👉 Higher TSS = greater overall demand

But not all training stress is equal. An easy aerobic ride and a harder run can contribute similar training load… yet place very different demands on your fuel systems.

👉 The higher the intensity and mechanical load → The more your body relies on carbohydrates (glycogen)

👉 It's training with significant and varied carbohydrate demand

This week is a good example of what most endurance athletes are actually doing — not purely aerobic work, but a mix of different metabolic demands across sessions.

The bike made up the largest portion of total load and was mostly aerobic. This means a greater reliance on fat oxidation — but still with meaningful carbohydrate use, especially as duration increases.

The run, however, shifts that balance. Even at moderate intensities, running has a higher cost due to more muscle recruitment, greater impact forces, and more eccentric loading.

👉 We are not operating in one energy system → We're constantly shifting between them

👉 Glycogen becomes the limiting factor — not total calories

Fueling isn't just about total intake — it's about matching the type of fuel to the type of work being performed.

Lower intensity work relies more heavily on fat oxidation, but still requires carbohydrates. As intensity increases, the body shifts toward greater carbohydrate use because it can produce energy more quickly.

👉 Performance is limited by fuel availability — not just calories

Total caloric intake alone does not determine performance. Substrate availability is far more relevant.

Carbohydrate availability plays a central role in sustaining performance. Glycogen is the primary substrate supporting moderate-to-high intensity exercise.

Energy production is governed by overlapping metabolic pathways. The aerobic system supports sustained energy production and utilizes both fat and carbohydrate. The glycolytic system provides rapid ATP production through glycogen breakdown, supporting higher intensity efforts.

As intensity increases, carbohydrate becomes dominant due to its ability to support higher ATP production rates.

The idea that low-intensity training is fueled almost entirely by fat is an oversimplification. Even in Zone 1-2, carbohydrate can contribute 30–50% of total energy production.

Glycogen usage is continuous. Over long sessions or repeated daily training, even modest carbohydrate utilization accumulates into meaningful glycogen depletion.

Higher intensity work depends on glycogen for rapid energy production. Running increases carbohydrate demand due to greater muscle recruitment, higher impact forces, and greater neuromuscular stress.

These sessions drive significant glycogen depletion in a relatively short time.

Even during low-intensity exercise, carbohydrate plays a critical role. The brain relies on glucose for function, and muscles continue using glycogen to support contraction.

When carbohydrate availability is low, both physical and cognitive performance are affected.

Stored in: Muscles and Liver

Glycogen is the storage form of glucose and the most readily available source for energy during exercise. Muscle glycogen is used locally within working muscles, while liver glycogen maintains blood glucose levels.

During exercise, muscle glycogen is progressively depleted, particularly in actively recruited fibers.

Even under optimal conditions, glycogen storage capacity is relatively small compared to total energy expenditure during endurance training.

For athletes training daily, glycogen must be restored repeatedly. This makes carbohydrate intake a critical determinant of performance.

Fueling is not about isolated sessions—it's about maintaining availability over time. Each session creates both fatigue and glycogen depletion.

If carbohydrate intake is slightly below requirement, that deficit carries forward. Over several days, these small deficits accumulate.

Each day represents a balance between glycogen depletion during exercise and restoration through diet. When intake is slightly below requirement, a small deficit remains.

Over multiple days, these deficits accumulate. Glycogen stores remain in partial depletion—not producing immediate failure but gradually reducing performance capacity.

Most athletes operate in a subclinical state of low glycogen availability. Heart rate increases at given workloads due to greater physiological strain.

These changes are often interpreted as normal fatigue but are directly linked to fuel availability.

At surface level, this intake appears appropriate for a high-volume endurance athlete. However, when contextualized against the actual metabolic demand of a 1311 TSS week, limitations become apparent.

The key issue is not whether intake is "good" in absolute terms, but whether it's sufficient relative to glycogen turnover across consecutive days.

When carbohydrate intake is consistently 50–150g below optimal, the impact isn't immediately obvious. However, over multiple days, this deficit compounds.

Protein intake exceeding ~2.2 g/kg doesn't provide additional recovery benefit. More importantly, this excess occupies caloric space that could be allocated to carbohydrate.

On this day, total intake appears high, but protein is significantly elevated relative to requirements. Additional protein doesn't provide further recovery benefit.

Even though carbohydrate intake is relatively high in absolute terms, it may still fall short when matched against training demand.

Fat intake is elevated while carbohydrate intake drops. Because total caloric intake has limits, higher fat often leads to reduced carbohydrate intake.

Over time, this contributes to incomplete glycogen restoration and reduced performance capacity.

Day-to-day variability in carbohydrate intake leads to inconsistent glycogen restoration. Glycogen stores remain in partial depletion—not producing immediate failure but gradually reducing performance capacity.

Without full restoration, each session is performed at slightly reduced capacity, limiting performance and adaptation.

This is a critical distinction: the difference between energy availability and fuel availability.

I can consume sufficient calories to maintain energy balance, yet experience reduced performance due to inadequate carbohydrate availability.

This is often misinterpreted as fatigue, lack of fitness, or overtraining. In reality, it's a fueling mismatch.

Carbohydrates serve multiple roles beyond energy provision. At the muscular level, they support glycolysis and rapid ATP production essential for moderate-to-high intensity efforts.

In multi-day training, carbohydrates are essential for glycogen restoration and consistency.

Fat is important, particularly for lower intensity exercise. However, it cannot support rapid energy production or replenish glycogen stores.

Relying too heavily on fat at carbohydrate's expense impairs both performance and recovery.

Protein intake beyond physiological requirements doesn't enhance muscle protein synthesis or recovery. More importantly, it reduces carbohydrate that can be consumed within total caloric intake.

For a 1311 TSS week, carbohydrate requirements fall into upper recommended ranges—typically 8–12+ g/kg.

Ensuring glycogen is fully restored requires sufficient carbohydrate plus appropriate daily distribution around training.

The effects of inadequate carbohydrate intake are cumulative. Each day of insufficient intake contributes to reduced glycogen stores.

Performance declines gradually, not suddenly.

These are real-world signs of suboptimal fueling. Performance doesn't collapse—it's consistently below potential.

Sessions feel harder than they should. This reduces both training quality and adaptation over time.

These physiological principles apply across all training loads and athlete levels. Glycogen availability applies across all endurance sports and experience levels.

Carbohydrate requirements increase proportionally to training demand. High-load weeks require upper-range carbohydrate intake to maintain performance and recovery.

Fueling should be dynamic, adjusting to daily demands. This approach allows better alignment between intake and demand, improving both performance and recovery.

Fuel timing plays a critical role in managing glycogen availability. Starting sessions with adequate glycogen improves performance and reduces early fatigue.

Post-exercise intake is equally important. Rapid carbohydrate intake supports glycogen resynthesis for the next session.