Most runners nail their training paces and their sleep, then completely ignore what happens to their glycogen stores at kilometre 25 of a 32-kilometre long run. The research on endurance fueling is unusually clear: for any run lasting longer than 90 minutes, on-run carbohydrate intake is not optional — it is the difference between a quality training stimulus and a depleted shuffle home that takes three days to recover from. This guide covers the physiology behind bonking, the specific quantities and timing that prevent it, and how to practise your race-day fueling strategy on training runs so nothing surprises you on race morning.
Your muscles store carbohydrate as glycogen. At the start of a long run, a well-fueled athlete has roughly 400–500 grams of glycogen across muscle and liver — enough to power approximately 90–120 minutes of running at moderate intensity. When those stores run low, your body cannot sustain pace. Output drops sharply, legs feel concrete-heavy, and mental sharpness collapses. Runners call it hitting the wall, or bonking. Exercise scientists call it hypoglycaemia-mediated fatigue.
The key detail: glycogen stores are fixed regardless of your fitness level. A VDOT 60 runner and a VDOT 40 runner carry roughly the same glycogen. The faster runner burns through it more quickly in absolute terms because they are covering more distance per unit of time. Fitness does not solve the fueling problem — strategy does.
Fat stores are, technically, unlimited. Even a lean athlete carries enough body fat to run hundreds of kilometres. The problem is the rate at which fat can be converted to usable energy. At race-relevant intensities — anything above easy pace — fat oxidation alone cannot keep up with demand. Carbohydrates are still the limiting fuel. Training in fat-adapted states (fasted long runs) can improve fat oxidation slightly, but the research does not support skipping carbohydrates on long or race-intensity runs.
Below 90 minutes of running, glycogen stores are typically sufficient, provided you start the run reasonably fueled. A normal pre-run meal two to three hours before departure tops off muscle and liver glycogen, and you will cross the finish with reserves intact. Fueling during the run adds negligible benefit at this duration and may cause unnecessary GI distress if you have not practised it.
At the 90-minute mark, the calculus changes. Glycogen stores are beginning to deplete in earnest, and without exogenous carbohydrate input, performance will decline in the final third of the run. For any run planned at 90 minutes or longer — marathon-pace long runs, long easy runs, race day — plan your fueling before you step out the door.
The threshold shifts based on intensity. Running at marathon pace burns more carbohydrate per minute than an easy shuffle. A 90-minute marathon-pace run will hit glycogen limits faster than a 90-minute easy run. For higher-intensity long sessions, lower your threshold to 75 minutes and plan accordingly.
The current sports science consensus for endurance fueling sits at 30–60 grams of carbohydrate per hour during sustained effort. The lower end (30 g/hr) is appropriate for runs at easy to moderate intensity or for runners who are newer to on-run fueling and still building gut tolerance. The upper end (60 g/hr) is appropriate for marathon-pace efforts and experienced runners with trained guts.
Trained gut matters. The intestine has a limited capacity to absorb glucose — roughly 60 grams per hour for a single carbohydrate source. Exceed that and the unabsorbed sugar sits in the gut, draws in water, and causes cramping and nausea. Athletes who train their gut to absorb higher volumes — by consistently fueling on long runs — can push to 60 g/hr without GI problems. Athletes who race on gels they never practised with frequently experience GI distress at exactly the worst possible moment.
A second mechanism extends the ceiling. The gut also has separate transporters for fructose (a different sugar). Products that blend glucose and fructose — most modern gels use a 2:1 maltodextrin-to-fructose ratio — can push total absorption to 90 g/hr in well-trained athletes. Elite marathon runners routinely consume 80–90 g/hr on race day. For most recreational runners targeting sub-4 to sub-3 marathon, 45–60 g/hr is realistic and effective.
Quick reference: one standard running gel (25 g carbs) every 30–35 minutes from minute 40 onwards delivers roughly 45–50 g/hr — within the effective range for most runners.
The most common fueling mistake is waiting until you feel hungry or fatigued before taking a gel. By that point, blood glucose is already falling and glycogen is significantly depleted. It takes 15–20 minutes for carbohydrate consumed on the run to be digested and enter the bloodstream as usable fuel. Starting too late means the rescue fuel arrives after the damage is done.
The correct approach is to begin fueling proactively at 40–45 minutes into any run that will last longer than 90 minutes, regardless of how you feel. Take your first gel at 40 minutes, your second at 75 minutes, your third at 110 minutes, and so on. Set a watch alert if you know you forget. Do not rely on hunger as the trigger — hunger at long-run intensity is a lagging indicator of a problem, not an early warning system.
Gels are the most practical format for running — compact, fast-absorbing, and available in every road race. Standard gels deliver 20–27 grams of carbohydrate in a 40–45 ml packet. Most require chasing with 100–150 ml of water; taking gels without water slows absorption and increases GI distress risk. Caffeinated gels (typically 30–75 mg caffeine) are useful in the second half of a marathon but can cause nausea if taken too early or without adequate food in the stomach.
Chews offer a more food-like texture that some runners find more palatable than gel consistency. Carbohydrate content is similar. They require chewing while running, which takes coordination that degrades when fatigued. Work well for runners who struggle with gel texture; require practice in training.
A 500 ml sports drink at 6–8% carbohydrate concentration delivers 30–40 grams of carbs plus sodium and fluid simultaneously. Useful when carried in a vest or when course aid stations stock isotonic drinks. The fluid co-delivery helps with hydration and absorption. The downside is carrying weight — gels are more calorie-dense per gram carried.
Dates, banana pieces, rice balls, and boiled potatoes are used by ultramarathon runners and some elites. Real food digests more slowly than gels and is harder to eat at marathon pace, but provides a psychological break from synthetic flavours on very long runs. For marathons and half marathons, gels are faster and more reliable. For runs longer than three hours, real food may be more tolerable.
| Format | Carbs per serving | Best for | Watch out for |
|---|---|---|---|
| Standard gel (40 ml) | 20–27 g | Marathons, half marathons | Take with water; practice required |
| Caffeinated gel | 20–25 g + 30–75 mg caffeine | Second half of marathons | Nausea if taken too early |
| Chews / blocks | 24–28 g per pack | Runners who dislike gel texture | Requires chewing while tired |
| Isotonic drink (500 ml) | 30–40 g | Vest-based fueling, ultras | Weight penalty; dilution varies |
| Real food (2 dates) | ~18 g | Long training runs, ultras | Slower digestion; harder at pace |
Carbohydrate fueling and hydration are separate problems that interact. Dehydration impairs carbohydrate absorption and accelerates fatigue. Hyponatraemia — dangerously low sodium from over-drinking plain water — is a risk for slower runners who drink large volumes without sodium replacement.
The practical target for most runners: drink to thirst rather than to a fixed schedule, and ensure every fueling stop includes some sodium. Most gels contain 50–100 mg of sodium per serving — useful but not sufficient for runs in heat exceeding 90 minutes. Add an electrolyte tab or capsule (300–500 mg sodium) every 45–60 minutes in warm conditions or if you are a heavy sweater. The signs of inadequate electrolytes are muscle cramps, persistent thirst despite drinking, and a bloated feeling — all of which are preventable.
Pre-run hydration matters too. Arrive at your long run well-hydrated but not waterlogged. A 500 ml drink of water or dilute sports drink 60–90 minutes before departure is a reliable starting point.
Build a fueled training plan on Get-Split →The single most important fueling principle for race day is this: never try anything new on race day. Every gel brand, every flavour, and every timing protocol you plan to use in the marathon should have been rehearsed on at least three long training runs. The gut is trainable — consistent fueling on long runs improves absorption capacity and tolerance over a 12–16 week training block.
Get-Split's training plans include fueling cues in the notes for long runs. When you push a long run workout to your Garmin device, the structured workout step notes specify the intended fueling protocol for that session. As your race approaches, the plan automatically scales fueling rehearsal volume to match what you will need on the day.
If you have historically skipped fueling on long runs, start with half the target dose — one gel at 45 minutes for your next long run — and gradually increase over four to six weeks. The gut adapts, but it adapts slowly.
Any run planned at 90 minutes or longer benefits from on-run carbohydrate. Shorter runs at easy pace generally do not need fueling if you start reasonably topped up. The exception is a long run deliberately done fasted for fat-adaptation purposes — but this is an advanced protocol that should not replace fueled long runs more than once every two to three weeks.
GI distress from gels usually has one of three causes: taking them without water (slows absorption, concentrates sugar in the gut), taking too many calories at once (gut overload), or simply not having trained the gut. Start with half a gel at a time rather than a full packet, always chase with water, and practise consistently. If a particular brand causes issues, try switching to an isotonic gel formulation, which contains less concentrated sugar. Chews or real food may also be better tolerated by runners with sensitive guts.
For most runners, a half marathon under 1:45 can be completed on pre-run glycogen stores alone if you start well-fueled. Runners targeting sub-1:30 or running their half at near-threshold intensity benefit from one or two gels (at minutes 35 and 60) to top off blood glucose. The full marathon requires full fueling strategy — start at minute 40 and continue every 30–35 minutes through to the finish. Calculate your expected finish time and plan gel count accordingly: a 4-hour marathon needs roughly six to seven gels after the first 40 minutes.
Yes, but with care. Caffeine is one of the few legal ergogenic aids with strong evidence behind it — 3–6 mg/kg body weight improves endurance performance by 2–4% in controlled trials. For a 70 kg runner, that is 210–420 mg caffeine. A caffeinated gel typically delivers 50–75 mg. Taking two to three caffeinated gels in the second half of a marathon delivers a meaningful dose without excessive intake. The mistake is taking caffeinated gels too early (before km 25) when the effect is wasted, or stacking them at the same time and spiking HR uncomfortably.
Estimate your total run time. Subtract 40 minutes (the pre-fueling window). Divide the remainder by 35 minutes per gel. Round up. For a 3:30 marathon: (210 - 40) / 35 = 4.9, so carry five gels. Add one spare. For a 4:30 marathon: (270 - 40) / 35 = 6.6, so carry seven gels plus one spare. Get-Split's training plan notes include gel-count recommendations calibrated to your target finish time once you set a race goal in the app.
Not reliably. The intestinal transporters that absorb carbohydrate at high rates are upregulated with repeated exposure — they improve with practice. Runners who never fuel in training and then take six gels on race day frequently experience GI distress from kilometre 25 onwards, exactly when they need the fuel most. Training your gut is as important as training your legs. Treat your fueling protocol as a non-negotiable part of your long-run preparation.