Why the marathon requires fueling that shorter races don't
Your body stores glycogen — carbohydrate held in the liver and muscles — as its primary fuel source for moderate-to-hard exercise. Total stores are roughly 400–500g in muscle and 90–120g in the liver, translating to somewhere around 1,800–2,000 kcal depending on body size and training status.
At marathon effort (roughly 75–80% of VO₂max), the body burns through this store at a rate that typically exhausts it around the 30–32 km mark for a runner who hasn't fueled during the race. This is the physiological basis of 'the wall' — it isn't fatigue in a vague sense, it's a specific, measurable depletion event, and it's largely preventable with adequate fueling.
Shorter races don't hit this wall because they're either too short in duration (a 5K or 10K finishes well before glycogen stores are meaningfully depleted) or, in the case of a half marathon, close enough to the edge that most runners finish before it becomes critical — though under-fueled half marathon runners can still feel the early effects in the final few kilometres.
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Carb-loading: the modern protocol
The old 'depletion' method — deliberately depleting glycogen with hard exercise and low-carb eating several days before the race, then loading — has largely been abandoned. It's stressful, disruptive to training in race week, and modern research shows it isn't necessary to achieve full glycogen supercompensation.
The current standard approach: increase carbohydrate intake to roughly 8–12g per kilogram of body weight per day for the final one to three days before the race, while reducing training volume (which the taper is already doing). For a 70 kg runner, that's approximately 560–840g of carbohydrate per day — a noticeably higher intake than normal training days, achieved through more rice, pasta, bread, and fruit rather than more fat or protein.
This isn't about eating more food overall — fat and fibre should actually decrease slightly in the final 24–48 hours, both to prioritise carbohydrate intake and to reduce GI distress risk on race morning. Carb-loading is a shift in composition, not just a calorie increase.
In-race fueling: how much and how often
During the race itself, the goal is to supplement — not fully replace — glycogen burn. Current sports nutrition guidance suggests 30–60g of carbohydrate per hour for most runners, achievable through gels, chews, or sports drink, though well-trained guts can sometimes handle more (up to 90g/hour using multiple carbohydrate sources).
Take your first fueling around 45–60 minutes into the race — before you feel like you need it. Gels and chews take 10–15 minutes to enter your bloodstream, so waiting until you feel depleted around kilometre 28–30 means the fuel won't help until it's almost too late.
Practice your race-day fueling plan on long training runs before race day. The marathon is not the place to try a new gel brand or a new fueling frequency for the first time — GI distress from an untested fueling strategy is one of the most common causes of a marathon going wrong despite adequate training.
Hydration and electrolytes
Drink to thirst rather than following a fixed volume schedule — overdrinking (leading to hyponatremia, a dangerous dilution of blood sodium) is a more common problem in mass-participation marathons than dehydration, particularly among slower runners spending more total time on course.
In hot conditions or for runners who sweat heavily, an electrolyte-containing sports drink or a salt tablet alongside water helps replace sodium lost through sweat, which plain water does not address. This matters more as race duration increases, which is another reason marathon nutrition strategy differs from shorter races.
Common fueling mistakes marathoners make
Waiting too long for the first fuel intake is the most common mistake — by the time you feel like you need it, the deficit is already building and the fuel takes time to take effect. Fuel on a schedule, not on feeling.
Trying something new on race day — a new gel, a new pre-race breakfast, an unfamiliar sports drink at aid stations — removes the benefit of having trained your gut to handle a specific fueling strategy. Whatever you use on race day should be identical to what you practiced on your longest training runs.
Under-fueling in the days before the race because of race-week nerves or a desire to 'feel light' works against you. Carb-loading isn't optional padding — it's the mechanism that pushes your glycogen depletion point past the finish line instead of into it.