Burn

1. Metabolism Isn't a Simple Engine: It's an Evolved System

Our bodies don’t work like simple fuel-burning machines because they aren’t products of engineering, they’re products of evolution.

Evolutionary perspective. Metabolism isn't a static process; it's a dynamic, adaptable system shaped by millions of years of evolution. Our bodies aren't designed like simple machines with inputs and outputs, but rather as complex, interconnected systems that respond to changes in diet and activity in ways that make evolutionary sense, even if they frustrate our attempts to stay trim.

• Our bodies prioritize survival and reproduction, not necessarily weight loss or health.
• Metabolic processes are not always linear or predictable.
• Our bodies have evolved to be incredibly efficient at storing energy.

Beyond simple calories. The traditional view of metabolism as a simple input-output system is flawed. Our bodies don't just burn calories; they also convert them, store them, and prioritize their use based on a complex set of signals. This means that the source of calories (carbs, fats, proteins) and the context in which they are consumed can have a significant impact on how our bodies respond.

Dynamic and adaptable. Our metabolic engines are incredibly dynamic and adaptable, able to respond to changes in exercise and diet in ways that make evolutionary sense even if they frustrate our attempts to stay trim and healthy. This adaptability is a key reason why simple approaches to weight loss often fail.

2. Energy Expenditure is Constrained, Not a Simple Calculation

More exercise doesn’t necessarily mean more energy burned per day, and burning more energy doesn’t protect against getting fat.

Fixed daily budget. Daily energy expenditure is not simply the sum of its parts (BMR + activity + digestion). Instead, our bodies seem to operate on a constrained energy budget, adjusting other metabolic processes to keep total daily energy expenditure within a relatively narrow range.

• Increased physical activity doesn't always lead to a proportional increase in calories burned.
• Our bodies compensate for increased activity by reducing energy spent on other tasks.
• This constraint is a key reason why exercise alone is often ineffective for weight loss.

Beyond the armchair engineer. The traditional view of metabolism as a simple engine where more activity equals more calories burned is an oversimplification. Our bodies are not simple machines; they are complex, evolved systems that prioritize survival and reproduction over our desire to be thin.

• The body is not a simple fuel-burning machine.
• Our bodies are products of evolution, not engineering.
• We need to move beyond the simplistic view of metabolism.

Implications for weight loss. The constrained model of energy expenditure suggests that focusing solely on increasing physical activity to lose weight is unlikely to be effective. Instead, we need to focus on managing our energy intake, as our bodies will adjust to keep daily energy expenditure within a narrow range.

3. Humans Evolved to Be High-Energy, Fat-Storing Apes

In the human lineage, our cells evolved to work harder, to do more, and to burn more energy.

Metabolic acceleration. Humans evolved a faster metabolic rate than other apes, allowing us to fuel our large brains, active lifestyles, and high reproductive rates. This metabolic acceleration is a key factor in what makes us human.

• Humans burn about 20% more energy than chimpanzees and bonobos.
• Humans burn about 40% more energy than gorillas.
• Humans burn about 60% more energy than orangutans.

Fat storage. Along with our faster metabolism, humans also evolved a greater capacity to store fat. This is likely an adaptation to protect against starvation, providing a fuel reserve to draw upon when food is scarce.

• Humans carry twice as much fat as other apes.
• Our increased body fat evolved hand in hand with our faster metabolic rate.
• Fat provides a larger fuel reserve to guard against starvation.

Trade-offs and adaptations. Our evolved metabolism is a trade-off, allowing us to do more but also making us more vulnerable to overeating and weight gain in environments where food is plentiful. Our bodies are not designed to be lean; they are designed to survive and reproduce.

4. Sharing, Not Just Hunting, Fueled Human Evolution

Energy is everything, and it’s worth risking everything to get it.

Beyond individual effort. The success of the human lineage is not solely due to our hunting prowess or tool-making abilities. It's also due to our unique capacity for sharing and cooperation.

• Humans are social foragers, routinely bringing home more than they need.
• Sharing creates a safety net, allowing individuals to take risks and specialize.
• Sharing is the glue that binds hunter-gatherer communities together.

The power of cooperation. Sharing food and resources allowed our ancestors to diversify their foraging strategies, with some focusing on hunting and others on gathering. This division of labor increased the overall efficiency of the group and provided a more reliable food supply.

• Hunting and gathering are complementary strategies.
• Sharing allows for specialization and risk-taking.
• Cooperation is a key human adaptation.

Metabolic consequences. The shift to a cooperative foraging strategy, with its emphasis on sharing, fueled the evolution of our faster metabolism and larger brains. It also led to a greater reliance on social connections and a more complex social structure.

5. Dietary Extremes Are Not the Answer: Balance Is Key

You are what you eat isn’t just a well-worn cliché, it’s how life actually works.

Omnivorous nature. Humans are evolved to be opportunistic omnivores, eating a wide variety of plant and animal foods. There is no single "natural" human diet, and our ancestors thrived on a diverse range of foods.

• Humans are not naturally carnivores or herbivores.
• Our digestive systems are adapted to process both plant and animal matter.
• We have a 65-million-year history of relying on carbohydrates.

The problem with processed foods. Modern processed foods are often engineered to be highly palatable and easy to overeat, disrupting our natural satiety signals and leading to overconsumption.

• Processed foods are often high in sugar, fat, and salt.
• They are designed to bypass our natural satiety mechanisms.
• They are often low in fiber and other nutrients.

Focus on whole foods. A balanced diet that emphasizes whole, unprocessed foods, including a mix of carbohydrates, fats, and proteins, is the best approach for long-term health and weight management.

• Prioritize fruits, vegetables, whole grains, and lean proteins.
• Limit processed foods, sugary drinks, and unhealthy fats.
• Listen to your body's hunger and satiety cues.

6. Exercise Is Vital, But Not for Weight Loss Alone

Our metabolic engines were not crafted by millions of years of evolution to guarantee a beach-ready bikini body, to keep us fit, or even necessarily to keep us healthy.

Beyond calorie burning. While exercise is essential for health, it's not a particularly effective tool for weight loss. Our bodies adapt to increased activity by reducing energy expenditure on other tasks, keeping daily energy expenditure within a narrow range.

• Exercise does not significantly increase daily energy expenditure.
• Our bodies compensate for increased activity by reducing energy spent on other tasks.
• Exercise alone is not a reliable way to lose weight.

Metabolic benefits. The real benefits of exercise lie in its ability to improve our metabolic health, reducing inflammation, improving stress response, and regulating reproductive hormones.

• Exercise improves cardiovascular health.
• It reduces the risk of type 2 diabetes and other metabolic diseases.
• It improves mood, stress, and cognitive function.

The importance of movement. Our bodies are built to move, and regular physical activity is essential for maintaining optimal health. We need to move beyond the idea that exercise is just about burning calories and embrace it as a fundamental part of a healthy lifestyle.

7. Our Guts, Not Just Our Muscles, Limit Endurance

We all get about one billion heartbeats.

Beyond muscle fatigue. The limits of human endurance are not solely determined by muscle fatigue or fuel depletion. Instead, our brains play a crucial role in regulating how hard we can push ourselves.

• Our brains integrate signals from all over the body to regulate endurance.
• We don't simply stop when we run out of fuel.
• Our brains shut us down before we cause serious damage.

The role of the gut. The maximum amount of energy our bodies can absorb is limited by our digestive capacity. Even when we're eating a lot, our bodies can only process so many calories per day.

• The body can only absorb about 2.5 times BMR per day.
• This limit is a key factor in determining endurance.
• Our guts, not just our muscles, limit our performance.

Metabolic trade-offs. The body prioritizes essential functions, like brain function and reproduction, over nonessential ones, like immune response and stress reactivity. This means that when we push our bodies to the limit, we may experience a decline in other areas of our health.

8. External Energy Use Defines Modern Human Life

Like the Earth’s tectonic plates, metabolism is the unseen foundation underlying everything, slowly shifting and shaping our lives.

Beyond internal metabolism. Humans are unique in our ability to harness external energy sources, from fire to fossil fuels. This has allowed us to transform our world and achieve unprecedented levels of productivity and comfort.

• Fire was the first major external energy source.
• Domestication of plants and animals increased our food supply.
• Fossil fuels have powered the Industrial Revolution and modern life.

Dependence on external energy. Our modern lifestyles are completely dependent on our ability to access and use external energy sources. This dependence has made us vulnerable to the depletion of fossil fuels and the consequences of climate change.

• We consume far more energy than our bodies produce.
• Our food systems are heavily reliant on fossil fuels.
• Our transportation and infrastructure depend on external energy.

The need for sustainability. To ensure our long-term survival, we need to transition to a more sustainable energy economy, one that relies on renewable sources and reduces our dependence on fossil fuels. We also need to be mindful of the ways that our modern lifestyles are affecting our health and well-being.

Last updated: April 7, 2025