How do I fuel?

Your muscles and liver hold roughly 90 minutes to two hoursof hard running as stored carbohydrate (glycogen). Run a marathon and you’ll need far more than that. “The wall”, that mile-20 lurch where the pace falls off a cliff, is simply the moment the tank runs dry and your body is forced onto fat, which can’t release energy fast enough to hold race pace.

The fix isn’t to run slower. It’s to keep topping the tank up from the start, so you never reach empty. Get this right and the last 10K stops being a survival exercise. Get it wrong and no amount of training saves the day.

Fuelling advice has changed more in the last decade than in the thirty years before it. If you learned the old numbers, they’re holding you back.

Carbohydrate need is driven by time on your feet, not your bodyweight, and capped by what your gut can take. Pick the row that matches your finish time, then choose a target you’ve actually trained.

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:

Here’s the quiet secret of sports nutrition: race fuel is maltodextrin, fructose, salt and flavour. The science is in the ratio, not the packet. Both ingredients cost a few dollars a kilogram from any homebrew or bulk-food store, and a kitchen scale gets you the same numbers as the premium mixes:

Any calculator that tells you to drink exactly 721 ml an hour is guessing with a straight face. 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. That’s why the tool above shows a band for your conditions and bodyweight. Two things narrow it to you:

• The 60-minute sweat test. Weigh yourself undressed, run an hour at marathon effort without drinking, towel off, weigh again. Every kilogram lost is a litre an hour of sweat. Do it once in cool weather and once in warm and you know your own range better than any formula.
• Read your salt. White crust on your cap and kit, stinging eyes, a history of late-race cramps: signs you sit at the salty end. Fuel at the top of the sodium band; everyone else does fine in the middle.

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. That tops up muscle glycogen above its normal level and is worth a couple of percent on race day: a few free minutes.

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:

• Go white, not wholemeal. White rice, pasta, bread, bagels and potatoes. This is the one week to skip the fibre; it just fills you up and risks race-morning trouble.
• Drink some of it. Fruit juice and sports drinks hit the target with far less bloat than another plate of pasta.
• Five or six small feeds beat two big blowouts. Keep fat and heavy protein down on loading days, they slow you up and crowd out carbs.
• Nothing new.Race week is not the time to try a cuisine your gut hasn’t met. Hydrate well, glycogen is stored with water.

Big-city marathons put water and sports drink roughly every mile or two, twelve to fourteen-plus stations. Some place gels at set points (Boston and New York stock Maurten depots around miles 12–21), but supply runs out and brands vary. The rule: carry 70–80% of your own fuel and use the course for fluid.

• Take a gel just before a water station, not after. Gels are concentrated, the water right behind it helps it empty from your stomach and heads off nausea.
• Pinch the cup and walk a few steps. Fold the rim to a spout, take ten seconds, sip rather than gulp. The time lost is nothing; the fluid you actually keep down is everything.
• Don’t skip sodium. Water alone over four hours invites cramp, and, if you over-drink it, something more dangerous. Make sure your plan includes salt, from the sports drink or your own mix.
• Never try anything new on race day.If a station offers a gel you haven’t trained with, skip it. The only thing that’s always safe is water.