Most SA runners use energy gels wrong. They take them too late, take too few, or take the wrong kind for the distance. The result: bonking at km 30, GI distress, sugar crashes, and a finish-line that feels worse than it should. This is the practical "how energy gels work, and how to use them properly" guide we wish someone had given us before our first marathon.
What Energy Gels Actually Do
Your muscles store glycogen (the carbohydrate form of energy) in two places: your muscles themselves and your liver. Combined, that's roughly 1,800 to 2,000 calories of stored fuel for a fit runner. Sounds like a lot. It runs out around the 90-minute mark of running.
That's the wall. When you've burned through stored glycogen and you haven't replaced it, your body switches to burning fat for fuel. Fat burns slower and gives you less power per unit of effort. Your pace drops, your legs feel heavy, your brain gets foggy. Welcome to bonking.
Energy gels prevent this by feeding fast-absorbing carbohydrate into your bloodstream while you run. Done right, your blood glucose stays stable, your muscles keep getting fuel, and you finish strong. Done wrong, you get a sugar spike followed by a crash, GI distress from concentrated sweetness, or simply not enough fuel to matter.
How Energy Gels Are Absorbed
The science is simpler than the marketing makes it sound:
You take a gel. The carbs hit your stomach. Your stomach passes them to your small intestine. The small intestine absorbs them into the bloodstream through specific transport proteins. Within 10 to 15 minutes, blood glucose rises and your muscles can use it.
The catch: those transport proteins have a maximum throughput. For glucose-only gels, the limit is around 60g of carbs per hour. Push past that and the excess sits in your gut, causing the bloating, cramping and runner's trots that everyone talks about.
Modern dual-carb gels (glucose plus fructose) use two different transport pathways. By layering them, the absorption ceiling rises to 90g or even 120g of carbs per hour. The SIS Beta Fuel at a 1:0.8 maltodextrin-to-fructose ratio is the SA-available example of this dual-carb approach.
For deeper science on which gel suits which distance, see our best carb gel guide.
When Should You Take Your First Gel?
The classic mistake: waiting until you feel tired.
By the time you feel the bonk coming on, your blood glucose has already dropped, your muscles are running low on glycogen, and a single gel won't reverse the trend fast enough.
Take the first gel before you need it. The right window:
Half-marathon: first gel at 8 to 10km (about 45 minutes in).
Marathon: first gel at 30 minutes, then every 30 to 45 minutes after.
Comrades or ultra: first gel at 45 minutes. Then continuous from there until the back end of the race.
Trail or multi-stage: first gel by the 60-minute mark, even if you've eaten before the start.
The principle: glycogen replacement during exercise is much more efficient than waiting and trying to recover. Drip-feed early, drip-feed often.
How Many Gels Do You Actually Need?
The maths is simpler than you might expect. Aim for around 60g of carbs per hour for moderate efforts and 60 to 90g per hour for race-pace efforts.
A standard gel delivers 22 to 25g of carbs. A high-carb gel like SIS Beta Fuel delivers 40g. A 100g serving like a flapjack or sports bar delivers around 30g.
Practical loading by distance:
Half-marathon (~2 hours): 2 to 3 standard gels total. One at 8km, one at 15km, optional one at 18km if you're racing hard.
Marathon (~4 hours): 5 to 8 gels. One every 30 to 45 minutes from the 30-minute mark. Mix gel types to avoid flavour fatigue.
Comrades (8 to 12 hours): 8 to 12 gels plus liquid carb fuel like Tailwind Endurance Fuel in your bottle. Plus real food (potatoes, bananas) at aid stations. The fuel-mix variety keeps your gut happy.
Trail or 50km+: mixed format. High5 Gummies when you need a chew, gels when you need a fast hit, real food when you need a mental break.
How to Take a Gel Without Feeling Sick
Six rules that fix nearly all gel-related GI problems:
1. Always take a gel with water. 150 to 200ml minimum. The exception: isotonic gels like the High5 Aqua are pre-diluted and don't need extra water.
2. Don't take gels too close together. 30 to 45 minutes apart for standard gels. Two gels in 10 minutes is a recipe for cramping.
3. Train your gut. Use gels in long training runs, not just on race day. Your gut adapts to higher carb intake the same way your legs adapt to longer distance.
4. Avoid stacking high-fructose gels. If you've taken one dual-carb gel (like SIS Beta Fuel), wait 45 minutes before the next one. Stack them too tight and the fructose load overwhelms your gut.
5. Match gel temperature to conditions. Warm gels in summer races sit heavier. Cold gels in winter races can shock your stomach. Room-temperature is forgiving.
6. Never try a new gel on race day. Test every gel during long training runs first. Race day is the worst possible time to discover that one specific brand makes you sick.
What About Caffeinated Gels?
Caffeine genuinely improves endurance performance. Multiple studies confirm a 2 to 5% improvement in time-trial performance with appropriate caffeine dosing. For a 4-hour marathon, that's 5 to 10 minutes faster.
The dose: 3 to 6mg of caffeine per kg of bodyweight, taken 45 to 60 minutes before a hard effort or strategically during a long event.
Most caffeinated gels deliver 50 to 100mg of caffeine. A higher-strength gel like SIS Beta Fuel +Nootropics delivers 200mg per serving, roughly equivalent to a strong coffee in one gel. For an 80kg runner, the optimal total dose is 240 to 480mg across a race day, which works out to one strong coffee plus one or two +Nootropics gels, or three to five lower-strength caffeinated gels spread over a marathon.
The classic strategy: caffeine-free gels for the first half, switch to caffeinated for the back end of long races. The caffeine boost arrives just as fatigue is setting in. Of the SIS range RBR stocks, only the SIS Beta Fuel +Nootropics gel is caffeinated. The plain SIS Beta Fuel has no caffeine, so use it for the first half of your race and switch to the +Nootropics variant in the back end. SIS does sell other caffeinated gel options that RBR doesn't currently stock.
Don't go crazy on caffeine. Stack 5 caffeinated gels with morning coffee and you can push your heart rate uncomfortably high while dehydrated. 200 to 400mg total for the day is the safe sweet spot for most runners.
Common Gel Mistakes SA Runners Make
The patterns we see week after week:
Waiting until they feel tired. Already too late. Take the first gel before you need it.
Taking only 2 gels for a marathon. Massively under-fuelled. You'll feel it from km 28 onwards.
Not drinking water with gels. Concentrated carbs without fluid sit heavy and slow absorption. Always pair with water.
Trying a new gel on race morning. Test in training first. Always.
Buying the lowest-priced gels available. Sometimes fine, sometimes the GI nightmare you didn't plan for. We've tested most SA-available options. The honest comparison is in our best energy gels for running in South Africa guide.
Stacking too many in a short period. Two gels in 15 minutes can cause GI distress. Keep them 30 to 45 minutes apart.
Building Your Gel Strategy for Race Day
The night before:
Lay out exactly the gels you'll take, in the order you'll take them. Number them on the wrapper if you're a planner. The last thing you want at km 25 is to be fishing in a race belt for the right one.
Race morning:
Pre-race breakfast 3 hours before the gun. One gel 15 minutes before the start (some prefer this, some don't). Your first race gel at the time you've practised in training.
During the race:
Stick to the schedule, not how you feel. The body is bad at signalling fuel needs in real-time during exercise.
For Comrades, Two Oceans and ultras specifically, our Comrades nutrition plan has the full carb-per-hour breakdown for sub-10, 10-11 and slower finishes.
Where to Start If You're New to Gels
Pick one gel and use it consistently in training for two months. Don't sample a different brand every week. Find what works for your stomach, then stick with it.
Three good starting options:
32Gi Sports Gel (R32): the gentlest on SA stomachs, low-GI, vegan. Best for sensitive stomachs and entry-level runners.
High5 Energy Gel (R38): thinner texture, easier to swallow at pace, mango flavour that's actually good. Best all-rounder for half-marathon and marathon training.
SIS Beta Fuel (R75): dual-carb, 40g per gel, race-day specialist. Use for serious marathon racing and Comrades.
Test each in training. By race day you'll know exactly which ones sit well and which ones don't. Browse the full gel range and start the gut-training process now if your next race is more than 8 weeks away.