Herman Pontzer's "Burn" takes readers on a fascinating journey into the world of human metabolism, challenging long-held beliefs about how our bodies use energy. Drawing from cutting-edge research and his own experiences studying hunter-gatherer societies, Pontzer presents a new understanding of how our bodies burn calories and what this means for weight loss, exercise, and overall health.
Introduction: The Currency of Life
At the heart of human existence lies a fundamental process: metabolism. Our bodies are composed of roughly 37 trillion cells, each one a miniature factory tirelessly producing the substances that keep us alive. From enzymes to neurotransmitters to hormones, these cellular products are the building blocks of life itself.
To fuel this constant cellular activity, we rely on the calories we consume through food and drink. Every day, our cells burn enough energy to bring eight gallons of ice water to a roiling boil – a staggering amount of power when you stop to think about it. Energy, then, is truly the currency of life.
Despite its critical importance, metabolism is often misunderstood. Popular notions about how our bodies use energy are frequently based on outdated science or misconceptions. Pontzer's book aims to set the record straight, offering readers a clear and engaging look at the latest research on human metabolism.
In the pages that follow, we'll explore fascinating insights such as:
- What we can learn about human evolution from studying Tanzanian hunter-gatherers
- How the practice of sharing food set humans apart from our ape cousins
- Why it's possible to eat nothing but junk food and still lose weight (though it's certainly not recommended!)
By the end of this journey, you'll have a new appreciation for the incredible ways our bodies use energy and what this means for our health, weight, and overall well-being.
You Are What You Eat – Literally
To understand metabolism, we need to start with a fundamental truth: you are, quite literally, what you eat. This isn't just a catchy phrase – it's a scientific fact that would have astounded our ancestors.
In 1859, the French scientist Louis Pasteur made a groundbreaking discovery. He created a special broth that wouldn't spoil, thanks to a two-step process: first, boiling the liquid to kill any germs, and second, keeping it in an airtight container to prevent new contaminants from entering. This process, which we now call pasteurization, wasn't just a practical breakthrough. It also put to rest an ancient theory known as spontaneous generation.
For centuries, people believed that life could spontaneously arise from non-living matter. This idea tried to explain phenomena like maggots appearing on rotting meat. Without powerful microscopes, it was hard to understand where these creatures came from, so many assumed they simply generated from the meat itself.
While we now scoff at such notions, the truth about how our bodies work is even more remarkable. In a sense, we are all spontaneous-generation machines. Every part of our body – from our bones and blood to our fingernails and hair – is constructed from the food we eat and the air we breathe. We transform inanimate matter into living tissue through the miraculous process of metabolism.
Here's how it works: Our bodies are composed of thousands of different molecules that interact in complex ways. These include enzymes, hormones, neurotransmitters, DNA, and more. However, very few of these molecules arrive in our bodies in a usable form through our diets. Instead, they must be converted through cellular processes.
Each cell in our body acts like a tiny factory. It pulls useful molecules from the bloodstream through its membrane, converts them into something new, and then often releases the end product back into the bloodstream to affect other parts of the body. For example, ovary cells take in cholesterol, transform it, and release estrogen – a hormone that influences the entire body.
This constant cellular work is what keeps us alive, but it requires an enormous amount of energy. That's where metabolism comes in. It's the body's life-preserving furnace, "burning" the food we eat and unlocking its energy to fuel all these vital processes.
Understanding this fundamental truth – that we literally build our bodies from what we consume – gives us a new appreciation for the importance of nutrition and the intricate workings of our metabolism.
Metabolism: Measuring the Body's Energy Use
To truly grasp the concept of metabolism, we need to understand the relationship between energy and work. In physics, these terms are actually interchangeable. When we talk about doing work, we're really talking about using energy.
Think about throwing a baseball. The work you do to accelerate the ball is converted into kinetic energy – the energy of motion – as the ball flies through the air. Another everyday example of energy is heat. When you warm up milk in a microwave, the increase in temperature is a measure of how much electromagnetic energy the milk has absorbed.
A fundamental law of physics states that energy consumed is always equal to work done plus heat gained. This law governs everything in the universe, including our bodies.
Energy can be stored in various forms, all of which have the potential to do work or create heat. A few examples include:
- Gasoline in a fuel tank
- A stretched rubber band (which has "strain energy")
- A heavy plant pot teetering on a window sill (which has potential kinetic energy)
At the molecular level, the bonds holding molecules together store chemical energy. This energy can be converted from one form to another, but it can never be lost. When you release a stretched rubber band, for instance, the molecular bonds snap apart, releasing that stored energy into the surrounding environment.
Explosions provide a dramatic illustration of this principle. Take nitroglycerin, for example. When detonated, its molecular bonds break apart into simpler molecules like nitrogen, carbon monoxide, oxygen, and water, releasing an enormous amount of energy in the process. If all the energy stored in a pound of nitroglycerin were converted to heat, it could vaporize a human instantly. If converted to kinetic energy, it could launch a 165-pound adult two and a half miles into the sky!
So, what does all this have to do with metabolism? Well, if energy and work are interchangeable, then the work our cells do and the energy they consume are two ways of measuring the same thing. When we talk about "metabolism," we're really talking about energy expenditure – the amount of energy our bodies use to fuel all cellular activities.
We can take this concept a step further by considering the rate at which energy is used. This gives us the metabolic rate – the amount of energy the body expends every minute to keep all its cellular processes running.
Understanding metabolism in these terms helps us see it for what it truly is: a measure of the body's fundamental activity, the energy that fuels life itself.
Tracking Energy Expenditure: Counting Atoms
Now that we understand what metabolism is, how do we actually measure it? In theory, the concept is simple: we follow the CO2.
Whether we're talking about burning coal or metabolizing carbohydrates, the process of using fuel creates a byproduct – carbon dioxide. When our bodies burn energy, they produce CO2, which we then exhale. By measuring how much CO2 a person produces, we can calculate precisely how much energy their body is burning.
One way to do this is by using a metabolic chamber – a sealed room equipped with sensors that analyze oxygen and CO2 levels. While this method provides accurate results, it doesn't tell us much about how people expend energy in their everyday lives.
In the 1950s, physiologist Nathan Lifson at the University of Minnesota made a breakthrough that allowed researchers to track CO2 production in people going about their normal routines. His insight began with a simple observation: the human body, which is about 65 percent water, is essentially a large pool with constant inflow and outflow.
Hydrogen and oxygen atoms enter our bodies through food and drink, and they leave as urine, feces, sweat, and the water vapor in our breath. While hydrogen atoms always leave the body as part of water molecules, oxygen atoms have another exit route. When carbon-based molecules are metabolized, CO2 is formed. The oxygen atom in this CO2 molecule comes from the body's water, and it's then exhaled.
Lifson realized that by tracking the rate at which hydrogen and oxygen atoms leave the body, he could calculate the rate of CO2 production, which in turn reveals how much energy has been burned.
To trace these atoms, researchers use a clever technique involving isotopes – heavier forms of hydrogen and oxygen. Here's how it works:
- Introduce hydrogen and oxygen isotopes (deuterium and oxygen-18) into the body.
- Analyze urine samples to count how many of these "labeled" atoms leave the body over time.
- Use this data to calculate the rate at which hydrogen and oxygen atoms are being lost.
- From this, determine the rate of CO2 production.
- Finally, use the CO2 production rate to calculate how many calories the body has burned.
For example, if 10 percent of the hydrogen atoms in a person's body were deuterium on Monday, but only 5 percent were deuterium on Wednesday, we know that half of the body water has been expelled and replaced with regular H2O in that time. The same principle applies to oxygen-18.
This method, known as the doubly labeled water technique, allows researchers to measure energy expenditure in free-living humans over days or weeks. It's become the gold standard for metabolic studies, providing invaluable insights into how our bodies use energy in real-world conditions.
We're Just Like Our Ancestors, Metabolically Speaking
For years, a common theory has tried to explain the prevalence of obesity in Western societies. It goes something like this: Our bodies, including our metabolic systems, evolved to cope with the environment of our early human ancestors. These hunter-gatherers lived in a world where food was scarce, and they had to expend enormous amounts of energy just to find enough to eat.
According to this theory, the advent of industrialization – which brought us cars, office jobs, and supermarkets – created a mismatch between our bodies and our environment. We're no longer as physically active as our ancestors, so we're not using our bodies as they were "designed" to be used. This, the theory claims, is the root of our modern metabolic issues and weight problems.
It's a compelling narrative, but new evidence suggests it's fundamentally flawed.
To test this hypothesis, researchers needed to compare the energy expenditure of modern Westerners with that of our prehistoric ancestors. While we can easily measure how many calories the average American or European burns in a day, we can't travel back in time to study early humans. However, we can do the next best thing – examine the energy expenditure of modern people who live similarly to our ancestors.
This is where the Hadza people of northern Tanzania come in. The Hadza are one of the world's few remaining populations of hunter-gatherers. Their lifestyle is physically demanding in ways that mirror what we imagine early human life to have been like.
Hadza women spend most of their day digging tubers out of the rocky soil and picking wild berries. Men cover an average of twelve miles per day in the scorching savanna, hunting game and scaling 40-foot trees to harvest wild honey. In the evening, the Hadza gather around campfires to share the fruits of their labor.
The author and his team collected urine samples from Hadza individuals and sent them to a specialized lab in Texas for analysis using the doubly labeled water technique. According to the prevailing theory, Hadza men and women should expend far more energy than their sedentary Western counterparts.
The results, however, were surprising. On average, Hadza men eat and burn around 2,600 calories a day, while women consume and expend about 1,900 calories a day. These figures are virtually identical to the average energy expenditure of men and women in Europe and the United States.
This finding is remarkable when you consider the stark differences in lifestyle. On the surface, the daily routines of a Hadza hunter-gatherer and someone commuting to an office job in New York or Naples couldn't be more different. Yet in terms of total energy expenditure, they're essentially the same.
These results aren't an anomaly. Other studies have found similar patterns. For instance, a 2008 study by Amy Luke, a researcher at Loyola University Chicago, compared the energy expenditure and physical activity of women living in rural Nigeria to African American women in Chicago. Despite their vastly different lifestyles, both groups expended the same amount of energy per day.
Another researcher, Lara Dugas, also from Loyola, analyzed data from 98 studies from around the world. Her conclusion? Sedentary populations in developed countries burn roughly the same amount of energy on average as people with much more physically demanding lives in developing nations.
These findings paint a clear picture: in terms of energy use, humans are remarkably similar wherever you look. Our daily energy expenditure seems to have remained largely unchanged since our hunter-gatherer days, despite the dramatic changes in our lifestyles and environments.
The Constrained Metabolism: How Our Bodies Budget Energy
The discovery that hunter-gatherers like the Hadza burn the same number of calories as sedentary Westerners raises an important question: How is this possible? How can people who spend their days foraging, hunting, and climbing trees not burn more calories than those of us who spend most of our time sitting?
The answer lies in what researchers call "constrained" or "fixed" metabolism. Our bodies have evolved strategies to keep our daily energy expenditure within a certain range, regardless of our activity level. There are likely several factors at play here:
Behavioral changes: Highly active people like the Hadza may subtly alter their behavior to conserve energy when they're not engaged in intense physical activity. This might involve sitting rather than standing when possible, or sleeping longer.
Energy reallocation: The body appears to "budget" its energy expenditure differently when we engage in lots of physical activity. Typically, most of the calories we burn go toward fueling basic cellular work and conducting "housekeeping" tasks like repairing wear and tear in the body. When we're very active, it seems the body makes room in its limited energy budget by cutting down on some of these tasks. For example, there's evidence that exercise reduces the immune system's inflammatory response and the production of certain hormones like estrogen.
Plateauing effect: Research shows that energy expenditure tends to plateau at higher levels of activity. In a study conducted by the author and Amy Luke, they used the doubly labeled water technique on 300 participants while monitoring their activity with fitness trackers for a week. The results showed that people with the most intensely active daily lives burned roughly the same number of calories each day as those with moderately active lives.
This constrained metabolism has significant implications for how we think about health, exercise, and weight management. If our daily energy expenditure hasn't changed much over the course of human history, we can't blame our current obesity epidemic solely on our more sedentary lifestyles.
In other words, it's not laziness (or "sloth") that's primarily responsible for making us overweight – it's overconsumption (or "gluttony"). This insight challenges many common assumptions about diet and exercise, and suggests we may need to rethink our approaches to weight loss and public health.
Our Evolutionary History and the Obesity Paradox
To understand why humans are so prone to obesity, we need to look at our evolutionary history. Charles Darwin observed that natural history is shaped by the struggle for resources. Because there's never enough food to go around, species typically evolve under conditions of scarcity.
This scarcity leads to evolutionary trade-offs. With limited energy available, a species can't have it all. You might end up with a creature like the Tyrannosaurus rex, which had enormous, powerful jaws but tiny arms. As Darwin put it in "The Origin of Species," "to spend on one side, nature is forced to economize on the other side."
However, humans seem to be an exception to this rule. We're remarkably extravagant when it comes to energy use. Consider the differences between humans and our closest relatives, the great apes. Once you account for variables like body size and activity level, humans consume about 400 more calories per day than chimpanzees and bonobos.
What do we do with all these extra calories? We use them to fuel a variety of energy-intensive traits:
- Our large brains: The human brain is so energy-hungry that every fourth breath we take goes toward feeding this three-pound organ.
- More frequent reproduction: Humans have babies more often than apes.
- Larger babies: Human infants are bigger relative to adult body size than ape babies.
- Longer lifespans: Humans live longer than apes.
- Greater mobility: We move around more than apes do.
There are some trade-offs – our digestive tract is smaller and less costly than that of most apes, for instance – but overall, humans evolved to burn more energy at a cellular level. This was nothing less than a metabolic revolution.
However, this revolution came with a significant downside. As our ancestors developed a faster metabolism, their risk of starvation increased. After all, the more energy you need to function, the worse it is when food runs out.
The evolutionary solution to this problem is what haunts us today: fat storage. The simplest way of keeping an energy-guzzling machine like the human body fueled in an environment defined by scarcity is to store energy for later use. The body's fuel-storage system is the fat cell.
This trait sets us apart from apes. If you keep a chimpanzee in a zoo with abundant food, it will get bigger than its wild counterparts, but it will remain lean. Extra calories build bigger muscles and organs rather than creating fat deposits. Humans, on the other hand, readily store excess calories as fat.
This evolutionary adaptation was crucial for our survival in times of scarcity. However, in our modern world of caloric abundance, it's become a liability. We've evolved a response to food scarcity, but we live in a world where food – especially calorie-dense, processed food – is readily available in many parts of the world.
This mismatch between our bodies' expectations (based on millions of years of evolution) and our current environment (shaped by rapid technological and cultural changes) is at the heart of our struggle with obesity. Our bodies are still preparing for famines that, in many parts of the world, no longer come.
Understanding this evolutionary context helps explain why losing weight and keeping it off is so challenging for many people. We're not just fighting against bad habits or lack of willpower – we're fighting against our own biology, shaped by millions of years of evolution in environments very different from the one we now inhabit.
The Power of Sharing: How Cooperation Fueled Human Evolution
To understand how humans developed such an energy-intensive lifestyle, we need to look at a key behavior that sets us apart from our ape cousins: food sharing. This seemingly simple act had profound implications for human evolution and metabolism.
Apes and humans have much in common. Both are social animals capable of forming complex, even lifelong relationships. However, when it comes to food, apes are rugged individualists. They don't share their meals, which profoundly shapes their approach to finding calories.
For an ape, survival depends entirely on its own ability to find food. There's no safety net, no one to help if the search comes up empty. This leads apes to focus on easily accessible, low-risk food sources – the proverbial "low-hanging fruit." There's little incentive to cooperate with others to hunt big game or gather large quantities of food because the spoils wouldn't be shared anyway.
Humans, on the other hand, evolved as social foragers. Our ancestors didn't stop searching for food once their own stomachs were full – they brought back extra to share with others in their group. This practice created a crucial safety net. If you returned to camp empty-handed after a day of foraging or hunting, you wouldn't starve because others would share their finds with you.
This safety net changed human behavior in significant ways:
Risk-taking: With the assurance that others would share food, individuals could take more risks in their foraging strategies. For example, a group of hunters could spend days tracking large game, knowing that even if they failed (which was often the case), they wouldn't go hungry thanks to the gathered foods brought in by others.
Specialization: Food sharing allowed for a division of labor. Some group members could focus on hunting while others gathered plants, each playing to their strengths and increasing the overall efficiency of the group's food acquisition.
Innovation: The security provided by food sharing gave people more time and energy to experiment with new tools and techniques, driving technological advancement.
While we don't know exactly when food sharing emerged in human evolution, evidence suggests it was a key feature of hominin life by about two and a half million years ago. Archaeological finds, such as zebra bones with cut marks from stone tools, provide tangible evidence of cooperative hunting and food sharing. Bringing down a large, fast animal like a zebra requires teamwork, which only makes sense in a context where the spoils are shared.
The practice of food sharing set humans on a new evolutionary path. It meant there was more energy available for life's essential tasks. More people survived, more babies were born, and more time was spent developing primitive technologies. Groups that shared food outcompeted those that didn't, driving the evolution of social and cognitive traits that supported cooperation.
Ultimately, this shift in behavior allowed for the development of the human body and brain as we know them today. Our faster metabolism created the energy budget necessary to support our energy-hungry brains, setting the stage for the cognitive leap that would define our species.
The legacy of food sharing is still with us today. It's reflected in our social norms around meals, our traditions of hospitality, and even in modern economic systems based on the division of labor and exchange. Understanding this aspect of our evolutionary history provides insight into not just our metabolism, but our fundamental nature as a cooperative, sharing species.
The Truth About Dieting: It's All About the Calories
Given what we've learned about human metabolism – that our daily energy expenditure has likely remained unchanged since Paleolithic times, and that increasing exercise doesn't significantly affect the number of calories we burn – it's time to rethink our approach to weight loss and obesity prevention.
The key takeaway is this: while exercise has numerous health benefits, it's not particularly effective for weight management. As the saying goes, "You can't outrun a bad diet." This brings us to the topic of dieting, which is often surrounded by conflicting information and fad approaches.
Let's cut through the noise and focus on the fundamental principle of weight loss: if you want to lose weight, you must burn more calories than you consume. This is a basic law of physics, and no diet can circumvent it.
The good news is that this principle gives you the freedom to choose a diet that works for you, as long as it results in a calorie deficit. A 2005 study by Michael Dansinger, now the head of the Diabetes Reversal Program at Tufts Medical Center, illustrates this point well. Dansinger's team randomly assigned 160 adults in Boston to follow one of four popular diets for twelve months:
- Atkins (low carb)
- Ornish (low fat)
- Weight Watchers (balanced approach)
- Zone (balanced approach)
The results were clear: regardless of which diet participants followed, those who stuck to their assigned plan lost weight, while those who didn't adhere to their diet failed to lose weight. This suggests that the specific "philosophy" behind a diet is less important than simply reducing overall calorie intake.
To drive this point home even further, consider the extreme experiment conducted by Mark Haub, a professor of human nutrition at Kansas State University. Frustrated with the pseudoscientific claims surrounding many popular diets, Haub decided to prove that weight loss is fundamentally about calorie balance, not the specific foods eaten.
For ten weeks, Haub ate nothing but junk food – candy, sugary cereals, chips, and cookies. However, he strictly limited his daily calorie intake to 1,800 calories. The result? After two and a half months, Haub had lost 27 pounds.
It's crucial to note that neither Haub nor any nutrition expert would recommend such a diet. It's clearly detrimental to overall health, lacking essential nutrients and potentially leading to various health problems in the long term. However, Haub's experiment serves as a stark illustration of the fundamental principle of weight loss: create a calorie deficit, and you will lose weight, regardless of where those calories come from.
This doesn't mean that the source of your calories is irrelevant for overall health. A diet rich in fruits, vegetables, whole grains, and lean proteins will provide essential nutrients and support better health outcomes than a diet of processed junk food. But when it comes to weight loss specifically, calorie balance is the key factor.
Understanding this principle can be liberating. It means you don't have to adhere to a highly restrictive diet that eliminates entire food groups or requires you to eat foods you don't enjoy. Instead, you can focus on finding a sustainable way of eating that allows you to maintain a modest calorie deficit over time.
Some strategies that can help include:
Portion control: Using smaller plates, measuring servings, and being mindful of portion sizes can help reduce overall calorie intake without drastically changing what you eat.
Increasing protein and fiber intake: These nutrients help you feel full and satisfied, potentially reducing overall calorie consumption.
Mindful eating: Paying attention to your food, eating slowly, and listening to your body's hunger and fullness cues can prevent overeating.
Planning meals: Preparing meals in advance can help you avoid impulsive, high-calorie food choices when you're hungry.
Tracking calories: While not necessary for everyone, keeping a food diary or using a calorie-tracking app can help you understand your intake and make informed choices.
Remember, sustainable weight loss is typically slow and steady. Aim for losing 1-2 pounds per week, which requires a daily calorie deficit of about 500-1000 calories. This moderate approach is more likely to result in long-term success than crash diets or extreme restrictions.
Ultimately, the most effective diet is one that you can stick to over the long term. By understanding the basic principle of calorie balance and finding an eating pattern that works for your lifestyle and preferences, you can achieve and maintain a healthy weight without resorting to fad diets or extreme measures.
Conclusion: Rethinking Energy, Evolution, and Health
Herman Pontzer's "Burn" challenges many of our long-held beliefs about metabolism, exercise, and weight loss. By examining the latest research and drawing insights from both modern societies and hunter-gatherer groups like the Hadza, Pontzer presents a new understanding of how our bodies use energy.
Key takeaways from the book include:
Constrained metabolism: Our daily energy expenditure is remarkably consistent across different lifestyles and activity levels. This suggests that our bodies have evolved mechanisms to keep our energy use within a certain range.
The limits of exercise for weight loss: While exercise has numerous health benefits, it's not as effective for weight loss as many believe. Our bodies adapt to increased activity by reallocating energy from other processes.
The power of food sharing: The practice of sharing food was a crucial development in human evolution, allowing for greater risk-taking, specialization, and ultimately, the development of our energy-intensive brains.
The fundamental importance of calorie balance: When it comes to weight loss, the most important factor is creating a calorie deficit, regardless of the specific diet followed.
The mismatch between our evolutionary past and present environment: Our bodies evolved strategies for coping with food scarcity, which can work against us in our current environment of caloric abundance.
These insights have significant implications for how we approach health, weight management, and public policy. They suggest that efforts to combat obesity should focus more on reducing calorie intake rather than simply promoting exercise. They also highlight the need for strategies that work with our evolved tendencies rather than against them.
At the same time, Pontzer's work underscores the incredible adaptability of the human body. Our metabolic flexibility allowed our ancestors to thrive in a wide range of environments and continues to serve us today. Understanding this flexibility can help us develop more effective, personalized approaches to health and nutrition.
Ultimately, "Burn" invites us to marvel at the complex, finely-tuned machine that is the human body. By understanding how our metabolism really works, we can make more informed decisions about our health, appreciate the intricate balance of our bodily systems, and gain a new perspective on our place in the natural world.
As we face global challenges related to obesity, nutrition, and health, the insights from "Burn" provide valuable guidance. They remind us that while our modern environment may be new, our bodies are the product of millions of years of evolution. By aligning our lifestyles and health strategies with this evolutionary heritage, we can work towards better health outcomes for individuals and societies alike.