Original episode & show notes | Raw transcript
Welcome to this detailed exploration of metabolism and fueling for endurance athletes. Drawing from the provided podcast transcript, this guide aims to deconstruct prevalent myths and replace them with a robust, science-based understanding of how the human body manages energy. We will move beyond simplistic “fat vs. carbs” debates and delve into the more holistic and critical concept of total energy balance.
Our goal is to understand not just what happens, but why it happens from a physiological and evolutionary perspective. We will address three primary myths and, in doing so, build a more accurate model of energy expenditure, weight management, and performance fueling.
This is a common misconception, often rooted in the idea that since endurance exercise burns a mix of fats and carbohydrates, one only needs to worry about replenishing the carbohydrate (glycogen) stores. The reality is far more complex.
The most critical concept to grasp is that the body is an excellent energy accountant, not a substrate accountant. It is primarily concerned with the overall energy balance—the total calories in versus the total calories out—rather than meticulously tracking whether those calories came from fat or carbohydrates.
Let’s use the podcast’s powerful example to illustrate this:
Imagine a five-hour ride where you burn 3,500 kilojoules (which we can approximate as 3,500 kcal, or dietary calories).
Let’s say a metabolic cart shows that a significant portion of this energy came from fat stores.
Following the myth, you meticulously replace every single carbohydrate calorie you burned during the ride.
The result? You are still in a 3,500 kcal energy deficit. This is roughly the energy equivalent of one pound of body fat.
To put this in perspective, a sustainable and recommended rate of fat loss for an athlete is around 0.5 to 1.0 pounds per week. In this single five-hour ride, you have created a deficit that is 30 to 60 times greater than the recommended hourly rate for a sustainable diet.
When the body detects such a massive and rapid drop in energy availability, it doesn’t celebrate the “fat burn.” It perceives a threat—a state of famine. This triggers a cascade of protective, and ultimately performance-harming, responses:
Metabolic Disturbance: The body enters a state of alarm. Research shows that even when total daily calories are sufficient, spending prolonged periods within a day in a large deficit is associated with signs of metabolic disturbance and low energy availability (LEA).
Impaired Recovery: Recovery is an energy-intensive process. It involves repairing muscle tissue, synthesizing new proteins, and replenishing energy stores. A large deficit starves these processes of the energy they need, leading to poor recovery. Studies have shown that simply delaying a post-ride meal by a few hours can measurably impair performance the very next day.
Hormonal Disruption: The body begins to down-regulate non-essential functions to conserve energy. This includes suppressing key hormones like thyroid hormone (which controls metabolic rate), testosterone, and IGF-1 (both crucial for recovery and adaptation).
In short, by only replacing carbs, you place your body in a severe acute energy deficit. This is a state you want to avoid, as it compromises recovery, health, and the very training adaptations you’re working to achieve.
This myth is a direct consequence of the first. If the goal is to lose fat, it seems logical to maximize fat burning during exercise. However, as we’ve established, the body’s response to the resulting energy deficit makes this a counterproductive strategy.
It’s crucial to stop thinking of body fat (adipose tissue) as a passive storage depot. It is an active endocrine organ that communicates with the rest of the body, particularly the brain, about your energy status.
Leptin: This is a key hormone produced by fat cells. Its primary role is to signal satiety and energy abundance to the brain. When you are well-fed, leptin levels are high, telling your brain, “We’re full, energy stores are plentiful.”
Ghrelin: Conversely, ghrelin is the “hunger hormone,” primarily produced by the stomach, which signals an empty stomach and the need for energy.
When you induce a large, acute energy deficit (by, for example, not fueling a long ride adequately), your fat cells shrink, and leptin levels plummet. The brain interprets this as a sign of starvation, leading to:
Increased Hunger: Ghrelin and other hunger signals go into overdrive. This is why you might feel ravenously hungry for hours or even days after a poorly fueled epic ride.
Metabolic Slowdown: To conserve energy in this perceived famine, the body reduces its Basal Metabolic Rate (BMR). This is partly mediated by the drop in thyroid hormone. You may feel cold, lethargic, and mentally foggy as your body dials down its energy expenditure.
The famous Minnesota Starvation Experiment provides a stark, real-world example of these effects. Participants on a severe semi-starvation diet experienced dramatic drops in metabolic rate, became obsessed with food, and suffered from numerous psychological and physiological issues. While an extreme case, it illustrates the principles at play.
The takeaway is that a successful fat loss plan relies on a small, consistent, and sustainable energy deficit, not a massive, acute one. The goal is to coax the body into giving up its fat stores without triggering its powerful anti-starvation alarms.
This is perhaps the most common calculation error made by athletes. It drastically underestimates the true energy cost of a training lifestyle. To understand why, we need to break down Total Daily Energy Expenditure (TDEE).
TDEE is composed of three main components:
Basal Metabolic Rate (BMR): The energy required to keep you alive at rest (breathing, circulation, cell production). This is what online calculators attempt to estimate.
Thermic Effect of Food (TEF): The energy used to digest, absorb, and metabolize the food you eat. This typically accounts for about 10% of your total energy intake.
Activity Thermogenesis (AT): All the energy you expend through movement. This is further divided into:
Exercise Activity Thermogenesis (EAT): The energy burned during planned exercise. This is your “bike kilojoules.”
Non-Exercise Activity Thermogenesis (NEAT): The energy burned from all other daily activities—walking, fidgeting, standing, cooking, etc.
The equation BMR + Bike kJ completely ignores TEF and, critically, NEAT. For an active person, NEAT can be a substantial contributor to TDEE. Furthermore, research on elite athletes reveals another layer.
Studies using the gold-standard “doubly labeled water” method on Tour de France riders found that their energy expenditure, even after subtracting their on-bike work, was 1.6 to 2.3 times their predicted BMR. This suggests that the body’s metabolic rate is significantly elevated to support the immense stress and recovery demands of high-volume training, far beyond just the work done on the bike.
Therefore, your actual daily energy needs are significantly higher than a simple calculator would suggest. Under-fueling, even unintentionally, is a common result of using this flawed equation, leading to the same issues of poor recovery, LEA, and performance plateaus.
By dismantling these three myths, we can construct a more accurate and effective framework for thinking about fueling and metabolism:
Prioritize Total Energy Balance: Focus on matching your total daily energy expenditure with your intake. Don’t get lost in the weeds of “fat burning” vs. “carb burning.” The body needs energy, and carbohydrates are the most practical, efficient, and performance-enhancing fuel source during exercise.
Fuel Your Work (and Your Recovery): Fueling properly on the bike is not about “defeating the purpose” of the ride. It is about minimizing the acute energy deficit, enabling higher quality training, and kick-starting the recovery process so you can adapt and get stronger.
Recognize Your True Energy Needs: Understand that your energy needs extend far beyond your BMR and what your power meter reads. A training lifestyle is metabolically expensive. If you are constantly feeling fatigued, hungry, or seeing performance stagnate, under-fueling is a likely culprit.
Ultimately, the body is a complex, adaptive system that prioritizes survival. Working with its systems by providing adequate energy will always yield better results than trying to “trick” it with extreme deficits or flawed fueling strategies.