Carb loading is the single highest-leverage thing you can do in race week before an ultra. This guide walks through the 1-2 day glycogen-loading protocol our nutritionists use, the 8-12g/kg target, the low-fibre staples that prevent GI distress, hydration timing, and the mistakes most ultra runners make.

This guide is part of our Ultra-Trail Snowdonia fuelling master series — read the master guide first for the full UTS protocol, then come back here for the deep dive on this single piece.

Key points

  • Carb load for 2 to 3 days before an ultra.
  • Aim for 8 to 12 grams of carbs per kg of body weight per day.
  • Low-fibre, easy-to-digest sources: white rice, pasta with simple sauces, white bread, bagels, white potatoes, bananas, honey, and sports drinks.
  • A 1 to 2 kg gain on the scale is normal and expected.
  • Yes - over 12 g per kg per day adds GI distress without adding usable glycogen.

Why carb loading matters for ultras

Ultras live or die on glycogen. Muscle glycogen is the body's preferred fuel at race pace, and once it's depleted, performance falls off a cliff. Untrained muscles store roughly 400 to 500 g of glycogen — enough for 90 to 120 minutes of high-intensity work. Carb-loaded muscles store 600 to 800 g — enough to push the wall back by an extra 60 to 90 minutes.

For UTS, that extra 60 to 90 minutes is the difference between cresting the final climb on full power and hiking it in survival mode. Carb loading is the single highest-leverage nutrition decision of race week.

How many carbs per kg per day

Aim for 8 to 12 grams of carbs per kilogram of body weight per day across the loading window. A 70 kg runner targets 560 to 840 g daily. A 60 kg runner: 480 to 720 g. A 80 kg runner: 640 to 960 g.

That sounds enormous on paper. In practice it requires steady portion sizes throughout the day — breakfast, mid-morning snack, lunch, mid-afternoon snack, dinner, and a small carb snack before bed. Three giant plates is uncomfortable and slower to digest.

The 1-2 day protocol

48 hours out: drop fibre. White bread instead of wholegrain, white rice instead of brown, peeled potatoes. Add a STYRKR MIX+ drink mix once or twice a day to top up carbs without filling the stomach.

24 hours out: stick to fully refined, well-tested carbs. Pasta with tomato sauce for the night-before dinner. White toast and honey for breakfast. No surprises.

Best foods for ultra carb loading

The carb-loading staples our nutritionists recommend:

  • Rice — white, basmati, jasmine
  • Pasta with simple tomato or olive oil sauces
  • White bread, bagels, English muffins
  • Potatoes — white, peeled
  • Bananas, white-fleshed fruits
  • Honey, jam, maple syrup
  • Sports drinks and drink mix — useful for hitting daily targets without filling the stomach

Foods to avoid in the 48 hours before race day: high-fibre cereals, large salads, lentils, beans, fatty meats, cream sauces, alcohol, and anything you have not eaten before.

Hydration during the carb load

Glycogen storage is fluid-dependent. Every gram of stored glycogen binds 2.7 to 4 grams of water. Without enough fluid, you can't fully load — and the load you do achieve is unstable.

Increase fluid intake by 500 ml to 1 L per day during the load. Pale-yellow urine within 4 hours of waking signals proper hydration. Add 300 to 700 mg of sodium per litre of fluid via electrolyte tabs to help retain the water you take in.

Avoiding common carb-loading mistakes

Three patterns trip athletes up most often:

  1. Adding fat alongside carbs. Cream sauces, fatty meats and rich desserts slow gastric emptying. Keep meals carb-dominant and lean.
  2. Skipping fibre too late. If you eat your usual high-fibre cereal 24 hours before the start, expect a difficult race morning. Switch to refined carbs at the 48-hour mark.
  3. Trying new foods. Race week is not the time to test out the hotel breakfast buffet. Stick to staples you've eaten before long runs.

Final word

Carb loading is one of the few endurance nutrition interventions where the research has converged for decades. Hit 8 to 12 g per kg per day for 2 to 3 days, use low-fibre sources, drink fluid alongside, accept the water-weight gain, and arrive at the start line with the extra hour of fuel that makes the difference.

This guide pairs with our Ultra-Trail Snowdonia fuelling master guide — the carb load is one piece of the larger UTS protocol that covers race-morning fuelling, in-race carb targets, sodium loading and crewing.

Related deep dives in this UTS series

Read the master overview at How to Fuel for UTS Snowdonia: An Ultra Trail Guide.