Carb Loading — Topping Off for Long Races

By Runsense · Reviewed by Raphael Crawford-Marks, Founder · Last reviewed June 9, 2026

Carb loading means topping off your muscles' stored carbohydrate (glycogen) in the days before a long race, so you start with a fuller tank. It's one of the best-supported pre-race nutrition strategies — and it's simpler than the week-long rituals of the past.

What it does

Your muscles store only a limited amount of glycogen, and running it down is what causes the late-race "wall." Loading raises how much you bank beforehand, extending the distance you can run before fuel becomes the limiter. It pairs with — but doesn't replace — fueling during the race itself.

Who needs it

Carb loading earns its keep for hard efforts lasting roughly two hours or more — the marathon and longer, and many long-course events. For a 5K, 10K, or even most half marathons, normal good eating is plenty; you won't run the tank dry in under ~90 minutes, so there's little to gain from loading.

How to do it

You don't need the old "depletion" phase. Muscle glycogen can be nearly maxed out with about a single day of very high carbohydrate intake while resting1 — convenient, since you're tapering and training little anyway. In practice, shift your last day or two heavily toward carbohydrate (the supercompensation target is roughly 10–12 g per kg of body weight per day in that window)2, leaning on easy-to-digest foods you tolerate well.

Common mistakes

Three traps: (1) eating *more* total food rather than shifting *toward* carbs — you want a higher carb proportion, not a feast. (2) Trying new or very high-fiber foods that upset your stomach on race morning — rehearse your pre-race meals in training. (3) Mistaking the temporary weight bump for fat: glycogen binds water, so a couple of pounds up is the tank filling, not a problem.

Sources

  1. Bussau VA, et al. Carbohydrate loading in human muscle: an improved 1 day protocol. European Journal of Applied Physiology 87(3):290-295 (2002). (Endurance-trained male athletes (1-day carb-loading protocol)) ↩
  2. Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics 116(3):501-528 (2016). (Joint position stand on sports nutrition) ↩

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