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Carb loading for a marathon: what the science says
Carb loading—increasing carbohydrate intake in the days before a marathon—does enhance endurance performance, but only under specific conditions and for specific distances. The science shows that timing, amount, and your individual gut tolerance matter more than the myth of eating pasta the night before.
What does carb loading actually do?
Carb loading works by increasing muscle glycogen stores above their normal resting levels. Glycogen is your muscles' preferred fuel during sustained aerobic exercise. A fully loaded muscle can store roughly 500–600 mmol/kg of dry muscle, whereas a normally fed athlete stores around 350–400 mmol/kg. During a marathon, this extra reserve can delay the point at which your glycogen stores deplete, which typically coincides with fatigue and a performance drop.
The effect is most pronounced in efforts lasting 90 minutes or longer. For a marathon—typically 2.5 to 4+ hours depending on pace—this matters. Studies using trained runners show a 2–3% improvement in time-trial performance when glycogen is maximally loaded compared to normal intake, though real-world gains vary.
When should you start carb loading?
The traditional 'pasta night' approach is outdated. Current evidence supports a shorter, more aggressive window: start 24 to 48 hours before the race, not a full week. This reduces the risk of stomach discomfort and bloating on race day.
- 24–48 hours out: increase carbohydrate to 10–12g per kg of body weight per day
- Reduce fibre and fat slightly to minimise digestive distress
- Spread carbs across 4–5 meals rather than loading one big meal
- Hydrate normally; overhydrating doesn't amplify the effect
How much carbohydrate is enough?
Individual tolerance varies, but the research-backed target is 8–12g per kg of body weight daily during the loading window. A 70 kg runner would aim for 560–840g of carbohydrate per day. This is significantly higher than normal training intake but still achievable without discomfort if you're strategic.
Focus on easily digestible sources: white rice, pasta, bread, bananas, honey, sports drinks, and low-fibre cereals. Avoid wholemeal, beans, and high-fibre vegetables during the loading phase, even though they're normally good choices.
Does carb loading work for everyone?
Not equally. Carb loading has the strongest effect if you're running a marathon at a moderate-to-fast pace (sub-3:30) where glycogen depletion is likely. If you're running significantly slower, pacing is conservative, or you're fuelling during the race with sports drinks or gels, the benefit shrinks. Some runners report no noticeable improvement; others feel tangibly stronger in the final kilometres.
Equally important: gut tolerance varies. A few days of higher carbohydrate intake can bloat some athletes or cause loose stools. Test your carb-loading strategy during a long run or practice race first. Never experiment on race day.
The role of hydration and sodium
Carbohydrate storage requires water—roughly 3–4g of water binds to each gram of glycogen. This means carb loading causes a temporary weight gain of 1–2 kg, mostly water. Some runners find this noticeable; most don't. Maintain steady hydration throughout the loading period rather than overloading fluids on race morning.
Sodium helps retain fluid and supports absorption of carbohydrates. Include salt in your pre-race meals and drinks—there's no need to avoid it during the loading window.
Carb loading and RunV-style coaching
The most effective approach isn't a one-size-fits-all schedule but one tailored to your individual glycogen needs, digestive comfort, and race pace. Data-driven training apps can help you log what you eat during the loading phase and how you feel during long runs, revealing patterns specific to your physiology. If you know your expected race pace and distance, you can calculate whether carb loading is worth the effort—or whether focus on pacing and race-day fuelling matters more.
The bottom line
Carb loading works. It's a legitimate, evidence-backed strategy for marathoners, particularly if you're targeting a time where glycogen depletion is a risk. But the window is short (24–48 hours), the amount is substantial (8–12g/kg), and the benefit is individual. Test it beforehand, keep fibre and fat moderate, and remember that smart pacing and race-day nutrition often matter just as much.
FAQ
- Can I carb load if I'm doing a low-carb diet?
- Carb loading requires a temporary shift to high-carbohydrate intake. If you normally follow low-carb eating, your muscles may not respond as effectively because they're adapted to a different fuel system. This strategy works best if you eat a balanced, carb-inclusive diet most of the time. If you're committed to low-carb, focus instead on optimised pacing and on-race fuelling.
- What's the difference between carb loading and having a big meal the night before?
- One large meal provides calories but won't maximally fill muscle glycogen stores. Carb loading spreads high-carbohydrate intake across 24–48 hours and multiple meals, ensuring your muscles have time to absorb and store carbohydrate fully. A single pasta dinner is tradition, not science.
- Should I carb load for a half-marathon?
- Half-marathons typically last 90 minutes to 2+ hours depending on pace. Carb loading has a modest effect at this distance; normal training diet is usually sufficient unless you're running very fast or have known glycogen concerns. Save the strategy for full marathons or ultra-distances where the payoff is clearer.
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