Carbohydrate loading — the practice of elevating muscle glycogen stores above their normal resting level through dietary manipulation — has been a cornerstone of marathon preparation for decades. But when athletes cross into ultra-distance territory, the physiology changes in ways that demand a fundamentally different approach. Events lasting 10, 20, or 30+ hours at lower relative intensities burn proportionally more fat and less carbohydrate, glycogen depletion is more gradual, and the logistical and GI challenges of the pre-race days loom larger.
Yet dismissing carb loading as irrelevant for ultras is a mistake that costs athletes cognitive sharpness and early-race pace. Even at the lower intensities typical of 100-mile running, glycogen fuels the central nervous system, sustains pace at key race moments — start, technical sections, mountain passes — and delays the shift into severe metabolic fatigue. The question for ultra athletes is not whether to carb load, but how to modify the protocol to suit the demands of events that begin where the marathon ends.
Why Glycogen Still Matters in Ultras
At the effort levels common in ultramarathons (55-70% VO2max for most participants), fat oxidation contributes 60-70% of total energy expenditure — a much higher proportion than at marathon pace. However, carbohydrate remains indispensable. Fat cannot be metabolised fast enough to sustain even moderate effort without carbohydrate co-oxidation. Even well-fat-adapted athletes require approximately 30-40% of their energy from carbohydrate during sustained ultramarathon efforts.
Beyond direct energy supply, muscle glycogen plays a critical role in sustaining cognitive function. The brain is nearly exclusively dependent on glucose, and declining glycogen and blood glucose levels are associated with impaired decision-making, mood disturbances, and increased rate of perceived exertion. Research from the Australian Institute of Sport shows that cognitive test performance declines measurably when blood glucose falls below 4.0 mmol/L, a threshold that is reached much earlier in athletes who begin ultras with sub-optimal glycogen stores.
Traditional vs Modified Carb Loading Protocols
The classic 3-day carb loading protocol for marathons — 8-10g of carbohydrate per kilogram of body weight per day for 72 hours before race day — saturates muscle glycogen stores to approximately 150-200% of their normal resting level. For a 70kg athlete, this means consuming 560-700g of carbohydrate daily, a significant nutritional commitment that can cause GI bloating, water retention (3-4kg is common), and considerable discomfort if not practiced in training.
For ultra-distance events, the modified protocol prioritises maximising glycogen without the GI distress that can compromise the race itself:
- Duration: 36-48 hours rather than 72 hours. Full glycogen supercompensation can be achieved in 36-48 hours in well-trained athletes, reducing the period of GI discomfort and water retention before a race that is already logistically demanding.
- Carbohydrate target: 10-12g/kg/day (e.g., 700-840g for a 70kg athlete). This is at the upper end of what research supports for glycogen supercompensation and reflects the greater energy demands of ultra-distance effort.
- Fiber reduction: Begin reducing dietary fiber 48 hours before race start. High-fiber foods (wholegrains, legumes, raw vegetables) increase GI residue volume, contributing to discomfort and bathroom urgency during early race stages.
- Fat and protein moderation: High-fat and high-protein meals slow gastric emptying and occupy caloric space that should be occupied by carbohydrate. Reduce fat to below 30g/day and protein to 1.0-1.2g/kg/day during the loading period.
Low-Residue Meal Planning for Ultra Athletes
Practical meal planning for the 36-48 hour loading window might look like this for a 70kg athlete targeting 10g/kg (700g carbohydrate total per day):
- Breakfast (150g carb): 200g white rice with honey, 2 slices white toast with jam, 500ml sports drink, 1 banana
- Mid-morning (80g carb): 2 rice cakes with honey, 300ml fruit juice, 1 energy bar
- Lunch (180g carb): 250g cooked white pasta with tomato sauce (no high-fiber vegetables), 2 white dinner rolls, 500ml sports drink
- Afternoon snack (100g carb): 400ml fruit smoothie (banana, mango, juice — no seeds or skins), 2 NorthLine Gold Standard gels (44g carbohydrate combined) with water
- Dinner (180g carb): 300g white rice, 2 medium boiled potatoes, 120g grilled chicken breast, 500ml sports drink
This approach may feel uncomfortably large for athletes unaccustomed to carb loading. Practice the protocol twice in training before your target race to confirm GI tolerance and optimal daily volume.
Race-Morning Nutrition
Many ultra-distance races have pre-dawn starts, creating a logistical challenge for race-morning nutrition. The target is a meal providing 2-3g of carbohydrate per kilogram of body weight (140-210g for a 70kg athlete) consumed 3-4 hours before the start gun. This timing allows gastric emptying, prevents mid-race GI distress from undigested food, and tops up liver glycogen depleted overnight during sleep.
If the race start is too early for a full meal 3-4 hours prior, consume a smaller meal of 1-1.5g/kg 2 hours before start, and supplement with 30-40g of easily digestible carbohydrate (a NorthLine gel or banana) in the final 15-30 minutes before starting. Avoid high-fiber, high-fat, and high-protein foods on race morning.
Transitioning from Pre-Race Loading to During-Race Fueling
One of the most common ultra-nutrition errors is treating carb loading and during-race fueling as separate, disconnected strategies. In reality, they are a continuum. During-race carbohydrate targets for ultra events are lower than for marathons, reflecting the lower relative intensity and longer event duration:
- 50K-100K events (6-14 hours): 40-60g of carbohydrate per hour. At these intensities, the gut can comfortably absorb 40-60g/hr without GI distress.
- 100-mile events (14-30+ hours): 40-80g/hr with significant variability. During early stages when intensity is higher, target the upper end; during night sections or when nausea is a factor, targeting 30-40g/hr through easily digested sources (sports drinks, gels) is more realistic.
- Solid food integration: Unlike marathons (where gels dominate), ultra athletes increasingly rely on real food from aid stations. Rice balls, potatoes with salt, and fruit provide carbohydrate alongside the palatability variety needed to sustain eating for 20+ hours.
For the foundational marathon carb loading protocol and the science underpinning glycogen supercompensation, read our guide on carb loading before a marathon. For a complete race-day fueling strategy specific to ultra distances, see the ultramarathon nutrition guide. Build your personalised hour-by-hour carbohydrate schedule with the Race Day Nutrition Planner, which supports multi-day events and allows you to map NorthLine gel timing across an entire 100-mile course.