Upends a lot of things we “knew” (or assumed, more likely) about nutrition and dieting. Dr. Herman Pontzer’s research shows that there is a daily caloric expenditure limit, which means you can’t out-exercise a bad diet. (Exercise is, however, still useful, just not in the way we think.)
- Hadza men and women burn the same amount of energy each day as men and women in developed countries like the U.S. and England. This is despite the fact that the Hadza get more physical activity in a day compared to the typical American in a week.
- The body seems to maintain daily energy expenditure within a narrow window, regardless of lifestyle. Dr. Pontzer calls this “constrained daily energy expenditure.”
- Constrained daily energy expenditure means that increasing daily activity through exercise or other programs will ultimately have little effect on the calories burned per day.
- Our metabolism is tightly regulated by our hypothalamus, which constantly monitors the food we eat and the calories we burn to keep our bodies in energy balance. But something about our modern environment is causing the hypothalamus to misfire, leading us to consume more calories than we expend.
- Humans evolved as opportunistic omnivores and eat whatever’s available, which is almost a mix of plants and animals (and honey.) There is no singular natural human diet, and the typical diet in the past looked nothing like Paleo or vegan diets today.
- Returning to an ancestral diet is impossible anyway, because we can likely no longer find the undomesticated plants and animals that we ate in the past.
- All diets work if you stick to them, because all diets reduce calorie intake.
- When exercise takes up a large chunk of the constrained daily energy budget, there is prioritization at work. Essential activities are protected until the bitter end. As a result, exercise has wide-ranging effects on how our metabolism is managed and where our calories are spent, which has enormous effects on our health.
- For example, it helps explain why exercise is effective reducing inflammation.
- When a large portion of the daily energy budget is spent on exercise, the body is forced to be more frugal with the remaining calories at its disposal. Suppressing the inflammation response, limiting it to target real threats rather than sounding the alarm constantly, reduces the energy spent on unnecessary immune system activity.
What is metabolism?
- 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.
- 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
- Instead, when we talk about “calories” in food, we’re actually talking about kilocalories, or 1,000 calories.
- In all biological systems, including our bodies, energy has one fundamental, common form: adenosine triphosphate, ATP. ATP molecules are like microscopic rechargeable batteries, which are “charged” by adding a phosphate molecule onto a molecule of adenosine diphosphate, ADP.
- A gram of ATP holds about fifteen calories of energy (that’s calories, not kilocalories), and the human body only holds about fifty grams of ATP at any given time. That means each molecule cycles from ADP to ATP and back over three thousand times per day to power our body.
- This first stage of metabolism is called anaerobic because it doesn’t require oxygen.
- 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.
- The second stage, aerobic metabolism, is where we need oxygen. If there’s sufficient oxygen in the cell, the pyruvate produced at the end of the first stage is pulled into a chamber within the cell called the mitochondria. There are dozens of mitochondria in a typical cell, and they are known as the “powerhouse of the cell” because the bulk of ATP production happens within them.
- In the mitochondria, pyruvate is converted to acetyl coenzyme A, or acetyl CoA.
- Acetyl CoA is like a train car full of passengers—carbon, hydrogen, and oxygen atoms—without an engine to pull it. Along comes oxaloacetate, which is hitched to acetyl CoA and begins to pull it along a circular track called the Krebs cycle.
- The train will make eight stops, and at each, some carbon, hydrogen, and oxygen passengers get on or off. The coming and going of those atoms generates two ATP. By the final stop, only the oxaloacetate engine is left. It’s hitched up to another acetyl CoA and the cycle repeats.
- Importantly, some of the passengers are robbed as they get on and off the Krebs cycle train, their electrons stolen away by the molecules NADH and FADH. These NADH and FADH molecules scurry away to the back alleys of the mitochondria and unload their purloined electrons into a special receptor complex in the membrane—a door in the wall.
- When the stolen electrons are deposited into the inner membrane complex, positively charged hydrogen ions (which are in plentiful supply) chase the negatively charged electrons and end up trapped in the intermembrane space.
- With all the positively charged hydrogen ions packed together, there is an electrochemical force pushing them out to balance the charge on either side of the inner membrane. But there’s only one way for the hydrogen ions to escape the inner membrane space: a special portal in the inner membrane that’s built like a turnstile. The hydrogen ions stream through the turnstile, driven by the electrical charge. As the turnstile spins, it forces together ADP and phosphate molecules, making ATP. This is the real moneymaker, producing thirty-two ATP. The complex choreography of electrons and hydrogen ions dancing along the inner membrane, called oxidative phosphorylation, is the primary energy generator that powers your body.
- And that’s why we should be suspicious of any diets that target one specific nutrient as a hero or a villain for weight loss. 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.
What is this going to cost me?
- 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.
- It feels harder to run fast because fatigue is related to how hard we work (e.g., calories per minute), not just the total number of calories burned.
- The bigger you are, the bigger your organs are and the more work they do each day.
- It’s not surprising, then, that BMR (in kcal per day) increases with body weight (in pounds) as:
- 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 BMR equations above give a sense of your body’s background energy needs per day, but they’re just ballpark estimates. Your BMR could easily fall above or below the value predicted from the equations above by 200 kcal per day. Much of that variation has to do with your body composition.
- When our immune system responds to an infection, it makes a number of molecules (immunoglobulins, antibodies, and other proteins) that circulate in the blood—telltale signs of the battles fought against bacteria, viruses, and parasites. Sam found that Shuar kids with more of these markers in their blood grew slower than those who had fewer. The cost of immune response, in terms of both calories and growth, is probably one big reason that indigenous populations like the Shuar, Tsimane, and Hadza tend to be short-statured.
- The biology of death is an area of intense and active research, but researchers have long been aware of an apparent connection to metabolism: the slower a species burns energy, the longer it tends to live.
- Most important, fat cells burn a lot less energy each day than lean tissue, the cells that make up our muscles and other organs. If fat cells make up a larger proportion of your body weight, you will burn fewer calories each day than a person who weighs the same but is leaner.
How humans evolved to be the nicest, fittest, and fattest apes
- Humans combine social and foraging efforts, sharing surplus food energy with other members of their group. Sharing increases the energy available for all tasks including reproduction and maintenance, leading to longer lives, larger families, larger brains, and increased activity. Humans burn more energy each day than other apes to fuel these traits. Greater energy expenditure also favors directing extra calories to fat (far more than in other apes) to survive periods of energy shortage.
- The other downside of our evolved metabolic strategy is our evolved propensity for metabolic disease.
- These diseases aren’t inevitable. The Hadza don’t get them.
- Our hominin bodies are also evolved to support, and in fact depend on, the high levels of daily physical activity that were the norm throughout the past two million years of hunting and gathering. We have evolved to require daily exercise.
- Hadza men and women were burning the same amount of energy each day as men and women in the United States, England, the Netherlands, Japan, Russia. Somehow the Hadza, who get more physical activity in a day than the typical American gets in a week, were nonetheless burning the same number of calories as everyone else.
- Daily energy expenditure wasn’t simply responding to differences in daily activity. Instead, the body seemed to be maintaining daily energy expenditure within some narrow window, regardless of lifestyle. I call this view of metabolism “constrained daily energy expenditure.”
- First, the fact that hunter-gatherers burn the same amount of energy as urbanites in the developed world means that daily energy expenditure is likely unchanged from our Paleolithic past to the computerized present. The modern explosion in obesity and all its downstream effects can’t be blamed on decreasing energy expenditures in industrialized countries.
- Second, constrained daily energy expenditure means that increasing daily activity through exercise or other programs will ultimately have little effect on the calories burned per day.
- When we blame our metabolism for our struggles with obesity, or we rely on exercise to increase daily expenditure and lose weight, or we fall for the latest metabolism-boosting scam, we are making a fundamental mistake about the way metabolism works. The global obesity epidemic cannot be a problem of energy expenditure.
- In its purest form, the argument that calories don’t make you fat makes as much sense as the argument that money doesn’t make you rich. It’s magical thinking.
Diet, metabolism, and evolution
- Our metabolism is tightly regulated by our hypothalamus, which constantly monitors the food we eat and the calories we burn to keep our bodies in energy balance. But something—or, more likely, several things—about our modern environment is causing the hypothalamus to misfire, leading us to consume more calories than we expend.
- There are three lines of solid evidence that tell us something about the diets our ancestors ate: the archaeological and fossil record, ethnographies of living hunter-gatherers, and functional analyses of the human genome. The details differ and it’s easy to get lost in the weeds, but the overarching message from each is clear: we evolved as opportunistic omnivores.
- Humans eat whatever’s available, which is almost always a mix of plants and animals (and honey).
- Work with Inuit populations in Greenland and Canada has shown that the FADS genes have changed in these groups as well, presumably in response to the high fat content (particularly omega-3 fats) in their diet, which has traditionally included a lot of seal and whale blubber. With a diet so heavily dependent on meat and fat, these populations are often held up by Phinney and others as great examples of the benefits of a ketogenic diet. But remarkably, most people in these groups can’t go into ketosis. Instead, they carry a mutant variant of the gene CPT1A that essentially prevents the production of ketones.
- We are opportunistic omnivores, eating whatever is around. There is no singular, natural human diet, and the typical diet in the past looked nothing like the carnivore Paleo diets of today or their equally restrictive vegan counterparts.
- And even if we wanted to return to some version of an ancestral diet, we’d be hard pressed to find the undomesticated plants and animals that we ate in the past.
- When we look at the dozens of studies that have measured metabolic rates on different diets, it’s most likely that the ratio of carbs to fats has little or no effect on daily energy expenditure. If there is an effect, it appears to be much lower than the carbohydrate-insulin model predicts, and the potential gains from any metabolic bump seem to be offset by increased intake.
- All diets work if you stick to them, because all diets reduce calorie intake.
- As a general rule, we ought to seek out foods higher in fiber and protein that fill us up, and avoid processed foods with added sugars and fats that push our food reward systems over the edge. The diet that works for you is the one that allows you to achieve and maintain a healthy weight without feeling like you’re starving.
- Constrained daily energy expenditure changes the way we think about the role of exercise in our daily energy budget. With a fixed energy budget, everything is a trade-off. Instead of adding to the calories you burn each day, exercise will tend to reduce the energy spent on other activities. You can’t spend the same calorie twice.
- When exercise starts to take up a large chunk of the constrained daily energy budget, we see the same sort of prioritization at work. Other functions are squeezed out. Activities that aren’t essential—luxuries to be indulged only when energy is plentiful—are shut down first. Essential activities are protected until the bitter end. As a result, exercise has wide-ranging effects on how our metabolism is managed and where our calories are spent, which has enormous effects on our health.
- A constrained daily energy budget helps explain why exercise is so effective at reducing inflammation. When a large portion of the daily energy budget is spent on exercise, the body is forced to be more frugal with the remaining calories at its disposal. Suppressing the inflammation response, limiting it to target real threats rather than sounding the alarm constantly, reduces the energy spent on unnecessary immune system activity.