Carb Loading Calculator

The science behind carb loading

Your body stores roughly 500g of glycogen across muscles and liver. That's enough fuel for about 75-90 minutes of hard running. If your race is longer than that, you're going to dip into those stores, and when they run low, your body shifts to burning fat. Fat burns too slowly to hold race pace. That's the wall.

Carb loading delays the wall by supercompensating your glycogen stores before the gun goes off. Research shows it can improve marathon performance by roughly 3% (Burke et al. 2011). For a 3-hour marathoner, that's 5+ minutes.

The old protocol is dead

The 7-day depletion/loading protocol is outdated. Bussau et al. (2002) found that a single day of 10 g/kg with minimal exercise matched the old multi-day approach in trained athletes. No starving yourself. No depletion runs. Areta & Hopkins (2018) reviewed 181 muscle biopsy studies and confirmed that fitness level and carb intake are the two biggest predictors of glycogen storage.

The activation session

A short run or set of strides the morning before your race primes your muscles to absorb and store more fuel. Tim Podlogar, a sports nutrition researcher who advises Tour de France teams, uses this protocol with his athletes: a brief activation session in the morning, then aggressive carb intake the rest of the day.

Weight gain is normal

Water follows glycogen into your muscles. Expect 1-2 kg on the scale during loading. It's fuel, not fat. It disappears during the race. A single pasta dinner provides roughly 200g of carbs. A 70 kg runner targeting 10 g/kg needs 700g. That's an entire day of focused eating, not one meal.

For the full science on how your body absorbs and uses carbs during racing, read our guide to the science of race fueling.