There are a lot of misunderstandings in sports nutrition. In the social media era, more people simply repeat what they hear without critically examining the science behind it.
Let’s get the facts straight:
The body uses adenosine triphosphate (ATP) as its primary form of cellular energy. ATP is not directly consumed from food but is produced inside the body’s cells through metabolic pathways that convert macronutrients—carbohydrates, fats, and proteins—into ATP.
These macronutrients differ in how efficiently and quickly they can be converted to ATP, and their energy potential is measured in kilocalories (kcal):
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Carbohydrates provide ~4 kcal/gram
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Proteins provide ~4 kcal/gram
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Fats provide ~9 kcal/gram
What you look for in endurance sports is energy—not just carbs. Brands that tell you to consume a set number of grams of carbohydrates per hour are oversimplifying the complexity of human metabolism.
Each energy source has its own metabolic pathway, advantages, and limitations.
ATP Production Pathways:
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Carbohydrates are broken down into glucose, which is used in glycolysis and then either oxidized in the mitochondria (aerobic) or converted to lactate (anaerobic), depending on oxygen availability.
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Fats (fatty acids) undergo beta-oxidation in the mitochondria to produce acetyl-CoA, which enters the TCA cycle to generate ATP. In low-insulin states (e.g., fasting, low-carb, or ketogenic diets), excess acetyl-CoA is converted into ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone). These ketones travel through the bloodstream to fuel the brain, heart, and muscles.
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Proteins are generally spared but can be used for energy in extreme situations. Amino acids are deaminated and their carbon skeletons are used in gluconeogenesis or enter the TCA cycle directly.
The fuel mix your body uses during exercise depends on intensity, duration, diet, and metabolic state.
For most athletes on high-carb diets, the crossover point—where carbohydrate oxidation surpasses fat oxidation—occurs around 60–70% of VO₂max. This point can be identified via respiratory exchange ratio (RER) or ventilatory threshold testing.
Below this threshold, fat is the primary fuel. As intensity increases, the body leans more on carbohydrates due to their rapid availability. However, this crossover point is not fixed and can shift dramatically with:
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Diet (high-fat vs. high-carb)
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Training adaptation (especially in fat-adapted athletes)
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Glycogen availability (fasted vs. fed states)
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Insulin levels (which suppress fat oxidation)
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Mitochondrial density and metabolic flexibility
So, you don’t just burn more sugar because you had a gel 40 minutes ago—your fuel utilization is dictated by long-term metabolic conditioning, not momentary sugar intake.
What the Research Shows:
In the Faster Study, ultra-endurance athletes following a low-carb, high-fat diet had significantly higher fat oxidation rates and a crossover point closer to 70–75% VO₂max. This suggests that both diet and training can meaningfully influence your metabolic efficiency.
Historically, fats were considered a "slow" fuel because early studies looked at sedentary individuals who only oxidized fat at 0.2–0.31 g/min. But this doesn’t hold up under modern scrutiny.
Recent research shows that trained athletes can reach fat oxidation rates near 1.0 g/min, while fat-adapted endurance athletes have reached between 1.5 and 1.85 g/min, even during high-intensity sessions.
At these rates, a 70 kg athlete could theoretically complete a sub-3-hour marathon (~42 km at ~4:15 min/km) relying almost entirely on fat, with minimal or no carbohydrate supplementation.
This completely overturns the idea that fat is a "slow" or inadequate fuel source. When properly optimized, fat is both efficient and sustainable, even under competitive conditions.
BSE: Fueling Performance Through Fat Adaptation
At BSE, we are a fat-based nutrition company focused on metabolic health as much as athletic performance. Our products are built on real, whole-food ingredients without additives or artificial sweeteners, tailored for long-term health and endurance.
While carbohydrates may play a role in short bursts of high-intensity activity or race-day peaks, relying on sugar during daily Zone 2 training undermines the goal of enhancing fat oxidation and mitochondrial efficiency.
If 80% of your training is in Zone 2, your fuel should support fat metabolism, not override it.
Also, avoid consuming carb-heavy products just before a race. Doing so spikes insulin, impairs fat burning, and can lead to an energy crash. Instead, start your race with one of our nut-based blends, which promote satiety, stable energy, and controlled insulin response.
Train your metabolism the same way you train your muscles: with intent, consistency, and science.
Metabolic characteristics of keto-adapted ultra-endurance runners