Substrate & Time to Exhaustion Calculator
During running, your body burns a mix of carbohydrates and fat for fuel. The ratio depends on exercise intensity: at easy paces fat dominates, while at race pace carbohydrate oxidation provides the majority of energy. Since glycogen stores are limited (400–600 g in muscle, 90–120 g in liver), marathoners risk "hitting the wall" when glycogen runs out. This tool models your fuel burn rate at any pace, predicts when glycogen depletion occurs, and shows how exogenous carbohydrate intake (gels, drinks) extends your time to exhaustion.
Frequently asked questions
How many carbs per hour should I take during a marathon?
Most runners should target 60–90 g/hr of carbohydrates during a marathon. Glucose-only absorption is capped at ~60 g/hr by the SGLT1 transporter; adding fructose (dual-transport) pushes the ceiling to 90–120 g/hr. Start with 60 g/hr and train your gut to tolerate higher rates.
What causes hitting the wall in a marathon?
Hitting the wall occurs when muscle glycogen is depleted and the body must rely primarily on fat oxidation, which cannot sustain high-intensity running. At marathon pace, glycogen depletion typically occurs after 90–120 minutes without exogenous carbohydrate intake.
What is RER and why does it matter for racing?
RER (Respiratory Exchange Ratio) indicates your fuel mix: 0.70 = pure fat, 1.00 = pure carbohydrate. At marathon pace, RER is typically 0.85–0.92. A higher RER means you are burning glycogen faster and will deplete sooner. Better aerobic fitness lowers RER at the same pace.
Sources
- Jeukendrup (2011). Nutrition for endurance sports. Journal of Sports Sciences.
- Rapoport (2010). Metabolic factors limiting performance in marathon runners. PLoS Computational Biology.
- Areta & Hopkins (2018). Skeletal muscle glycogen content at rest and during endurance exercise. Sports Medicine.
Related glossary terms
- CHO Oxidation
- Fat Oxidation
- Glycogen
- RER (Respiratory Exchange Ratio)
- SGLT1
- Tlim (Time to Exhaustion)
- Dual Transport (Glc+Frc)