The science of carb loading: Why it matters for endurance
Carbohydrates are the body's primary and most efficient source of energy during exercise. When consumed, they are converted into glucose and stored in the muscles and liver as glycogen. These glycogen reserves act as your body's personal fuel tank, providing the power needed for high-intensity, sustained effort. For typical runs and daily activities, a balanced diet provides sufficient glycogen. However, during prolonged endurance exercise—generally over 90 minutes—these stores can become significantly depleted.
When your body runs out of available glycogen, it's forced to rely more heavily on less efficient fuel sources, like fat, which leads to a decrease in performance and the onset of fatigue known as "hitting the wall". Carb loading, therefore, is a strategic dietary manipulation designed to overfill the muscle and liver glycogen stores, ensuring you start an endurance event with a maximum fuel reserve. Studies have shown this can improve performance by 2-3% and endurance by up to 15-25% in events lasting longer than 90 minutes.
When you should carb load vs. when you don't
Not every long run requires a full-fledged carb-loading protocol. It's an energy-boosting strategy for specific situations, not a general approach to fueling. The primary determinant is the length of your run. Events lasting less than 60-90 minutes, such as a 5K or 10K, typically don't require carb loading because your body's existing glycogen stores are sufficient. In fact, for shorter distances, excessive carb intake might lead to a feeling of sluggishness.
Table: Carb Loading Needs Based on Run Duration
| Run Duration | Carb Loading Necessary? | Rationale |
|---|---|---|
| Under 60-90 minutes | No | Standard glycogen stores and mid-run fueling are sufficient. |
| Half Marathon (~90 mins+) | Yes, but less intense | Increased carb intake 24-48 hours prior can be beneficial to top off reserves. |
| Marathon and beyond (Ultra) | Yes, highly recommended | Maximizes glycogen for extended exertion beyond the 90-minute mark to prevent "hitting the wall". |
How to properly and safely carb load for a long run
Effective carb loading is a science, not an excuse for a junk food binge. The process typically involves increasing carbohydrate intake for 1-3 days before the event, while simultaneously reducing training volume, or tapering. A common mistake is simply increasing overall food consumption rather than focusing on carbohydrate-dense options, leading to bloating and discomfort.
Here’s how to do it right:
- Timing: Start increasing your carbohydrate intake 24 to 48 hours before your long run or race. There's no need for an old-school "carb depletion" phase.
- Quantity: Aim for 8-10 grams of carbohydrates per kilogram of body weight per day. This can feel like a lot of food and requires intentional planning.
- Focus on the right foods: Choose familiar, carbohydrate-rich foods that are low in fiber and fat to minimize gastrointestinal distress on race day.
- Stay hydrated: As your body stores glycogen, it also stores water, so proper hydration is critical during the carb-loading phase.
The right way to fuel up
To make your carb-loading efforts effective and comfortable, focus on high-carb, low-fiber options. You want to prioritize digestible energy sources that won't weigh you down. Here is a list of ideal carb-loading foods:
- White bread, bagels, or English muffins
- Pasta and white rice
- Plain oatmeal or low-fiber cereals
- Pancakes or waffles with syrup
- Potatoes (without the skin)
- Bananas and other low-fiber fruits
- Sports drinks and fruit juices
- Pretzels and rice cakes
The risks of improper carb loading
Just as proper carb loading can enhance performance, mistakes can jeopardize it. Eating the wrong types or amounts of food can lead to significant discomfort. The most common issues include:
- Digestive discomfort: Overconsuming high-fiber or high-fat foods during the loading period can cause bloating, gas, and stomach upset during the race. This is why lower-fiber options are often recommended for the final day or two.
- Weight gain: The extra glycogen and water stored in your muscles during carb loading can cause a temporary weight increase of 2-4 pounds. While this is a normal part of the process, panicking about the number on the scale can be psychologically unsettling. It's a sign that your body is storing fuel, not that you've gained fat.
- Blood sugar swings: For individuals with diabetes, carb loading requires careful management and consultation with a physician, as it can cause significant blood glucose fluctuations.
Alternatives to traditional carb loading
For runs that don't quite cross the 90-minute threshold but are still long enough to tax your energy stores, you can use in-run fueling strategies instead of a multi-day carb load. For runs lasting longer than 60-90 minutes, taking in 30-60 grams of carbohydrates per hour can be effective.
Examples of in-run fueling options include:
- Energy gels
- Energy chews or blocks
- Sports drinks
- Small amounts of easily digestible foods like bananas
Testing these during your training is crucial to prevent mid-race surprises. As a general rule, the shorter the run, the less you need to worry about intensive pre-race fueling. For longer events like marathons, however, carb loading is a non-negotiable part of your preparation.
Conclusion: Listen to your body and practice your plan
So, do I need to carb load for a long run? For events over 90 minutes, the answer is a resounding yes. It's a proven strategy to boost endurance and delay fatigue. However, it's not a one-size-fits-all solution and needs to be practiced during your training cycle. Never try a new fueling strategy on race day. By planning your carbohydrate intake, reducing your training volume, and focusing on low-fiber options, you can top off your energy reserves effectively and set yourself up for a successful long run. Remember to listen to your body and consult with a sports dietitian for personalized advice, as individual needs can vary. Sports Dietitians Australia has useful resources on this topic.
