How do I fuel?

Muscle and liver glycogen are finite. How long they support a given effort varies with starting stores, intensity, training status, pacing and carbohydrate taken during the race. A marathon is long enough for carbohydrate availability to matter for many runners.

“The wall” is not one simple switch. Pacing, muscle damage, heat, hydration, gastrointestinal tolerance and fuel availability can all contribute. A rehearsed carbohydrate plan can support performance, but it does not replace appropriate training or conservative early pacing.

Marathon fuelling has moved from one fixed ceiling to a range that depends on duration, carbohydrate type, gut tolerance and what the runner has rehearsed. More is not automatically better.

Duration informs broad sports-nutrition guidance, but finish time alone cannot show what your gut has been trained to tolerate. That is why the calculator returns 60 g/hr for every finish time when you choose the suggested option. Higher targets are explicit choices to make only after repeated, symptom-free rehearsal.

This is where targets become a race plan. A standard gel is about 22 grams; the “double” gels carry 40. Here’s what common marathon brands actually deliver:

A homemade drink can combine maltodextrin, fructose, salt and flavour. The table is a Clinic coaching recipe, not a universal prescription. Use a kitchen scale, check the sodium content of the salt you buy and rehearse both concentration and volume:

Any calculator that tells you to drink exactly 721 ml an hour is presenting a rough model as a measurement. Sweat rates between runners span half a litre to two-and-a-half litres an hour, and sweat sodium spans 200–2,000 mg per litre. The tool above uses conditions and bodyweight to show a rough Clinic starting band, not your sweat rate or a full-replacement target. These inputs can provide better individual context:

• The 60-minute sweat test. Weigh yourself privately before and after a representative steady run, account for anything you drink and any urine, and towel off before the second measurement. Do not deliberately dehydrate at marathon effort or perform the test alone in dangerous heat. Repeat in conditions you commonly race in. The result is still an estimate, but it is more useful than bodyweight and temperature alone.
• Read your salt. White crust on your cap and kit, stinging eyes and a measured high sweat-sodium result can suggest higher sodium loss. Cramps alone do not diagnose it. Use the calculator band as a starting point, then refine it with a sports dietitian or formal testing if losses are a concern.

The old “deplete yourself then load” ritual is gone. The current evidence is simpler and kinder: in the 1–2 days before the race, eat 8–12 grams of carbohydrate per kilogram of bodyweight a day while you taper. This can increase glycogen availability, but the individual performance effect is not guaranteed.

For a 70 kg runner that’s roughly 560–840 grams a day , more than it sounds. The trick is to do it without feeling stuffed:

• Keep fibre familiar. Do not cut fibre for a whole week. If you have rehearsed a lower-fibre approach and it helps your gut, use familiar white rice, pasta, bread or similar foods during the final one or two loading days only.
• Drink some of it. Fruit juice and sports drinks hit the target with far less bloat than another plate of pasta.
• Several smaller feeds are a Clinic coaching preference when large meals feel uncomfortable. Keep overall meals familiar and avoid abruptly restricting other nutrients.
• Nothing new. Race week is not the time to try a cuisine your gut hasn’t met. Hydrate well, glycogen is stored with water.

Aid-station spacing, products and availability vary by event and year, so check the current athlete guide. The Clinic coaching preference is to carry roughly 70–80% of your planned carbohydrate when practical and use verified on-course products for the rest. That percentage is a logistics buffer, not a physiological rule.

• One Clinic coaching option is a gel before a water station. Concentrated gels may be easier to tolerate with a small rehearsed amount of water. Do not force fluid if you are not thirsty.
• Pinch the cup and walk a few steps. Fold the rim to a spout, take a few controlled sips if your plan and thirst call for them, then move on.
• Use a rehearsed drink plan. Sodium can replace some sweat losses and improve palatability, but it is not a guarantee against cramp or hyponatraemia. The main safety rule is not to over-drink.
• Never try anything new on race day. If a station offers a gel you haven’t trained with, skip it. Water can also be harmful when it is forced in excess, so drink to thirst within a plan you have rehearsed and do not drink enough to gain body mass.