Carbohydrates

In the first part, we established that insufficient energy availability is the surest ticket to stagnation and illness. We now know: dropping below 30 kcal per kg of fat-free mass (FFM) puts us in a dangerous gray zone, while 45 kcal/kg FFM is the target for optimal performance and health.

But once energy availability is secured, the immediate next question arises:
What should these calories consist of?

Anyone wanting to be successful in endurance sports cannot avoid the targeted management of carbohydrates, proteins, and fats. Consider them as tools with which we can "tune" our metabolism. In this section, we will focus on the most important tool for intensity: carbohydrates.

Carbohydrates: The High-Performance Fuel

Carbohydrates (CHO) are by far the most important energy source for high-intensity efforts. Why? Because they produce more ATP (energy) per liter of oxygen consumed than fats. They deliver energy much faster and more efficiently—a crucial factor as soon as you cross your threshold or need to counter attacks on a climb.

The Limitation: Your Tank is Finite

Your glycogen stores in the muscles and liver are limited (approx. 400–600 g, depending on training status and muscle mass). As they run low, your body's signals to reduce intensity grow stronger. The brain protects you from a total metabolic catastrophe (central nervous system fatigue). When "bonking" (or "hitting the wall") occurs, your body pulls the plug to secure vital processes (see also our series Fascination with the Limit).

In Practice: Fuel for the Work Required

Today, we manage carbohydrates according to the principle of "Fuel for the work required" (Impey et al., 2018).

This means:

• On easy days: Reduced intake to promote fat oxidation and mitochondrial biogenesis.

• On interval days: Massive intake to ensure the quality of the session.

• Competition Loading: In the 48 hours prior to an event, enormous amounts should be consumed: 8–10 g per kg of body weight (Bussau et al., 2002) so that the glycogen stores are completely filled.

The 90 g/h Myth – Intake vs. Oxidation

For a long time, the mark of approx. 90 g CHO/h was considered the absolute limit of human physiology, based on the guidelines by Asker Jeukendrup (2014). However, in recent years, the amounts consumed at the world elite level have practically exploded. A prominent example is the Norwegian triathlete Casper Stornes, who reportedly ingested up to 182 g/h during the Ironman World Championship in Nice. Whether these extreme numbers correspond 100% to reality or also serve as a marketing tool for sponsors remains to be seen—but practically speaking, many pros today are safely operating in the range of 120 g to 150 g/h.

The major catch: There are currently no robust studies proving that an intake of 120 g/h provides a clear performance advantage over 90 g/h. An interesting study by Podlogar et al. (2022) showed: increasing the intake beyond 90 g/h did lead to higher oxidation rates of externally supplied (exogenous) carbohydrates, but the body's own (endogenous) glycogen stores were not additionally protected by this. The physiological consequence: More energy in the form of carbohydrates is ingested and oxidized, but in return, the entire metabolism shifts so heavily into "sugar mode" that more carbohydrates are burned overall.

The subtle distinction between mere intake, actual oxidation, and the effect on the body's own stores is therefore elemental.

The Tip of the Iceberg (Or the Drop That Spills the Cup):
The Fructose-to-Glucose Ratio

So far, we have only talked about the total amount of carbohydrates. The final tuning measure to even be able to absorb amounts beyond the 90 g/h mark lies in the correct ratio of glucose to fructose. Only in this way can we utilize two different "doorways" in the gut:

• SGLT1 Transporter: This pathway for glucose is saturated at approx. 60 g/h. A biological bottleneck.

• GLUT-5 Transporter: This pathway is reserved for fructose.

By combining them (e.g., in a ratio of 1:0.8 or 2:1), we bypass the SGLT1 bottleneck. But caution is advised here: High amounts of fructose demand significantly more from the gastrointestinal tract, and tolerability is extremely individual. In cases of fructose sensitivity, a conservative 2:1 ratio is often the safer choice.

At lower intensities, and thus lower carbohydrate consumption, it makes sense not only to reduce the total amount but also to adjust the composition. Instead of using the "turbo fuel ratio" of 1:0.8, switching to pure glucose (or maltodextrin) can even be advantageous in certain situations like base training. You take advantage of the slower absorption, ensure a more stable blood sugar level (less insulin release), and thus specifically promote the actual goal of base training: fat metabolism training.

Regeneration Begins in the Gut

Carbohydrates are your most important "anti-stress supplement" in a race. High availability during exertion dampens the release of the stress hormone cortisol and reduces muscle damage (EIMD – Exercise-Induced Muscle Damage). The groundbreaking study by Viribay et al. (2020) on professional cyclists showed: athletes who ingested 120 g CHO/h had significantly lower markers for muscle damage (creatine kinase) and a faster recovery of heart rate variability (HRV) than the groups consuming 60 or 90 g/h. Conclusion: Even if 120 g/h might not directly increase pure power output in a race compared to 90 g/h, it catapults you much faster back into your next training session.

Summary

Carbohydrate requirements are highly dynamic. Depending on the workload, they range between 3 g and 12 g per kg of body weight (Burke et al., 2011). This is exactly where the greatest potential for your performance progress lies: Those who consistently and sensibly apply the "Fuel for the work required" principle in practice will harness the full power of this fascinating macronutrient without overloading their metabolism.

References

• Burke, L. M., et al. (2011). Carbohydrates for training and competition. Journal of Sports Sciences.

• Bussau, V. A., et al. (2002). Carbohydrate loading in human muscle: an improved 1-day protocol. European Journal of Applied Physiology.

• Horowitz, J. F., et al. (1997). Lipolysis and fat oxidation extremely sensitive to small increments in plasma insulin. American Journal of Physiology.

• Impey, S. G., et al. (2018). Fuel for the Work Required: A Theoretical Framework for Case-specific Carbohydrate Periodization. Sports Medicine.

• Jeukendrup, A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine.

• Podlogar, T., et al. (2022). Increased exogenous but unaltered endogenous carbohydrate oxidation with combined fructose-maltodextrin ingested at 120 g h-1 versus 90 g h-1 at different ratios. Medicine & Science in Sports & Exercise.

• Viribay, A., et al. (2020). Effects of 120 g/h of Carbohydrates Intake during a Mountain Marathon on Exercise-Induced Muscle Damage in Elite Runners. Nutrients.