The 60-second version
Marathon carb loading has changed substantially since the classic 1970s “depletion-then-load” protocols, which produced GI distress and unreliable results. The current consensus, supported by 30+ years of trial evidence: simply consuming 8-12g of carbohydrate per kg of body weight in the 36-48 hours before a marathon achieves the same muscle glycogen super-compensation as the older depletion protocols, with much better GI tolerance. The depletion phase isn’t needed. The protocol that emerged: cut fat and fibre intake, eat carbohydrate-dense foods at 8-12g/kg in the 36-48 hours pre-race, distribute across 4-6 meals, finish carb-loading 12-16 hours before the start. For a 70kg athlete, that’s 560-840g of carbohydrate daily — a substantial amount that requires intentional planning. The most common errors are too little carbohydrate (the “pasta dinner” alone isn’t enough), too much fibre (causing GI distress), and carb-loading on race morning (too late). During the race itself: 60-90g of carbohydrate per hour, ideally using a multi-transportable mix (glucose + fructose) to maximise absorption beyond the single-carbohydrate ceiling of 60g/hr.
Why glycogen matters at marathon distance
Muscle glycogen stores in a normally-fed adult are roughly 1,500-2,000 kcal worth — sufficient for 90-120 minutes of running at marathon pace. Marathon pace times exceed this in most runners. Without carb loading, runners typically “hit the wall” (rapid pace deterioration as glycogen depletes) around 30-35km. With proper carb loading, super-compensated muscle glycogen extends the glycogen window to roughly 35-40km, and intra-race carbohydrate consumption fuels the remainder. The performance impact is large — well-loaded runners typically maintain pace where unloaded runners decelerate significantly Burke 2011.
The modern protocol
- 36-48 hours before race: begin carb loading. No depletion phase needed.
- 8-12g carbohydrate per kg body weight daily. For a 70kg athlete: 560-840g/day. For an 80kg athlete: 640-960g/day. These are large numbers requiring planning.
- Distribute across 4-6 meals. 100-150g of carbohydrate per meal, plus snacks. Eating one giant pasta dinner doesn’t deliver enough.
- Cut fat and fibre. Both slow gastric emptying and produce GI volume that interferes with race-day comfort. The pre-race diet should be lower than normal in vegetables, beans, whole grains; higher in white rice, pasta, bread, low-fibre cereals, juice, sports drinks.
- Finish loading 12-16 hours before the start. The night-before dinner can be normal-sized. Race morning should be a small, easy meal (200-400 kcal of simple carbohydrate, 2-4 hours before the start).
- Hydrate with electrolytes throughout the loading period. Glycogen storage requires water (3g water per gram of glycogen). Expect 1-2kg of water-weight gain from a successful load.
During-race carbohydrate
- 60-90g of carbohydrate per hour, beginning around 30-45 minutes into the race.
- Use multi-transportable mixes (glucose:fructose ratio of roughly 2:1). Glucose alone is absorbed at a maximum of 60g/hr; adding fructose uses a separate transporter, raising the absorption ceiling to 90+g/hr Jentjens 2004.
- Practice the strategy in training. Gut tolerance to high carbohydrate intake is trainable; race-day GI distress is usually from untrained gut handling.
- Sources: sports gels, energy drinks, banana, dates, hard candy. Solid food works for some runners but produces more GI issues at race pace.
“Modern carb-loading protocols using 8-12g/kg of carbohydrate over 36-48 hours achieve muscle glycogen super-compensation equivalent to the older depletion-loading protocols, with significantly better gastrointestinal tolerance and reduced training disruption.”
— Burke et al., J Sports Sci, 2011 view source
Common errors
- Carbohydrate dose too low. “Just eat more pasta” falls short of 8-12g/kg in most cases. Track the actual intake during the loading period.
- Too much fibre. Whole-grain pasta, brown rice, beans, vegetables — nutritious normally, problematic the day before a marathon. Switch to white versions.
- Race-morning carb-loading. Too late. The intestinal absorption and glycogen synthesis timeline doesn’t allow meaningful loading on race day.
- Untrained gut for intra-race carbohydrate. 60-90g/hr is well-tolerated only with prior training. Practice fuelling at race intensity during long training runs.
- Carb-loading for short events. 5k, 10k, half marathon don’t need carb-loading — normal mixed diet provides sufficient glycogen for events under 90 minutes.
Who actually needs this
- Marathon and longer (42km+): yes, carb-loading produces meaningful benefits.
- Half marathon (21km): probably not. Most well-trained athletes complete a half marathon within their existing glycogen window. A normal mixed diet pre-race is sufficient.
- Long-course triathlon (Olympic+): yes, similar protocol.
- Ultra-distance events: yes, with adapted nutrition during the event.
- Cycling stage races / multi-day events: daily carbohydrate intake during the event matters more than pre-event loading.
Practical takeaways
- Modern carb loading: 8-12g/kg/day of carbohydrate for 36-48 hours before the race. No depletion phase needed.
- Distribute carbohydrate across 4-6 meals; one large dinner isn’t enough.
- Cut fat and fibre during the loading period to reduce GI volume and race-day discomfort.
- During the race: 60-90g carbohydrate/hour using multi-transportable mixes (glucose:fructose 2:1).
- Practice intra-race fuelling in training — the gut handles high intake only with prior adaptation.
References
Burke 2011Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29 Suppl 1:S17-S27. View source →Jentjens 2004Jentjens RL, Moseley L, Waring RH, Harding LK, Jeukendrup AE. Oxidation of combined ingestion of glucose and fructose during exercise. J Appl Physiol. 2004;96(4):1277-1284. View source →