Humans spent most of our existence as a species hunting and gathering. Therefore, if you want to understand how human physiology evolved, it makes sense to study hunters and gatherers—the people whose lifestyle most closely approximates that of our remote ancestors over hundreds of thousands of years.
There are very few groups of true hunters and gatherers left in the world. One is them is the Hadza, a population of about 1,400 or so who live in northern Tanzania around the shores of Lake Eyasi.
Anthropologists who work with hunter-gatherers have long since verified the strenuousness of their lifestyle using things like pedometers and heart rate monitors. Furthermore, hunters and gathers remain active throughout their entire life. Old people don't "take it easy", but remain physically active almost until their death rather than riding around in scooters after age 50.
We've quantified the amount of physical activity that Hadza adults get each day, and the results are staggering: both men and women average more than two hours hard work each day, roughly ten times more than the average American. That's in addition to walking. They get more physical activity in a day than the typical Westerner gets in a week. The kids and old folks are active, too...
This impressive amount of physical activity isn't unique to the Hadza. All hunter-gatherers lead lives that would make Westerners melt. And while you wouldn't know it from our cushy, urbanized existence today, this extreme level of physical activity was the norm for all humans only a few thousand years ago. Our ancestors—all of them—were hunting and gathering only a few hundred generations ago, the blink of an evolutionary eye. We are a hunting and gathering species from hunting and gathering stock. (B: 156)
It was always just assumed that hunter-gatherers were burning massive amounts of calories every single day due to their intensely active lifestyles, and that was the reason why they didn't get fat. Furthermore, because they didn't get fat, it followed that they didn't get metabolic diseases like diabetes, heart disease, many cancers, and other so-called “diseases of civilization”. Everyone presumed that since they got ten times more exercise, they must be burning ten times more calories.
But no one had actually bothered to measure exactly how much energy their metabolisms were expending, because until the doubly labeled water technique was invented you had to use sophisticated equipment like gas masks and portable monitors to do it. And so, armed with doubly labeled water, Pontzer headed off to Tanzania to do the first-ever metabolic study of a hunter-gatherer population.
Hunter-gatherers and metabolism
Pontzer gave thirty Hadza subjects—both men and women—the doubly labeled water, collected their urine samples, and sent them off to a laboratory at Baylor College of Medicine. What he expected to find—what everyone expected him to find—was that the Hadza were burning hundreds of more calories per day (at least) than sedentary Westerners living cushy lives in post-industrial societies.
The results that came back instead, when checked and re-checked, made headlines all over the world.
What the results showed was that, when adjusted for factors like age, body size, body composition, and similar factors, Hadza adults weren't burning any more calories than people living in Westernized industrial societies! Here's how Pontzer describes this moment:
I had prepared for the Hadza data by putting together a big comparative dataset of daily energy expenditure measurements for adults from industrialized populations. Anyone who knows anything about energy expenditure...knows you have to account for body size. Bigger people burn more calories because they have more cells toiling away.
So I started my analysis of the Hadza data with a plot of daily energy expenditure and body size for well over a hundred men and women from the United States, Europe, and other industrialized countries. Specifically, I plotted daily energy expenditure against fat-free mass, since fat mass contributes very little to metabolic rate. Then I overlaid the Hadza data—we had measurements for seventeen women and thirteen men.
I expected the Hadza data to form a cloud hovering well above the United States and European data. Everyone knew that the Hadza had exceptionally high energy expenditures because they were so physically active.
Except they didn't. The Hadza data sat right on top of the measurements from the United States and Europe. Hadza men and women were burning the same amount of energy 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. (p. 158)
Here’s the original paper: Hunter-Gatherer Energetics and Human Obesity (PlosOne)
Were the Hadza simply some kind of physical freaks with lower metabolisms than everyone else on the planet? In the intervening years, it became clear that the answer was no—the Hadza aren't different than anyone else. Since the Hadza study, a number of other foraging and subsistence farming populations all over the world have been studied yielding the exact same results.
Studies of people living similarly active lifestyles in the rainforests of South America such as the Shuar people of Ecuador and the The Tsimane of Bolivia have found that their BMRs were slightly elevated because their immune systems had to cope with a greater burden of diseases and parasites. But once this was accounted for, their daily energy expenditures were in line with those of people living in Great Britain and the United States. Another study which compared the daily energy expenditure of women living in Maywood, Illinois and rural Nigeria found that they were the same: "The PAL ratio of daily energy expenditure to BMR was the same for Nigerian and U.S. women despite clear differences in lifestyle." (B: 163) Every study of hunter-gatherers, horticulturalists and subsistence farmers from all over the world has found the same results:
Lara Dugas, a postdoc working with Amy Luke at Loyola Medical School, analyzed daily energy expenditures reported for ninety-eight populations around the globe. There was a good deal of variability in daily energy expenditure—some were high, some were low. But the population in rural farming communities, who work hard each day to make a living, had the same daily energy expenditures as pampered urbanites in the industrialized world. Even among industrialized countries, there's no correspondence between measured physical activity and daily energy expenditure, activity energy expenditure, or PAL ratios. People who work harder don't necessarily burn more calories. (B: 163)
In the end, Pontzer concluded that physical activity levels were not directly correlated with how many calories we burn every day:
Results from the Hadza seemed to fly in the face of the factorial approach to estimating daily energy expenditure, which assumes that daily energy expenditure increases in response to daily physical activity...Somehow the Hadza were adapting to their strenuous lifestyles in a way that kept the total number of calories burned each day in check...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. (B: 160)
Our metabolic engines shift and change to make room for increased activity costs, ultimately keeping daily energy expenditure within a narrow window. As a result, physically active people—whether it's hunter-gatherers living today or in our collective past, or people in the industrialized world who exercise regularly—burn the same amount of energy as people who are much more sedentary...humans burn the same amount of energy whether we're living off the land as hunter-gatherers or cooped up in the industrialized zoos we've built for ourselves. (B: 166)
It's not just humans—even studies on animals show the same thing. The amount of energy burned by animals in the wild and their relatives living in zoos is exactly the same. When animals are made to exercise in a lab, their metabolism slows down to compensate. Each animal's evolved physiology tries to keep its daily energy expenditure in check: "The juggling act that our bodies do to keep daily expenditures within a narrow window is apparently an ancient and widespread evolutionary strategy." (B: 165)
How was this possible? These people were doing orders of magnitude more physical activity than we do every single day, and yet they were burning the exact same amount of calories as we do. How was this not a violation of the laws of thermodynamics‽
Constrained Daily Energy Expenditure and Metabolic Compensation
The answer turns out to be something Pontzer dubbed "constrained daily energy expenditure." The idea is that our metabolism adjusts to our activity level. The body wants to keep the amount of calories burned per day with a narrow range, and uses a variety of mechanisms to accomplish this. The ability of the metabolism to adjust itself in response to variable conditions of energy intake and expenditure is known as "metabolic compensation."
The predominant thinking until now has been (and in many quarters, still is) the energy balance model (EBM)—also known as "calories in, calories out," or CICO. In this model, "calories in" is the food we eat and "calories out" is our physical activity level. If there is an imbalance between these two, the thinking goes, the excess energy is stored cumulatively as fat. Conversely, if there is a negative energy balance through either reduced energy intake (i.e. caloric restriction) or increased energy output (i.e. exercise) the body will reduce its fat stores. Thus, obesity is classified as a mismatch between energy intake and output, and we are consequently told to "eat less and move more." In this view, the human body is modeled as a perfect thermodynamic engine.
There are several problems with this model, however. The first is that it assumes fat stores are effectively unregulated, and that every extra calorie goes straight into our "fuel tank" as additional fat in a 1:1 ratio. But there is no reason to believe this! Every single system in the human body is regulated through a complex biological feedback mechanism modulated by hormones and other signals. Recall from last time that most of your metabolic functions are outside of your conscious control.
With the model of the calorie-balancing scale, we assume that fat gain or loss is essentially unregulated, and that weight gain or loss is under conscious control. But no system in the body is unregulated like that. Hormones tightly regulate every system in the body. The thyroid, parathyroid, sympathetic, parasympathetic, respiratory, circulatory, hepatic, renal, gastrointestinal and adrenal systems are all under hormonal control. So is body fat. The body actually has multiple systems to control body weight.
If we eat an additional 200 calories today, nothing prevents the body from burning that excess for heat. Or perhaps that extra 200 calories is excreted as stool. Or perhaps the liver uses the extra 200...There are an almost infinite number of ways that the body can dissipate excess energy instead of storing it as body fat...We certainly don't mind if energy is used as heat or used to build new protein, but we do mind if it is deposited as fat. (OC: 34, emphasis in original)
Another is that it's virtually impossible to maintain that kind of perfect energy balance. This can be determined through simple calculation. If we assume a 2,000 calorie per day diet is ideal for the average person (in reality, this number is closer to 2,500 to 3,0001), that means we take in 730,000 calories over the course of a year.
One pound of fat contains 3,500 calories. That means that if we take in as little as 7,000 excess calories over the course of a year, we will gain two pounds of fat. That is less than a one percent deviation from our yearly caloric intake! No one is monitoring their caloric intake that closely, certainly not hunter-gatherers like the Hadza. No one even knew what a calorie was until the nineteenth century. People always had plenty of food available in North America, for example, and sedentary jobs and driving were already common by the 1950s. Yet obesity wasn't a significant problem until relatively recently. In fact, even throughout the twentieth century the biggest difficulty was maintaining enough weight, as these advertisements from yesteryear show.
With reflection, it is obvious that caloric expenditure must decrease. If we reduce daily caloric intake by 500 calories, we assume that 1 pound (0.45 kilograms) of fat per week is lost. Does that mean that in 200 weeks we would lose 200 pounds (91 kilograms) and weigh zero pounds? Of course not. The body must, at some point, reduce its caloric expenditure to meet the lower caloric intake. It just so happens that this adaptation occurs almost immediately and persists long term. (OC: 39)
A common retort is that excess calories were simply too difficult to obtain for nearly all of human existence until quite recently. But this assumes that everyone, everywhere in the world was in a permanent caloric deficit until roughly the last fifty years or so. That beggars belief. If the energy balance model is to be believed, people's weight and fat stores should have been constantly fluctuating day-in and day-out and from year-to-year. Yet we know that's not the case. As far as we know, excessive fat storage and persistent weight gain was not a major problem for most people throughout all of human history until the present day.
The other retort is that humans in the past were much more physically active. And, of course, that's true. But Pontzer's work shows—along with that of many other scientists—that the energy expenditure is not significantly higher for people today, whether making a living as farmers or hunter-gatherers, so it stands to reason that it was not significantly higher for people in the past, either.
For the energy balance model to be correct, we would have to believe that all people everywhere throughout history were perfectly balancing their energy intake and expenditure such that their weight did not fluctuate but remained stable throughout their entire lifetime. I'm sorry, but that is simply impossible to believe. Rather, it's much more likely that people's evolved biological mechanisms allowed them to maintain at a healthy and stable weight throughout their lives without starving themselves or counting calories, and that somehow this mechanism has broken down. As Ran Prieur puts it, “We think our bodies are stupid, and will inevitably get fat unless our heads intervene, but this has only been the case since around 1980.”
Of course, you may be wondering, if these biological and physiological systems are so sophisticated, then why we are so damn fat? The Hadza aren't overweight, and we now know it's not because of their activity level. Pontzer informs us that the Hadza effortlessly maintain the same weight throughout their entire adult lives:
Maintaining the same weight without really trying seems like an impossible dream to most of us, but it's far more common than you think. At least, it used to be.
Hadza men and women, for example, are incredibly weight-stable across the life span; body weights and BMIs hardly change from early adulthood to old age.
Ponder that for a moment. In the face of seasonal changes in food availability, through good years and bad, and despite the fact that men and women in their twenties and thirties (usually with young children) work a bit harder than older adults, their weight doesn't change. Presumably, this sort of effortless weight management was the norm in our hunter-gatherer past. In hunter-gatherer environments like those in which we evolved, our bodies are perfectly capable of managing our weight by adjusting our metabolism and hunger to suit the conditions.
Even in the industrialized human zoos we live in today, with unlimited delicious food always at our fingertips, our hypothalamus does a remarkably good job matching energy expenditure to intake. When we eat more calories than we burn, our metabolic rate increases as our body tries to make use of some of the surplus. When we burn more calories than we eat, hunger increases and expenditure goes down.
Sure, day to day, there's some mismatch between calories in and calories out...But over the long term, energy balance is incredibly precise. Today, in the grips of an obesity epidemic, the average American adult gains about half a pound a year, an error of around 1,750 kcal. That's only about 5 kcal per day, or less than 0.2 percent of daily energy expenditure. In other words, without thinking much about it, we match our daily energy intake to within 99.8 percent of our daily expenditure (and vice-versa). (B: 181-182)
Imagine that—not counting calories; not obsessing over every single morsel that goes into your mouth; not having to constantly motivate yourself to head to the gym and work out; not feeling self-conscious about your weight or aggressively bullied and shamed by people who think they're doing you a favor; not having to possess iron "willpower"—and your weight effortlessly remains the same throughout your entire life. If the Hadza are indeed representative of our deep evolutionary past, then this was the reality until only about a generation ago.
Yet today, in our supposedly wonderful civilization, obesity rates are at epidemic levels. Even though we're living longer, were' living sicker, plagued by a host of metabolic diseases that used to be rare even in the recent past. And even increasing lifespans are finally hitting a zenith or even declining in some places in the face of this unrelenting onslaught. The countries with the highest obesity rates in the world in the 1970s would be considered among the world's fittest and healthiest today.
We’re Not Steam Engines
The obvious conclusion is that humans are biological creatures and not simply walking thermodynamic engines. Stepping on the gas does not increase the output and extra fuel is not cumulatively stored as fat. Rather than a steam engine, metabolism is a homeostatic system. This illustrates a core mistake of the energy balance CICO model. Rather than calories in and calories out being independent variables, they are, in fact, dependent variables, and highly so.
The analogy here is a thermostat. If the temperature gets too low, compensation mechanisms kick in like turning on the furnace. If temperature gets too high, we use a heat pump (i.e. air conditioner) or ventilation to throttle it back down. If you leave the window open on a cold day, the thermostat will force your furnace to work harder (expend more energy) to keep the temperature within a narrow range. At the same time, if you are cooking food on a hot stove, your thermostat will cause the furnace to kick in less (expend less energy), since there is now an additional source of heat in the house.
Another common analogy is cash flow. If our income is drastically reduced, we do not continue to spend the same amount of money as if nothing has changed. That could be disastrous. We economize. We cut back on voluntary expenditures and prioritize fixed costs. On the other hand, when we are flush with cash, we tend to spend more, often on frivolities. Extra money often "burns a hole in our pocket." The body does the exact same thing. It's an intelligent system, honed by millions of years of evolution. Rather than a steam engine, the body is really more like an AI.
Metabolism is governed by the hypothalamus in the brain. The hypothalamus sends signals to the thyroid gland to ramp metabolism up or down in response to energy intake. Various hormonal signals are used by the thyroid gland to accomplish this task. Furthermore, our appetite is dialed up and down in response to energy intake and fat stores. Fat cells secrete a hormone called leptin, which suppresses appetite. Ghrelin, a hormone produced by the stomach when it is empty, increases appetite. Together, these signals allow energy intake and expenditure to be matched with a high degree of accuracy. The hypothalamus is the thermostat—when it is damaged, such as by a brain tumor, obesity is commonly the result.
Really, none of this should have been surprise. Numerous studies have been done trying to get people to lose weight exclusively through exercise, and every single one of them has been a failure. What typically happens is that weight loss is high in the beginning, plateaus, and then stops. More and more effort is needed to get it to continue, and the amount of weight lost given the increased activity level always falls far short of expectations based on the energy balance model, even when calories are kept constant. Because metabolism takes time to adjust, weight loss is high at the beginning of the study, but the longer it goes on, the more weight loss deviates from how much is anticipated. When the metabolisms of study participants are measured, they show a marked decrease over the course of the study. This has been shown time and time again. Contestants on The Biggest Loser had metabolisms that were 25 percent slower after 30 weeks, and their metabolisms were still slower that expected six years after the show. Their bodies were trying to regain the excess weight.
All studies that try to achieve weight loss through exercise show the same pattern: the longer the study lasts, the less that weight loss meets expectations. For the first couple of months in a new exercise program, results are all over the place. People generally lose weight, but there's a huge amount of variability in how they respond in the short term (some people even gain weight).
But after a year, even with someone watching them exercise so they don't skip or cheat, the average amount of weight lost is less than half of what's expected. By two years, the average amount of weight lost is less than five pounds, and many...will lose nothing. In other words, if you start a new exercise program tomorrow and stick to it religiously, you will most likely weight nearly the same in two years as you do right now. (B: 169)
Increased activity levels also cause us to want to eat more because the body wants to increase its energy intake to match increased expenditure. It's not called "working up an appetite" for nothing. The brain dials up our hunger levels using very ancient adaptations that tap straight into our "lizard brain"—the brain's deep core evolved structures which are very hard to override with "willpower." We consume more food in response to increased activity levels so that we don't lose weight—in fact, some people even gain weight! Simply put, if you burn an extra 200 calories a day, it does not necessarily increase your energy expenditure by the same amount because your metabolism slows down, and the longer you exercise and the more weight you lose, the greater the effects of lowered metabolism and increased appetite will be.
Such disappointing weight-loss results are due in part to...metabolic compensation to increased activity...but constrained daily energy expenditure isn't the whole story. The other important change is that exercising drives us to eat more. Our brains are exceptionally good at adjusting our hunger levels so that we make up for any increase in expenditure by increasing intake... (B: 169)
If our bodies were simple machines, small increases in daily energy expenditure would eventually lead to weight loss. But our bodies aren't simple machines. They're dynamic products of evolution built over hundred of millions of years to be agile and flexible and respond to changes in activity and food availability. Our bodies—or more the point, our brains—manipulate both our hunger and metabolic rate in ways that make it awfully hard to maintain weight loss.
Our metabolic engines are exquisitely tuned to match the energy burned each day with the energy we eat, and vice versa. (in fact, that's probably why animals evolved constrained daily energy expenditure in the first place: to match expenditure to the amount of food available.) Even transient increases in daily energy expenditure are met with increases in energy intake. When we burn more, we eat more. (B: 167-168)
A similar thing happens when we voluntarily restrict our caloric intake—our metabolism slows down and our hunger increases dramatically. The most extreme example of this is the Minnesota Starvation Experiment, referenced by all three books2.
The experiment was designed to study the so-called "starvation response" to help starving refugees from World War Two. A group of male conscientious objectors volunteered to have their calories cut in half (from about 3000 down to 1500 calories a day) while maintaining their previous activity levels. They did lose weight over the course of the study—a staggering 25 percent of their body mass. But what was more interesting is what happened to their bodies as they did so.
Their strength and physical endurance declined drastically. Their heart rate slowed down from 55 to 35 beats per minute, their heart stroke volume decreased by 20 percent, and their blood pressure dropped. Their internal organs shrank. Their body temperature went from a normal 98.6° to 95.8°F and they constantly complained of being cold. Their hair and nails grew brittle. They became moody and irritable, and lost their libido. They thought about food constantly, including in their dreams. They became neurotic, unable to focus, and a some even hoarded cookbooks and kitchen utensils. Food was the central topic of conversation to the point of obsession, and the main recreational activity was hanging out in diners to watch other people eat ("food porn" in the most literal sense.) (OC: 36-37; B:174-175; E: 33-37)
But the most striking thing was what happened to their metabolisms. Their metabolisms slowed down by a whopping 40 percent—far more than what would be predicted simply by their reduced body mass. The average metabolic rate of the volunteers measured after 24 weeks was that of an eight-year old child who weighed just 55 pounds! When the volunteers were finally allowed to eat normally, they overshot their initial weight, gaining several pounds of fat—likely an evolved response to prolonged starvation3. "The men in the Minnesota Starvation Experiment should have lost 78 pounds, but the actual weight lost was only 37 pounds—less than half of what was expected. More and more caloric restriction was required to keep losing weight." (OC: 39)
What was happening was that the body was devoting incoming energy to "core" functions and reducing energy expenditure anywhere it could. The body's evolved physiology has discretion over how energy—i.e. calories in—is spent, allocating more energy to certain metabolic functions in times of surplus and withdrawing it from others in times of deficit. Where it economizes depends on the organism's evolutionary history. The priority is offspring. For long-lived animals like us, it sacrifices reproduction for other core functions like the brain hoping for better days ahead. For shorter-lived animals, the emphasis is on reproducing right now, before it's too late.
While—unless you're a supermodel—most diet regimes aren't anywhere near that extreme, caloric restriction has the same poor track record as exercise for long-term weight loss. Metabolism throttles back and hunger surges to prod us to consume more calories.
Not surprisingly, there are ancient, evolved responses to negative energy balance in humans and other animals. When our bodies sense that we're not eating enough to meet our daily energy requirements, we start throttling things down. The body struggles mightily to balance its energy budget so that expenditure doesn't exceed intake. Our thyroid gland, the body's master controller of metabolic rate, reduces the amount of thyroid hormone produced, which is like taking your foot off the gas pedal. Our cells slow down, which lowers BMR and daily energy expenditure.
At the same time the hormones and the brain circuitry that control hunger increase our desire for food. We become ravenous, fixated on food as our body directs our mental energies toward finding something—anything—to eat. This is called the evolved starvation response, also known as dieting. (B: 174)
You may be wondering, if metabolism slows down when you get too little food, does it speed up when you take in too much food? In fact, the answer is yes! The body tries to burn off excess calories through the same methods: it increases metabolism and suppresses appetite.
Studies have been done overfeeding people like prisoners who were a normal BMI at the outset of the study. As the study went on, weight gain was always much lower than would be predicted by CICO. Subjects had to stuff themselves more and more in order to keep gaining weight—as much as 10,000 calories a day! Many subjects dropped out because they simply couldn't eat that much. In some cases, their metabolisms were spending up to 50 percent more calories per day to try and burn off the excess weight. When the study was concluded, the prisoners returned to their original BMIs: "Oveereating did not, in fact, lead to lasting weight gain. In the same way, undereating does not lead to lasting weight loss." (OC: 59-60)
An intriguing study was conducted by Dr. Ethan Sims. He took one group of people with a high BMI and dieted them down, and another group with a normal BMI and overfed them, until both groups ended up at more-or-less the same weight. He then measured their metabolisms. He found that people who were originally obese had metabolisms that were 50 percent slower than the people who were originally slim but had put on weight. Metabolisms were based on their initial weight, not on the current weight nor on food intake or activity levels. (OC: 60)
The Importance of Exercise
Even though the inhabitants of sedentary post-industrial societies and hunter-gatherers like the Hadza burn the same amount of calories per day, they are spending those calories in different ways! This, according to Dr. Pontzer, is the reason why exercise is so effective at maintaining health and vitality, even though it is ineffective at preventing obesity or leading to sustained weight loss. If "calories out" isn't based on the amount of physical activity we engage in, then where are the extra calories going that aren't being spent on exercise as they are with hunter-gatherers?
Earlier we learned that the vast majority of the calories your body burns daily are allocated to systems that you are unaware of and are outside of your conscious control. Metabolic adjustment means that when you are less active, your body is still spending those calories, but it's spending it on other things—things which are most likely detrimental to us. Just like excess income might cause you to spend money on things you probably shouldn't, the calories that are not being burned off through exercise are allocated to metabolic processes that are probably harmful and which might be behind a lot of the chronic health conditions that plague us in modern societies.
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 we burn each day, exercise will tend to reduce the energy spent on other activities. You can't spend the same calorie twice...Only recently, with the growth of doubly labeled water studies of daily expenditure across a wide variety of lifestyles, has the constrained model come to the fore. As a result, we're only beginning to understand the importance of metabolic trade-offs in exercise and health. (B: 239)
Perhaps the biggest of these is inflammation. Inflammation levels are far higher in sedentary people than in active people, and inflammation has been pinpointed as the underlying cause behind host of diseases too numerous to name. Chronic, low-grade inflammation is incredibly damaging to the body, and its results are often seen over time:
Instead of blazing acutely in one spot for a few days or weeks, as when we fight a cold, inflammation can smolder imperceptibly in many parts of the body for months or years. In a way, chronic, low-grade inflammation is like having a never-ending cold so mild you never notice its existence. But the inflammation is nonetheless there, and mounting evidence indicates that this slow burn steadily and surreptitiously damages tissues in our arteries, muscles, liver, brain, and other organs.
The discovery of low-grade inflammation and its effects has simultaneously created new opportunities to combat disease and unleashed new worries. In the last decade, chronic inflammation has been strongly implicated as a major cause of dozens of noninfectious diseases associated with aging, including heart disease, type 2 diabetes, and Alzheimer's. The more we look, the more we find the fingerprints of chronic inflammation on yet more diseases including colon cancer, lupus, multiple sclerosis, and just about every medical condition with the suffix "-itis" including arthritis. (E: 59-60)
Another place the body is allocating the extra calories is our sympathetic nervous system—specifically our stress reactivity. We are constantly on edge, and the lack of exercise may be contributing to it. We overreact to everything and are filled with anxiety. Studies have shown that stress response decreases with exercise. In addition, heightened stress reactivity floods our bodies with cortisol, a hormone which also contributes to weight gain.
Exercise is well known to reduce stress and improve mood, in part by reducing the magnitude of the stress response. A nice example of this comes from a Swiss study that used public speaking to induce a stress response in two groups of men: endurance athletes and sedentary non-exercisers. The groups were similar in age, height and weight, and general anxiety levels, but their reactions to stress were remarkably different. Both groups showed elevated heart rate and cortisol levels, but the athlete's response was smaller and dissipated more quickly. Their bodies invested less energy in the stress response, just as the constrained daily energy model would predict. (B: 242-243)
The third place Pontzer highlights is reproduction—a likely place for the body to allocate excess calories. Reproductive hormone levels are far higher in sedentary populations than in hunter-gatherers. While meatheads may think excess testosterone is a good thing, evidence indicates that unnaturally high levels in our body are damaging us in a variety of ways. Increased levels of estrogen and testosterone have been correlated with breast and prostate cancer, respectively. We know that hormone levels decrease with intensive exercise, especially aerobic exercise:
Exercise in one of the most effective ways to decrease the risk of cancers of the reproductive system (like breast and prostate cancer), in part because it keeps reproductive hormone levels in check. In fact, reproductive hormone levels in the sedentary industrialized world are likely much higher than they were in our hunter-gatherer past, judging from the levels seen in the Hadza and other, physically active, traditional populations. (B: 244-245)
Energy spent on physical activity is energy not spent on reproduction, a trade-off modulated by reproductive hormones like estrogen. Women who exercise moderately produce more than enough hormones to reproduce, but the bodies of sedentary women naturally shunt more energy toward reproduction, leading to 25 percent higher levels of estrogen. Because reproductive hormones like estrogen induce cell division in breast tissue, inactivity increases the risk of breast cancer, while exercise has the opposite effect. Levels of estrogen, hence breast cancer, are also elevated by obesity and by having fewer pregnancies. (E: 397)
Both Exercised and Burn repeatedly emphasize that while exercise is ineffective for long-term weight loss (note the qualifier), the evidence is overwhelming when it comes to its beneficial effects on the human body. Exercise affects just about every system in the body in beneficial ways. Just like our hunter-gatherer ancestors, we are built to move, but our poorly-designed human zoos take this possibility away from us far too often.
As our hominin ancestors evolved into hunter-gatherers, the body adapted to the incredible physical demands it entails. No part was left untouched. Muscles, heart, brain, guts—everything was affected...Those ancient adaptations have consequences for us today: our bodies are built to move. (B: 236)
The Takeaway
The most obvious takeaway is that the rising obesity rates are not caused by inactivity. This is an idea that deserves to die. Exercise is not a cure for obesity! It never was. Countless studies have confirmed it.
The Hadza energetics study...and all the other research we've covered in this chapter highlights how pointless it can seem to count calories: our bodies do such a good job of adjusting to the calories we expend that it can feel like calories aren't real at all. Our hypothalamus is a master of metabolic sleight of hand, altering our energy expenditure and hunger when we're not looking. Without the tools of metabolic science, keeping track of calories is a fool's errand, like trying to follow a magicians cards as they vanish and reappear.
Energy balance is the only thing that alters our weight. That''s the inescapable reality of physics. The problem is that we're atrocious at keeping track of the calories we consume, and our evolved metabolic trickery makes in nearly impossible to keep track of the calories we expend. (B: 186-187)
And really, we should have known this all along. All throughout the obesity crisis, especially the United States, activity levels have gone up, not down! We aren't more sedentary than our immediate predecessors who didn't struggle with their weight. In 1970 the New York Marathon began with just 126 starters. Today it has more than 50,000 and similar numbers are true for marathons and other athletic events across the country. Musclemen like Charles Atlas were eccentrics banished to the back pages of comic books and magazines, and bodybuilding was an underground hobby. Almost nobody included "working out" as part of their daily routine. The jogging craze that so perplexed Ron Burgundy in the 1970s was followed by the aerobics fad of the 1980s (as depicted in the current TV show Physical). In the 1970s, gyms and fitness studios were mostly shady backroom operations with just a few members. Today they are a multi-billion dollar industry, with outlets like Gold's Gym and Planet Fitness in every city throughout the country.
Yet all throughout this period obesity rates continued to climb relentlessly. Shouldn't the increased activity levels and gym memberships have done something? Anything??? This alone should have clued us in to the fact that our often-alleged laziness and sloth are not the real cause of obesity. Why, then, do so many people—including doctors—insist that getting more exercise will somehow solve it?
If the "calories out" part of the equation is more-or-less fixed, then what about the "calories in" part? Is that the ultimate cause of the crisis, and if so, is it part of the solution? Do hunter-gatherers’ eating habits offer us an indication as to what is making us fat and unhealthy? And do they give us an idea of what to eat for optimal health? And what's the deal with the Paleo diet, anyway?
That's what we'll be talking about next time.
Pontzer informs us that the 2,000 calorie a day number you see on food labels is wrong, closer to what a child would consume. What happened was that in the 1990s the FDA wanted a benchmark for food nutrition labels, so it conducted a huge survey of Americans' eating habits assuming that the average would be close to what an ideal diet should be. The problem was that people consistently underreport the amount of calories they consume by an enormous margin (just like they overreport height, income, and penis size): "In a recent study of 324 men and women across five countries, adults underreported actual food intake by 29 percent on average. That's the equivalent of forgetting an entire meal every day." (B: 103). The FDA took the erroneous self-reported averages and arbitrarily rounded them off for their dietary recommendations, which persist to this day.
This illustrates a problem with calorie counting for weight loss: people are terrible at keeping track of how many calories they eat, and even if they could, they don't know for sure how many calories they're burning as we saw. Of course, autistic CICO Nazis on the internet insist we must keep a calculator with us at all times and record every single calorie that passes through our lips 365 days a year because that's what they do. But I find it ridiculous to consider this a realistic solution for obesity. Nobody wants to live like that, and until very recently, nobody had to.
The study was conducted by Dr. Ancel Keys--the same Ancel Keys who led the crusade for low-fat diets in the 1960s and 1970s, making him the medical equivalent of Thomas Walter Midgley Jr.
Epigenetics indicates that this response is somehow passed down through generations. Studies have shown that people whose immediate ancestors starved are more likely to be obese in the present. We don't yet know the mechanism of how this is done, and it's currently one of the most intriguing mysteries in science.