Understanding the Science of Ultra Marathon Carb Loading
For ultra marathon runners, the primary fuel source for their working muscles is glycogen, a form of stored carbohydrates. During prolonged, high-intensity exercise, these glycogen stores become depleted, leading to the dreaded 'wall' and severe fatigue. Carb loading is a strategic nutritional approach to maximize these glycogen reserves in the muscles and liver, providing a more robust energy tank for race day. The goal is not to eat more food overall, but rather to shift the macronutrient balance of your diet to prioritize carbohydrates while reducing fat and protein intake.
Dispelling the Carb Depletion Myth
Historically, carb loading involved a glycogen depletion phase with a low-carb diet before the loading phase. However, modern sports science has shown this step is unnecessary and can cause undue stress on the body. The current, recommended approach focuses simply on increasing carbohydrate intake in the final days leading up to the event while tapering your training volume. This allows your body to effectively store more carbohydrates without the negative side effects of a depletion period.
The Optimal Timing and Intake
For an ultra marathon, the carb-loading phase typically spans 36 to 72 hours, beginning 2 to 3 days before your race. The amount of carbohydrates needed varies by individual but generally falls within the range of 8-12 grams per kilogram of body weight per day. This can be a substantial amount, so spreading it out over multiple meals and snacks is key to avoiding an uncomfortable, overly full feeling. Remember, you should also be significantly reducing your training load during this time, known as a taper, to ensure your muscles can effectively store the extra energy.
Making Smart Food and Drink Choices
Not all carbs are created equal when it comes to race-week fueling. The goal is to consume easily digestible, low-fiber carbohydrates to prevent gastrointestinal distress during the race. While complex, high-fiber carbs are excellent for everyday training, they can be problematic when consumed in high volumes close to race day. Lean on refined options to minimize bulk and digestive effort.
Foods to Prioritize:
- White pasta, white rice, and noodles
- White bread, bagels, and low-fiber cereals (like cornflakes)
- Pancakes, waffles with syrup or jam
- Potatoes (peeled), sweet potatoes
- Fruits like bananas, applesauce, and dried fruits (dates, raisins)
- Sports drinks, fruit juice, and liquid meal replacements for calorie density
- Energy gels and chews (practiced beforehand)
Foods to Limit or Avoid:
- High-fiber vegetables (e.g., broccoli, cauliflower, Brussels sprouts)
- Legumes and beans
- High-fat foods (creamy sauces, fried foods, excessive oils)
- Excessive protein intake
- New or unfamiliar foods that might upset your stomach
- Spicy foods
Carb Loading: A Comparison Table
| Feature | During Normal Training | During Carb-Loading (2-3 days pre-race) |
|---|---|---|
| Carb Intake | Moderate (5-7g/kg/day) | High (8-12g/kg/day) |
| Fat/Protein | Balanced with carbs | Reduced to make room for carbs |
| Fiber | High-fiber, nutrient-dense foods are encouraged. | Reduced to prevent GI issues and bloating. |
| Fluid Intake | Regular hydration | Increased, as glycogen binds with water. |
| Training Volume | Higher volume | Tapered and reduced significantly. |
| Food Focus | Whole grains, varied fruits, and vegetables | Easily digestible, low-fiber carbs. |
| Weight Fluctuation | Stable | Temporary weight gain (1-2kg) due to water retention. |
A Sample Carb-Loading Menu
To visualize what this looks like, consider a sample day for a 70kg runner needing 700g of carbs:
- Breakfast (7am): Large bowl of low-fiber cereal with low-fat milk and a sliced banana. A glass of fruit juice. (approx. 150g carbs)
- Mid-Morning Snack (10am): A large bagel with jam and a sports drink. (approx. 100g carbs)
- Lunch (1pm): A large plate of white pasta with a simple tomato-based sauce and a small portion of lean chicken. (approx. 175g carbs)
- Mid-Afternoon Snack (4pm): A handful of pretzels and a fruit smoothie made with banana and yogurt. (approx. 100g carbs)
- Dinner (6pm): White rice bowl with grilled white fish and a small serving of skinless potatoes. (approx. 150g carbs)
- Evening Snack (8pm): Applesauce and a handful of candy or gummy chews. (approx. 50g carbs)
Race Day Morning Nutrition
On the morning of your ultra, the goal is to top off your liver glycogen stores, which are partially depleted overnight. A light, high-carbohydrate breakfast should be consumed 3-4 hours before the race starts. Examples include a bagel with jam, oatmeal, or a banana. The amount should be personalized based on what you practiced during training. Avoid anything heavy or unfamiliar to prevent stomach upset.
Conclusion
Successful carb loading for an ultra marathon is a planned, multi-day process that prioritizes strategic carbohydrate intake over simply eating a lot. By following the modern approach of increasing carbs while tapering, focusing on low-fiber, easily digestible foods, and staying well-hydrated, you can maximize your glycogen stores and set yourself up for a powerful performance. Practicing this strategy during your training is the ultimate key to feeling confident and fueled at the start line. A well-fueled runner is a powerful runner, and with the right strategy, you can push past your limits and conquer the ultra marathon distance. For more information on evidence-based sports nutrition strategies, you can visit a reputable source like the National Institutes of Health.
