Carbohydrate Loading Duration for Marathon
For marathon runners, carbohydrate loading should be performed for 36-48 hours (approximately 1.5-2 days) prior to the race, consuming 10-12 g/kg/day of carbohydrates during this period. 1
Evidence-Based Timeframe
The most recent Sports Medicine guidelines (2025) specifically recommend a dedicated 36-48 hour period of high carbohydrate intake for events exceeding 90 minutes in duration, which clearly applies to marathon running. 2 This recommendation is consistently supported across multiple high-quality sources and represents the current standard of care. 1, 3
Key Implementation Details
Carbohydrate Intake Targets:
- 10-12 g/kg body weight per day during the 36-48 hour loading period for optimal glycogen supersaturation 2, 1, 3
- For shorter events (<90 minutes), habitual intake of 7-10 g/kg/day is sufficient without dedicated loading 2, 1
Practical Timing:
- Begin the loading protocol approximately 1.5-2 days before race day 2, 1
- Combine with exercise tapering during this same period to maximize glycogen storage 3
- The day-before plus morning-of carbohydrate intake (DBMC) has been shown to independently predict faster marathon times 4
Fiber Management Strategy
An important but often overlooked aspect: implement a low-fiber diet (48-72 hours before the race) to reduce gastrointestinal discomfort from the increased food volume required for carbohydrate loading and to minimize the need for bowel movements on race day. 2 This involves:
- Reducing fiber intake to manage the bulk of gastrointestinal contents 2
- Using a FODMAP management strategy (24-48 hours of low FODMAP foods) to offset increased food intake 2
- Favoring energy-dense, low-fiber carbohydrate sources like white bread, white rice, and refined pasta 5
Common Pitfalls to Avoid
Outdated Protocols: The older "depletion phase" approach (exhausting exercise followed by 3 days of high-fat/high-protein diet, then 2-3 days of high-carbohydrate) is no longer recommended. 6 There is no evidence that low carbohydrate diets enhance training outcomes or provide benefits as a depletion phase, and may actually impair performance if carried out for extended periods. 3
Practical Challenges: For athletes on relatively low-calorie diets, achieving 10-12 g/kg/day can be challenging. For example, a 55 kg athlete would need to consume carbohydrates representing 88% of a 2000 kcal/day diet. 2 In such cases, using energy-dense, low-fiber carbohydrate sources and sports foods becomes essential. 2
Individual Variation: Women may oxidize more fat and less carbohydrate than men at the same relative exercise intensities, and carbohydrate loading effectiveness may vary with menstrual cycle phase. 2 Mid-follicular phase loading has shown 17-31% improvements in muscle glycogen, while mid-luteal phase results are more variable. 2
Pre-Race Morning Strategy
On race morning (3-4 hours before the start), consume 1-3 g/kg body weight of carbohydrates to restore liver glycogen stores and ensure optimal muscle glycogen availability. 1 Avoid consuming large amounts of carbohydrates in the 60 minutes immediately before the race to minimize reactive hypoglycemia risk. 1