Carb loading vs. fat burning: What works best?

Endurance athletes, from marathon runners to triathletes, face the crucial decision of how to fuel their bodies for optimal performance.

Carbohydrate loading has long been a favoured strategy among endurance athletes to enhance performance by maximising glycogen stores before a race or competition. However, despite its popularity, the practice has sparked significant controversy in recent years. Fat burning, or fat-adaptation, is a strategy gaining traction among endurance athletes. By training the body to rely on fat rather than carbohydrates for energy, proponents argue that it provides a sustained energy source and reduces the need for frequent refuelling.

Carbohydrate Loading

Carbohydrate loading, or carb-loading, is a strategy that involves increasing carbohydrate intake to maximize the storage of glycogen in muscles and liver. Glycogen is the body's most readily available energy source during high-intensity exercise.

Advantages

Immediate energy source: Glycogen can be quickly converted to glucose, providing an immediate and efficient energy source during prolonged, high-intensity activities.

Improved performance: Studies have shown that carb-loading can enhance performance by delaying the onset of fatigue, allowing athletes to maintain a higher intensity for a longer period.

Mental edge: Knowing that they have maximized their glycogen stores can give athletes a psychological boost, improving confidence and focus.

Implementation

Carb-loading typically involves a few days of increased carbohydrate consumption (7-12 grams per kilogram of body weight per day) combined with a reduction in training intensity. This helps to maximize glycogen stores without the depletion that comes from continuous training.

Considerations

While effective, carb-loading isn't without its drawbacks.

Gastrointestinal discomfort: Some athletes experience gastrointestinal discomfort (including bloating and diarrhoea) due to the sudden increase in carbohydrate intake in the days leading up to an event. This is particularly problematic for athletes who already experience gastrointestinal distress during endurance events, as it can negatively impact performance.

Increase in body weight: Additionally, the extra glycogen stored can lead to a temporary increase in body weight due to water retention, as glycogen binds with water in the muscles.

Questionable value: One of the primary debates centres around the actual necessity of carbohydrate loading. Critics argue that while it may be effective for events lasting longer than 90 minutes, for shorter-duration activities, carbohydrate loading may offer little to no benefit. Studies have shown that glycogen depletion is less likely in shorter endurance events, suggesting that athletes may not need to drastically alter their diet to achieve optimal performance.

Sustainability: some researchers question the long-term sustainability of relying on carbohydrates as a primary energy source. High-carbohydrate diets can cause insulin spikes.

Fat Burning

The fat-adaptation or fat-burning strategy focuses on training the body to utilize fat as the primary energy source. This is achieved through a high-fat, low-carbohydrate diet (LCHF) or ketogenic diet, which aims to shift the body’s metabolism from relying on glycogen to using fat for fuel.

Advantages

Sustained energy supply: The body has a virtually unlimited supply of fat, providing a consistent and long-lasting energy source, which is especially beneficial for ultra-endurance events.

Reduced need for refuelling: Athletes on a fat-burning regimen may not need to refuel as frequently during long events, reducing the risk of gastrointestinal issues and the logistics of carrying and consuming carbs.

Stable blood sugar levels: A fat-adapted metabolism can result in more stable blood sugar levels, minimizing the risk of energy crashes and enhancing mental clarity.

Implementation

Adopting a fat-burning approach requires a significant dietary shift to increase fat intake (60-75% of daily calories) while reducing carbohydrates (typically less than 50 grams per day). This transition period, often referred to as “keto-adaptation,” can take several weeks and may initially result in a decrease in performance until the body fully adapts.

Considerations

There are some challenges experienced by athletes with this approach.

Adaption phase: The primary drawback of fat burning is the initial adaptation phase. When transitioning to a high-fat, low-carbohydrate diet, athletes may experience fatigue, headaches, irritability, and a temporary drop in performance. This phase, known as “keto flu”, can last several weeks.

Compromised high-intensity efforts: High-intensity efforts that rely heavily on glycogen stores can be compromised, as fat oxidation is slower and less efficient for quick bursts of energy. Studies suggest that while fat-adapted athletes can perform well in prolonged, steady-state efforts, they may struggle with high-intensity segments, including sprinting.

Individual variation: The effectiveness of fat-adaptation varies among individuals. Some athletes may experience significant benefits, while others may see minimal improvements or even negative effects. The variability in individual responses raises questions about the universal applicability of fat burning as a performance strategy.

Long-term health effects: While short-term benefits are noted, the long-term impact on overall health and metabolic function remains less clear.

Choosing the Right Strategy

The choice between carbohydrate loading and fat burning depends on various factors, including the type of endurance event, the athlete’s metabolic efficiency, and personal preference. For high-intensity, shorter-duration events, carbohydrate loading may be more beneficial. In contrast, ultra-endurance athletes might find fat burning advantageous for its sustained energy benefits.

Ultimately, the best approach is often individualised and may involve a combination of both strategies. Consulting with a sports nutritionist can help athletes tailor their diet and fueling plan to their specific needs, ensuring optimal performance on race day.

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Photo credit

Photo source iStock: Stockholm, Sweden Aug 23, 2015: Ron Darmon and a group of triathletes cycling in the Men's ITU World Triathlon series event August 23, 2015 in Stockholm, Sweden

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