Most runners do not blow up because they are unfit. They blow up because they ran out of fuel. You feel strong, the pace is comfortable, and then somewhere past the 30km mark the lights go out. Legs turn to concrete. The pace you held easily now feels impossible. That is the wall, and the good news is it is largely avoidable. Fuelling well is a skill, not luck, and you can learn it.
This is a practical guide to fuelling long runs and races. No hype, just what works in the field.
Why you hit the wall
Your body stores carbohydrate as glycogen in your muscles and liver. It is your fast, easy fuel. The problem is you do not have much of it. Most runners hold somewhere around 90 minutes to two hours of glycogen at a steady race effort. Run harder and you burn through it faster.
When glycogen runs low, your body shifts to burning fat. Fat is a huge fuel tank, but it burns slowly and cannot keep up with race pace. That mismatch is the bonk. The fix is simple in principle. Top up carbohydrate from outside before the tank runs dry, so your body never has to make that desperate switch mid-race.
The core rule: carbs per hour
Forget fuelling by feel. Fuel by the clock and by a target. The research here is settled and it is the foundation of everything else.
| Effort | Carbs per hour | Notes |
|---|---|---|
| Under 60 minutes | Little to none | Your stored glycogen covers it. Water is usually enough. |
| 60 to 90 minutes | 30 to 60g | Start fuelling. This is where most runners under-eat. |
| 2 to 3 hours | 60 to 90g | Aim higher. Use dual-carb products to absorb more. |
| 3 hours and beyond | 60 to 90g, sustained | Consistency matters more than peaks. Keep it coming. |
That upper end, 90g per hour, is only possible because of a trick called dual-carb fuelling. Your gut can only absorb about 60g of glucose per hour through one pathway. But fructose uses a different pathway. Combine glucose and fructose, usually around a 2:1 or 1:0.8 ratio, and you can push absorption higher without flooding your stomach. Most modern energy gels and carb drinks are built around this. Check the label for both sugars if you are going long or fast.
One practical note. A typical gel holds around 22 to 25g of carbs. So one gel every 30 to 40 minutes lands you in the 40 to 50g per hour range. To hit the higher numbers you will need to combine gels with a carb drink or chews.
When to take your gels
This is where good intentions fall apart. Runners carry the gels, then wait until they feel tired to take one. By then it is too late. A gel takes 10 to 15 minutes to hit your bloodstream. If you wait for the bonk, you are fuelling the recovery, not the run.
The rule is simple. Fuel before you need it, never after.
- First gel at around 45 minutes. You feel fine. Take it anyway. You are topping up while the tank is still full.
- Then every 30 to 40 minutes after that. Set a watch alarm if you have to. Do not rely on feel.
- Do not front-load. Taking three gels in the first half hour does not bank fuel. It just upsets your stomach. Spread it evenly.
How much you actually need by distance
Here is a rough guide to plan around. Adjust for your pace and how long you will be out there. A four-hour marathon needs more total fuel than a three-hour one.
| Distance | Rough time | Fuel plan |
|---|---|---|
| 10km | Under an hour for most | Usually nothing needed. Maybe one gel if you are slower or it is hot. |
| Half marathon | 1h45 to 2h30 | 2 to 4 gels, plus electrolytes in the heat. |
| Marathon | 3h30 to 5h | 5 to 9 gels, or a mix of gels, chews and drink. Add caffeine late. |
| Ultra / long trail | 5h plus | Keep fuelling every 30 to 40 min. Mix sources. Include real food and salt. |
If you want exact numbers for your event, weight and finish time, our nutrition calculator does the maths for you.
Take gels with water, not sports drink
This one trips up a lot of people. When you take a gel, wash it down with a mouthful or two of plain water. Not sports drink.
The reason is concentration. A gel is already a concentrated dose of carbohydrate. If you chase it with a sugary sports drink, you are dumping two big carb hits into your stomach at once. That high concentration pulls water into your gut, which is exactly how you end up with cramping, bloating and that sloshing feeling. Plain water dilutes the gel to the right concentration so it empties from your stomach and absorbs properly. Save the carb drink for steady sipping between gels, not on top of them.
Caffeine: save it for the back half
Caffeine genuinely works. It blunts perceived effort and sharpens focus when you are tired. But timing matters, and more is not better.
Take your caffeine in the second half of the effort, when the fatigue is real and you need the lift. Hitting it early wastes the effect, since you do not need help in the first hour, and it can leave you jittery. One or two caffeine gels, well timed, is plenty for most runners. Do not stack caffeine gels back to back. And if caffeine usually upsets your stomach in daily life, test it in training before you trust it on race day.
Electrolytes and sodium, especially in our heat
Gels handle your energy. They do not handle your salt. In a South African summer, or a coastal race in humidity, you sweat hard and you lose a lot of sodium. Replace carbs only and ignore sodium, and you set yourself up for cramps, fading energy and in bad cases hyponatremia from drinking plain water without salt.
For anything over about 90 minutes, especially in heat, add an electrolyte source. That might be electrolyte tablets in a bottle, a carb drink that already includes sodium, or salt capsules on longer efforts. As a rough target, aim for somewhere around 300 to 700mg of sodium per hour, leaning higher if you are a heavy or salty sweater. You can usually see it as white salt marks on your kit after a hot run. Our range of electrolytes covers both tablets and drink mixes.
Train your gut before race day
Here is the rule that saves more races than any other. Never debut a new product on race day.
Your gut is trainable. Practise taking gels and drinks at race intensity during your long runs, and your stomach adapts to processing fuel while you run. Skip that practice and try to slam 60g an hour cold on race morning, and your gut rebels. Use your long training runs to dial in exactly which products sit well, how often you can take them, and how much water you need with each one. By race day your fuelling plan should be boring and rehearsed, not an experiment.
Not sure which products to practise with? Our companion guide on the best energy gels for running in South Africa walks through the options we stock.
The most common fuelling mistakes
If you take nothing else from this, take these. They are the errors we see again and again.
- Waiting until you feel tired to fuel. Too late. Start at 45 minutes and stay ahead of it.
- Under-fuelling. One or two gels across a marathon is not enough. Hit your carbs per hour.
- Taking gels with sports drink. Double dose, unhappy stomach. Use plain water.
- Ignoring sodium. Carbs alone will not save your legs from cramps in the heat.
- Front-loading caffeine. Save it for when you actually need the lift.
- Trying new products on race day. The fastest way to a portaloo stop.
Where to start
Fuelling well is not complicated. Start your first gel around 45 minutes, take one every 30 to 40 minutes after that with a sip of water, add electrolytes in the heat, and practise the whole thing in training. Get those basics right and the wall stops being a thing that happens to you.
If you want a fuelling plan built around your exact race and pace, run the numbers through our nutrition calculator. When you know what you need, you can browse our energy gels and stock up to start training your gut. See you out there.
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