Amino acids are distinguished by their tails, which are always some configuration of carbon, hydrogen, and oxygen atoms.

From the blood, the amino acids are pulled into cells to construct proteins, which are chains of amino acids strung together. The construction of proteins from amino acids is one of the primary jobs of DNA.

Amino acids are also used to make a variety of other molecules like epinephrine, the fight-or-flight hormone; and serotonin, one of the neurotransmitters our brain cells use to communicate.

We pee out the equivalent of fifty grams (about two ounces) of protein each day. Exercise adds to that total by increasing muscle breakdown. We have to eat enough protein to replace what we lose each day, lest we find ourselves in protein deficit. If we eat more protein than we need, the extra amino acids are converted to urea and cleared out by the urine.

After the nitrogen-containing head is chopped off, converted to urea, and sent on its way, the tails are used to make glucose (a process called gluconeogenesis, which literally means “making new sugar”) or ketones, both of which can be used for energy, as we’ll see below. Proteins are typically a minor part of the daily energy budget, providing around 15 percent of our calories each day. But they are a vitally important emergency energy supply if we’re starving, a bit like setting the furniture on fire to heat your house.

If there’s not enough oxygen present, either because we’re not breathing effectively or (more likely) because our muscles are working too hard, too fast for oxygen supply to keep pace with all of the pyruvate being produced, the pyruvate gets converted to lactate. Lactate can be reconverted to pyruvate to be used for fuel, but if it builds up, it can also become the dreaded lactic acid, which makes our muscles burn when we’re working hard and pushing our limits.

If we’re burning a lot of fat, whether we’re on an extremely low-carb diet or starving, some of the acetyl CoA generated will be converted to molecules called ketones. Most ketone production occurs in the liver. Ketones are sort of a traveling version of acetyl CoA, and can travel in the bloodstream to other cells, be reconverted to acetyl CoA, and used to generate ATP. Like a lot of metabolic conversion, most ketone production is done in the liver, but they are used throughout the body. This is the pathway that popular ketogenic diets engage, promoting a system of eating all fats and proteins and almost no carbohydrates. With the carbohydrate train line essentially shut down, all traffic shifts to the fat and protein pathways.

The dark side of converting fats to energy is that the tracks run both ways. As you see in Figure 2.1, a sugar molecule (glucose or fructose) can be converted to acetyl CoA and then jump on the fatty acid track instead of entering the Krebs cycle, and voilà! You convert the sugar into fat. It’s the same process used to convert fat into acetyl CoA, just run in reverse.

In fact, like any good, flexible transit system, our metabolic pathways are evolved to respond to traffic conditions and send molecules to their most sensible destinations.* Got more sugars than you need? Send the extra glucose and fructose to glycogen. Glycogen stores full? Send the excess sugar to acetyl CoA. If the Krebs cycle train is overcrowded because energy demands are low, start sending acetyl CoA to fat. And there’s always plenty of space available in fat. Glycogen stores fill up, and you can’t store excess protein, but there’s no limit to how much fat you can layer on.

Nothing is innocent if eaten in excess. Any calories that aren’t burned, no matter if they come from starches, sugars, fats, or proteins, will wind up as extra tissue in your body. If you’re pregnant or bulking up at the gym, that extra tissue might be useful things like organs or muscle. But if you’re not, those extra calories, no matter their original dietary source, will end up as fat. That’s the foundation we need to understand to begin talking about all the real-world complexities of diet and metabolic health.

The main way that speed affects cost is straightforward: the faster we move, the faster our muscles have to do the work of moving our bodies, and the faster we burn calories.

That probably fits with your intuition (faster speed means faster expenditure), but there’s a surprising implication: regardless of how fast you run, you’ll burn the same number of calories per mile. That means you burn the same number of calories to run three miles at your fastest pace as you do to jog it casually—you just burn the calories faster (and finish sooner) when you run fast.

Infants (0 to 3 years): BMR = 27 × Weight − 30 Children (3 years until puberty): BMR = 10 × Weight + 511 Women: BMR = 5 × Weight + 607 Men: BMR = 7 × Weight + 551

The largest organs are the quietest. For a typical U.S. adult, muscle accounts for 42 percent of body weight but only 16 percent of BMR, about 280 kcal per day (about 6 kcal/day per pound). Your skin weighs 11 pounds but burns only 30 kcal per day; your skeleton weighs a bit more but burns even less. Fat cells are more active than you might think. They make hormones and traffic in glucose and lipids to maintain energy supply to the body. Still, each pound of fat burns only about 2 kcal per day, for a total of about 85 kcal per day for a typical 150-pound adult with 30 percent body fat.

With each beat, the heart pumps about 2.5 ounces (70 ml) of blood into the body via the aorta. That’s about five quarts (or about five liters) per minute, nearly all the blood in your body. And that’s just at rest! During exercise, you heart’s output can easily triple. Amazingly, all this work is done for the low, low cost of about 2 calories per beat.

The kidneys are your body’s housecleaning staff: tireless, essential, and underappreciated. In addition to maintaining precisely the right amount of water in your body, the kidneys handle the enormous task of clearing out waste and toxins, filtering 180 liters of blood a day. Millions of microscopic sieves (the nephrons) clean every drop of blood thirty times per day, pumping salts and other molecules in and out to eliminate the bad stuff and keep the good.

Together, your kidneys weigh only half a pound, but they burn about 140 kcal per day, accounting for 9 percent of BMR.

The liver breaks apart unused chylomicrons and stores the fat or repackages it into other lipoprotein containers (including the low-density lipoproteins, or LDLs, and the high-density lipoproteins, or HDLs, in your cholesterol report). The liver is the main site of gluconeogenesis, converting fats and amino acids to glucose when needed, and turning the nitrogen-bearing head of amino acids into urea to excrete in the urine. The liver is also the primary site of ketogenesis. Oh, and it breaks down a wide range of toxins, from alcohol to arsenic (but you should still definitely do that grapefruit and maple syrup cleanse . . .). All this ceaseless metabolic work burns about 300 kcal per day, 20 percent of BMR.

The human gastrointestinal tract weighs about 2.5 pounds and burns about 12 kcal per hour, and that’s just at rest on an empty stomach. Digestion costs much more, about 10 percent of the daily calories consumed, or 250 to 300 kcal per day for the typical adult. It’s unclear how much of the energy burned by the gut is attributable to the trillions of bacteria toiling away in our microbiome. A recent study in mice by Sarah Bahr, John Kirby, and colleagues suggests the calories burned by the microbiome might account for as much as 16 percent of BMR in humans, which would mean that the resting energy expenditure of the gastrointestinal tract (about 12 kcal per hour) is attributable almost entirely to gut bacteria.

The brain and liver share the title of “costliest organ.” Your brain weighs a little less than 3 pounds but burns about 300 kcal per day, accounting for 20 percent of BMR. The high cost of brain tissue is the main reason large brains are so rare among animals.