Burn

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where does the fat go when we “burn it off” with exercise? Think it turns into heat? sweat? muscle? Wrong, wrong, wrong. You breathe most of it out as carbon dioxide, and turn a small fraction of it into water (but not necessarily sweat).

For me, a typical overscheduled American, one of the most jarring culture shocks in living with the Hadza is their disinterest in time. It’s not that they have no concept of time. They live with the daily rhythms of light and dark, hot and cool; the lunar cycle; the seasonal cycles of rainy and dry. They’re fully aware of growth and aging and the cultural and physiological milestones that delineate our lives. After decades of visits from researchers and other outsiders, they even have a feel for Western measures of time, of minutes and hours, weeks and years. They get it, they just don’t seem to care

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.

The difference was huge. Azy, the 250-pound male, burned 2,050 kilocalories per day—the same as a 65-pound, nine-year-old human boy. The adult female orangutans, at 120 pounds, burned even less energy: 1,600 kilocalories per day, about 30 percent lower than expected for a human that size.

As we will discuss, our metabolism also responds to changes in exercise and diet in ways that thwart our attempts to lose weight. And our drive for food is ferocious, as we can see with the Hadza. If our evolved appetites can push us into a pride of hungry lions for breakfast, how can we keep ourselves out of the fridge?

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).

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.

Chewing and mixing the food with saliva is the first step in digesting your meal and its main constituent parts, the macronutrients.

Starch digestion starts in your mouth, with an enzyme in your saliva called amylase, which begins the process of breaking long amylose and amylopectin molecules down into smaller and smaller pieces.

The Hadza erupt with glee. Ha! The lions got ’em!
I’m stunned. What psychopaths root for the lions?
Then it begins to sink in. Feeling bad for the elephants is a luxury of life in the industrialized world, experiencing nature through a television screen.*

energy expenditures are reported in metabolic equivalents, or METs. One MET is defined as 1 kilocalorie per kilogram of body mass per hour, roughly the energy cost of resting. The Compendium of Physical Activity, compiled every few years since 1993 by Barbara Ainsworth and her team, is the definitive resource for anyone wondering about the caloric cost of a particular activity.

For the most part, the costs of physical activity are disappointingly small. Consider a typical 150-pound adult. Even if they get their recommended daily allowance of 10,000 steps per day (about five miles), that’s only about 250 kcal—just about the same number of calories as a 20-ounce bottle of soda (240 kcal) or half of a Big Mac (270 kcal). Climbing one flight of stairs (about 10 feet of ascent) burns about 3.5 kcal, less energy than they’ll get from an M&M

Background energy expenditure goes by several names: basal metabolic rate, basal energy expenditure, resting energy expenditure, resting metabolic rate, and standard metabolic rate, among others.

With all of your organs toiling away all day, it’s little wonder that BMR accounts for most of the calories you burn each day, around 60 percent for most of us

People living in the Arctic tend to have about 10 percent higher BMRs than those in warmer climates, which is probably due in part to brown fat activity.

Mild cold exposures, like hanging out in shorts and T-shirt in a 65°F room, can raise your metabolic rate 25 percent above your BMR

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.

daily energy expenditure is surprisingly tough to pin down, and after more than a half century of trying, we still get it wrong. The problem, as we began to discuss in Chapter 1, is that our bodies are not simple machines. Our metabolic engines are dynamic, adaptive products of evolution. Daily expenditure is not simply the sum of its parts.

the factorial method hide its fundamental flaw. It assumes that daily energy expenditure is simply BMR plus the costs of physical activity and digestion. That view has become so accepted and widespread that it’s hard to imagine any other perspective

Humans are social foragers. We routinely bring home more than we need, with the intention of giving it away to our community.

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.

they were burning around 360 kcal per day running, based on their body weight and mileage. If the factorial model were correct, we’d expect their daily energy expenditures to be at least 360 kcal per day higher by the end of the study, and closer to around 390 kcal/day if we factor in the calories consumed at rest by their increased muscle mass (Chapter 3). Instead, at week 40, their daily energy expenditure was only about 120 kcal higher. These women went from never exercising to running 25 miles per week, fit enough to run a half-marathon, and their daily energy expenditure was essentially the same as when they started (Figure 5.2). Men in the study showed similar results.

Constrained daily energy expenditure seems to be the rule among warm-blooded animals. Several laboratory studies in rodents and birds have measured daily energy expenditures while increasing daily physical activity—not so different from Westerterp’s half-marathon study. Again and again, we see the same result: daily energy expenditure doesn’t change even as the animals work harder and harder. The juggling act that our bodies do to keep daily expenditures within a narrow window is apparently an ancient and widespread evolutionary strategy.

Doubly labeled water studies in the industrialized world, which stretch back to the 1980s, seem to confirm this: daily energy expenditures and the PAL ratio have stayed the same in the United States and Europe for the past four decades, even as obesity and metabolic disease have skyrocketed.

Faced with increasing daily physical activity, the body adjusts, saving energy elsewhere to keep daily energy expenditure in check. Any lasting increase in daily expenditure is matched by increased intake, nullifying the potential for weight loss.

Our research with the Hadza shows that men and women rack up about five hours of physical activity each day. A third of that—around one to two hours—is what physiologists call “moderate and vigorous” activity like fast walking or digging tubers, the kind of exertion that really gets your heart rate up. The rest is “light” activity, like strolling around camp or picking berries. Daily workloads for groups like the Tsimane and Shuar are similar

Sports scientists and physiologists still argue about the mechanisms in our body that set the limits (for ringside seats to this scientific brawl, check out Alex Hutchinson’s excellent book, Endure).

First, it became clear from lab studies and lived experience that a person who feels completely exhausted still has plenty of fuel on board. Even when we feel like we’ve reached our absolute limits, there’s still plenty of ATP in our tired muscles and glucose and fatty acids circulating in our blood.

For truly limitless endurance you need to be able to maintain body weight, and to do that you have to keep your daily energy expenditure at around two and a half times BMR

human digestive efficiency—the ratio of energy absorbed over energy eaten—is about 95 percent.

Generated at: 2023-08-01-08-55-17