Carb loading: the complete guide
The difference between blowing up at km 30 and crossing the line with energy. Everything you need to know about carb loading: who it serves, how much and when to eat, and how not to gain unnecessary weight.
What carb loading is and why it works
Carbohydrate loading (carb loading) is the strategy of increasing muscle and liver glycogen stores in the 1 to 3 days before a long race by eating more carbohydrates and tapering training. Glycogen is how the body stores fast-access energy: roughly 1 g of glycogen per 4 g of muscle mass under normal conditions, scaling up to 1.5 to 2 times that ceiling with proper loading. In a marathon or any 90+ min race, energy fatigue (the famous "wall" or "bonk") happens when muscle glycogen drops below about 25% of max and the body is forced to rely more heavily on fat oxidation, which cannot match the pace your aerobic engine demands. Loading beforehand delays that moment and consistently improves race time by 2 to 3% in trained endurance athletes (Hawley 2018, Burke 2021). For context, 2 to 3% off a 3:30 marathon is roughly 4 to 6 minutes for zero extra training.
Who it makes sense for (and who it does not)
Carb-load protocols make sense for races with an effective duration above 90 minutes: marathon, half marathon at hard pace, trail running above 25K, granfondo, Olympic-distance triathlon or longer, long Hyrox Doubles, and any cycling event over three hours. Does not make sense (and may actively hurt) in short races (parkrun, 5K, a fast 10K, Hyrox Pro Solo) because the water weight gain leaves you 1 to 2 kg heavier with no compensating energy benefit. Also skip carb loading if you have uncontrolled diabetes, undiagnosed coeliac disease, or if the "race" is really a short training session. When in doubt, estimate your predicted finish time: if it is under 90 min, skip the load and stick with your normal eating pattern plus a sensible race-day breakfast.
Three protocols: 1 day, 3 day, 7 day classic
One-day protocol (Fairchild 2002): 10 g/kg of carbs in 24h, training only a 3-min max sprint the day before. Result: glycogen +90%. Impractical day to day, but very useful in last-minute emergencies when you forgot to start loading earlier. Three-day protocol (Sherman 1981, modified): 7 to 10 g/kg/day for 3 days alongside reduced training volume (a normal pre-race taper). This is the modern standard for marathon, half marathon and Olympic-distance triathlon. Seven-day classic protocol (Bergstrom 1967, original): 3 days of glycogen depletion (exhaustive training plus a low-carb diet) followed by 4 days of loading at 10 g/kg/day. Achieves extreme supercompensation but demands genuinely hard training in the depletion phase right before a goal race, which carries a high illness and injury risk. Rarely recommended to amateurs today, mostly retained as a historical reference.
How much to eat: g/kg/day by protocol
For the 3-day protocol (most common in amateur races), the ISSN 2018 position stand and the Sport Nutrition Group 2023 consensus recommend 7 to 10 g/kg/day. For a 70 kg athlete that is 490 to 700 g of carbs per day, and for a 60 kg athlete it is 420 to 600 g. In real plates: 600 g of cooked pasta delivers roughly 180 g of carbs, so to reach 600 g you need around 3 to 4 full plates per day, spread across 5 to 6 meals plus snacks. Alternatively, count via 4 g per main meal across 5 to 6 meals plus 2 snacks of 60 to 80 g. Use a food tracking app for the first 2 days to calibrate your intuition; most people underestimate their carb intake by 20 to 30% when they go off feel alone.
How not to gain unnecessary weight
Common myth: carb load means stuffing pasta and ending up 3-4 kg heavier. Reality: weight gain is expected (1-2 kg) and is mostly metabolic water (3 g per 1 g of stored glycogen). That water is good: it is released through sweat during the race and acts as available energy. Excess gain (3+ kg) happens when you add carbs instead of substituting macros. The rule: during the 3 loading days, cut fat below 20% of total calories (lean fish instead of salmon, chicken breast instead of thigh, avoid abundant olive oil) and reduce protein to 1.2-1.5 g/kg (not 2+). Do not eat extra dessert, substitute the fat component of the plate with more simple carbs (more rice, more bread).
Top foods vs foods to avoid
Top carbs for loading: white pasta (NOT wholegrain in this phase, since fibre causes GI distress on race day), white rice, boiled or mashed potato, white bread with honey, ripe banana, no-pulp orange juice, dates, toast, oats in water, baby cereals (high density, easy digestion). Caffeinated carbs in the last 2 days (tea with honey, coffee with bread). Avoid in the last 24-48h: legumes (beans, lentils, chickpeas), brassicas (broccoli, cabbage), nuts, wholegrain bread, avocado, fatty salmon, fried food, alcohol. These may be healthy in daily diet but the high fibre + fat creates gut fermentation and risk of GI distress in race.
Hydration during the load
Each gram of stored glycogen retains about 3 g of water. To load 500 to 600 g of extra glycogen, the body needs to absorb roughly 1.5 to 1.8 L of additional water on top of normal daily needs. Practical solution: drink 30 ml/kg of water per day (2.1 L for a 70 kg athlete) plus 500 ml extra per hour of training, and add a small pinch of salt (around 0.5 g) per 500 ml to retain the fluid in the body instead of flushing it through urine. Check urine colour as a quick gauge: straw yellow means you are on target, completely transparent means you are over-drinking and losing electrolytes, and dark yellow means you are behind. Poor hydration silently aborts a carb load because glycogen physically cannot be stored without enough intracellular water alongside it.
Sample 3-day plan for 70 kg (8 g/kg/day = 560 g carbs)
Day 1 and 2: breakfast 1.5h post-short-training = 60g oats in plant milk + banana + spoon of honey (90 g carbs). Mid-morning = white bread toast with jam (40 g). Lunch = 200g cooked pasta with tomato sauce + grilled chicken breast (90 g). Afternoon snack = 1 banana + 200 ml orange juice + 2 toasts with honey (60 g). Dinner = 300g white rice with lean fish (100 g). Before bed = tapioca porridge with honey + tea (60 g). Total: 440 g. Day 3 (eve): same but + 1 extra snack late afternoon (40 g) and bump lunch/dinner portions by 20%. Total: 560+ g. Race-morning breakfast: covered in our race-day plan.
The 5 most common myths about carb loading
1) "I have to eat pizza the night before": false. High fat + cheese delays gastric emptying up to 8h. Simple pasta is better. 2) "More pasta, better": false. Above 12 g/kg/day the body cannot store more, excess turns to fat. 3) "Carb load only matters for marathon": false. Works for any race above 90 min where glycogen will be limiting. 4) "I eat wholegrain because it is healthier": false in this phase. Fibre (>30 g/day) in the last 2 days substantially increases GI distress risk in race. 5) "I do not need carb load because I already eat a lot": check. Average carbs in European amateur athlete diet is 4-5 g/kg/day, not 8-10. Without measuring, you are probably under-loaded.
Apply this to your race
Carb loading only matters above 90 min of racing. See the specific guide for your discipline:
Trail and ultra-trail
Load at 8-12 g/kg/day, 2 days before. Stack with T-3h breakfast.
Marathon and half marathon
8-10 g/kg/day in the 3 days before, substituting fat. No extra weight.
Granfondo and cycling
Classic 7-10 g/kg/day load. Critical for >3h events.
Triathlon (70.3 and Ironman)
Loading mandatory above 70.3. Pair with gut training.
Frequently asked questions
From how long a race is it worth loading?
Above 90 minutes. In shorter races, normal glycogen is enough, and the weight gain (1-2 kg of metabolic water) does not pay off.
How much weight will I gain with the load?
Around 1-2 kg, mostly water. That water leaves through sweat during the race and acts as available energy. If you gain 3+ kg, you are adding calories instead of substituting macros.
Can I carb load if I am diabetic?
Only with medical supervision and glycaemia monitoring. Loading raises post-meal peaks and requires insulin adjustment for type 1, and extra attention for type 2.
Can I do it vegan?
Yes, easily. Pasta, rice, potato, bread, fruit, honey or agave syrup. Add tofu/seitan for lean protein.
Do I still need a gel in the morning if I already carb loaded?
Yes. Loaded glycogen is your base reserve, but during the race you still need exogenous carbs at 30-90 g/h depending on duration.