Race packs at major marathons are littered with discarded gels — athletes carrying ten, using five, or carrying four and bonking at mile 22. The question of how many energy gels to take is not answered by distance. It is answered by time.
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Why Duration, Not Distance, Determines Gel Count
Carbohydrate depletion is a function of intensity and duration, not kilometres covered. A 4-hour marathoner running at 65% VO₂max is burning approximately 800–950 kcal/hr, of which 55–65% comes from carbohydrate — roughly 110–130g/hr. Endogenous glycogen stores supply approximately 300–400g of carbohydrate. Without exogenous carbohydrate intake, glycogen becomes critically depleted at roughly 2.5–3 hours for most runners at this intensity.
A 3-hour marathoner is running at approximately 80% VO₂max. Glycogen utilisation is higher per unit time — but the event duration is shorter. The risk window is different. The gel protocol is different.
The baseline formula: > Total carbs needed (g) = carb rate (g/hr) × duration (hrs) > Gels needed = total carbs needed ÷ carbs per gel
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Carb Rate Targets by Duration
Sports dietetics research, synthesised in the ISSN 2023 position stand, establishes the following evidence-based targets:
| Marathon Duration | Carb Rate | Example Gel Protocol |
|---|---|---|
| Sub 2:30 | 30–45 g/hr | 1 gel per 30–35 min |
| 2:30–3:30 | 45–60 g/hr | 1 gel per 25–30 min |
| 3:30–4:30 | 60 g/hr | 1 gel every 22–25 min |
| 4:30+ | 60 g/hr | 1 gel every 20–22 min |
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The Timing Problem: Front-Loading vs Even Distribution
Many runners take their first gel at the start or within the first 10 minutes — a timing error. Glycogen stores are full at the start. Plasma glucose is adequate. A gel consumed in the first 15 minutes provides no performance benefit and may trigger an insulin response that briefly suppresses fat oxidation.
Optimal first gel timing: 20–30 minutes into the race. This pre-empts early glycogen depletion without flooding the system at a point where it has no shortage.
Subsequent gels: evenly distributed from the first gel through to approximately 15–20 minutes before the finish (taking a gel in the final mile produces no benefit — glucose absorbed post-finish line contributes nothing to that performance).
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The 25g Assumption and Why It Matters
Not all gels are equal in carbohydrate content:
| Gel Brand | Carbs per serving |
|---|---|
| GU Energy | 21g |
| SiS Go Isotonic | 22g |
| Clif Shot | 24g |
| Maurten Gel 100 | 25g |
| High5 Energy Gel | 23g |
| Maurten Gel 160 | 40g |
| SiS Beta Fuel | 40g |
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The Weight Penalty of Over-Carrying
For competitive runners, gel weight is a real consideration. A standard 32g gel packet — approximately 25g carbohydrate, 7g packet weight — weighs 32g. Carrying 10 gels represents 320g of additional weight, or roughly 0.3–0.4% of bodyweight for a 75kg runner. At elite pacing, 0.3% additional weight costs approximately 5–8 seconds per mile. Over a marathon, this is 2–3.5 minutes of time penalty from over-fuelling weight.
The correct approach: carry the minimum number of gels required to meet the carb target for your predicted finish time, plus one contingency gel for pacing slippage. Not four contingency gels.
For runners calculating the exact number of gels based on their target race duration, effort level, and gel carbohydrate content — the energy gel calculator at winsport.uk/tools/nutrition/energy-gel-calculator generates a gel count, total carb target, and a timed schedule showing when to take each gel during the race.
What is your standard gel protocol for marathon racing — do you calculate a precise intake target, or has your gel count been the same since your first race?