On-the-Run Fuelling: Gels, Hydration and Timing
Why Fuel Matters
Most marathoners have heard of “the wall”, it’s that point around mile 20 where your body runs out of stored energy and every step feels impossible, like a bear has jumped on your back, but it’s not because of a lack of fitness, it’s quite simply a fuelling problem.
Your body stores about 2,000 calories of glycogen (carbohydrate) and the majority of runners burn roughly 100 calories per mile and if you quickly do the maths it’s easy to see why you run out of energy just as you’re approaching the final 10k stretch.
The good news? You can delay the wall, or if you’re really savvy, you can avoid it altogether, you just need to be smart with fuelling and hydration.
Gels and Calories
The research is everychanging, practise what works for you because it’s not an exact science, 1 energy gel can give you anything from 30-60 grams of carbs per serving
1 energy gel (around 100 calories) every 40–60 minutes.
For a 4-hour marathon:
6 gels × 100 calories = 600 calories.
Add in an energy drink and you’ll easily top up another 400 calories.
Total: ~1,000 calories replaced — enough to keep you moving strong.
Yes, gels taste grim. But they’re light, easy to carry, and they work. The key is to find brands and flavours that sit well with you and then stick to them.
Hydration
Hydration is just as important as carbs because being dehydrated makes your heart work harder to pump blood around your body which can raise your effort level when your pace doesn’t change.
Basic guidelines:
Aim for 150–200ml every 20 minutes (roughly a few good gulps).
In warmer weather, you’ll need more.
Try out different types of sports drinks, some have added carbs and/or electrolytes and this can be counterproductive by increasing your thirst!
Read more on Electrolytes here
Tip: practise drinking on the run. It sounds simple, but it takes coordination to sip without choking or spilling half of it down your top!
Train Your Gut
GI distress (runner’s trots, nausea, cramps) is one of the leading causes of marathon DNFs. The good news is that your gut can be trained just like your muscles.
Start fuelling early in your long runs, don’t wait until you’re tired, a gel isn’t any good if it’s sat in your pocket.
Stick to the same gels/drinks you’ll use on race day.
Practise carrying fuel: belt, vest, or stash points.
If you know your stomach struggles, start small (half a gel at a time).
Don’t Try Anything New on Race Day
Marathons are stressful enough without gambling on a new gel or drink you’ve never tried. Use your long runs as dress rehearsals — same breakfast, same fuel, same kit.
Fuel little and often: 1 gel every 40–60 mins.
Drink regularly: don’t wait until you’re thirsty.
Train your gut: practise in training so there are no surprises.
Keep it familiar: no new foods or drinks on race day.
Do this, and the wall doesn’t have to be part of your marathon story.