Smart Fueling Calculator

Your muscles and liver store enough glycogen for well over 45 minutes of running, so extra carbs don't improve a run that short. Between 45 and 75 minutes they only help at hard effort (>75% VO₂max), where a small 20–30g intake or a sports-drink mouth rinse can help performance via neural signaling.

The intestine has separate transporters for glucose and fructose (SGLT1 and GLUT5). Using them simultaneously, roughly in a 2:1 ratio (e.g., maltodextrin + fructose), allows absorption up to 90g/hr without GI distress. Single-source carbs max out at ~60g/hr due to transporter saturation. This is the Jeukendrup (2011) multiple-transportable-carbohydrate model. The calculator only goes above 60g/hr for tempo and hard efforts; easy long runs stay at 45–60g/hr.

Start with 30–40g/hr and increase by 10g/hr every 2–3 weeks of long runs. Practice fueling during training runs, never try a new strategy on race day. GI distress from carbs is often a trainability issue, not just individual tolerance.

Weigh yourself without clothes before and after a one-hour run, and note how much you drank. Every 1 kg lost is about 1 L of sweat, so sweat rate (ml per hour) = weight lost in grams + fluid drunk in ml. Repeat it in different weather. ACSM's goal is to drink enough that you lose no more than 2% of body weight, without drinking so much that you gain weight.