Why Fasted Training Isn’t the Key to Better Endurance Performance

In recent years, fasted training has gained a lot of popularity. The idea behind it is that exercising on an empty stomach might stimulate fat oxidation – the breakdown of fat into energy – and, in turn, improve endurance by teaching the body to rely on fat as a primary fuel source. This concept sounds appealing—especially to endurance athletes looking to optimize performance—but does the science back it up? In this post, we’ll dive into what current research says about fasted training and endurance, as well as some potential risks associated with this approach.

The Theory Behind Fasted Training

Fasted training, typically performed after an overnight fast, is rooted in the idea that a lack of carbohydrate (glycogen) availability will encourage the body to tap into fat stores for energy. Supporters of fasted training believe this can increase the body’s fat oxidation capacity, potentially delaying fatigue and extending endurance. However, it’s crucial to examine whether this theory translates into measurable improvements in endurance performance.

🔍 What Does the Research Say About Fasted Training and Endurance?

Let’s look at what scientific studies reveal about fasted training and its effects on key endurance indicators, including aerobic capacity (VO2max), efficiency, and time-trial performance.

1. Limited Impact on Endurance Markers

Studies show that fasted training has little to no effect on improving aerobic capacity (VO2max), efficiency, or time-trial performance. In fact, several studies suggest that fasted training may even decrease performance for endurance athletes. One study showed a slight improvement in peak power associated with body weight reduction, but did not find any improvements in aerobic capacity. 

1. Fat Oxidation and Endurance

Contrary to popular belief, there’s minimal evidence to support that fasted training significantly increases fat oxidation or enhances endurance. While some adaptations in fat oxidation may occur, studies suggest they are not sufficient to improve overall endurance performance, especially in high-intensity scenarios where glycogen remains the preferred fuel source.

1. No Real Endurance Benefits from Time-Restricted Eating

Much of the research around fasting is conducted in populations who observe fasting for religious reasons, like Ramadan, or using time-restricted eating (TRE), when the eating window is confined to a specific period of time. The most common form of TRE is known as “16:8,” which involves eating in an 8-hour window and fasting for the other 16 hours of the day. In studies on fasting, the findings remain mixed, with little evidence that fasting has a meaningful impact on endurance performance. While TRE may contribute to weight management and metabolic health in some cases, its effect on endurance training is not well-supported.

❗ Risks of Fasted Training: Do They Outweigh the Benefits?

Beyond the lack of clear endurance benefits, fasted training carries several risks, particularly for endurance athletes who already place significant demands on their bodies. Here are some of the primary concerns:

1. Reduced Endurance Capacity

Training in a fasted state means your body has limited glycogen stores, which are essential for maintaining energy levels during endurance workouts. This lack of glycogen can result in shorter, less effective workouts and diminish your body’s ability to sustain higher intensity efforts.

1. Higher Fatigue and Perceived Effort (RPE)

When glycogen is low, your body tends to feel like it’s working harder to maintain the same pace or power output, leading to increased fatigue and higher Rate of Perceived Effort (RPE). This heightened fatigue can make training sessions feel more challenging than necessary, potentially reducing the quality and enjoyment of your workouts.

1. Dehydration Risks

Fasting can disrupt your hydration balance, especially in longer workouts. If you’re fasting and exercising without sufficient fluids, you’re more likely to become dehydrated, which can further impair performance and recovery.

1. Altered Sleep Patterns

The timing of meals and overall calorie intake throughout the day can significantly impact sleep quality, with fasting sometimes delaying bedtime or reducing total sleep duration and sleep efficiency. Sleep is pivotal in recovery, so fasting may not be the best idea for endurance athletes who require optimal sleep.

1. Increased Cortisol Levels

Fasted training elevates cortisol, a stress hormone that, when chronically elevated, can promote muscle breakdown and increase fat storage. Over time, high cortisol may compromise muscle repair and growth, leading to higher injury risk and potentially counterproductive results for endurance athletes aiming to build strength and stamina.

⚠️ The Bottom Line: Fasted Training Isn’t the Magic Bullet for Endurance

Despite the hype surrounding fasted training, research does not support it as an effective method for improving endurance. For athletes looking to maximize performance, relying on fasted training could be counterproductive, especially for high-intensity sessions or longer workouts that demand sustained energy. Here’s a summary of why fasted training may not be worth the risks:

• No significant improvements in endurance, indicated by measures like VO2max, efficiency, and time-trial performance, have been consistently observed with fasted training.

• Increased risks of fatigue, dehydration, poor sleep, and heightened cortisol levels, all of which can negatively affect performance and recovery.

• For high-intensity workouts, fueling properly before training is essential, as these sessions demand glycogen—a fuel source that fasted training lacks.

In short, the scientific consensus suggests that the risks of fasted training far outweigh any potential benefits for endurance. Athletes should focus on fueling properly before training sessions to ensure they’re adequately supported, avoid unnecessary fatigue, and keep their bodies performing at their best.

🥑 Fueling Strategies for Endurance Athletes

Here are some evidence-based tips for fueling to optimize endurance performance:

1. Pre-Workout Nutrition

Aim to consume a balanced meal or snack with carbohydrates and some protein 1–3 hours before training. Depending on how much time you have, this could be as simple as a banana with a dollop of peanut butter or a bowl of oatmeal with fruit. Providing your body with the energy it needs can help delay fatigue and improve training quality.

1. Stay Hydrated

Start your training sessions hydrated, especially if they’re long or intense. Water is sometimes sufficient, but electrolyte beverages can be beneficial for extended or intense sessions.

1. Mid-Workout Fuel for Longer Sessions

For endurance sessions lasting 60 minutes or longer, consume easily digestible carbohydrates, such as a sports drink or energy gel, to maintain energy levels and support performance.

1. Recovery Nutrition

After training, focus on replenishing glycogen and supporting muscle repair by consuming carbohydrates with protein. This can help reduce recovery time and ensure you’re ready for your next workout.

1. Focus on Consistent Fueling

Rather than relying on occasional fasted training, prioritize a well-balanced diet that includes carbohydrates, proteins, and healthy fats throughout the day to keep energy levels stable and support endurance training goals.

🚀 Ready to Maximize Your Endurance? Fuel Up for Success

While fasted training may sound appealing as a way to enhance endurance, the evidence doesn’t support its effectiveness for performance. For endurance athletes, maintaining consistent energy levels by fueling up pre-workout is the most effective strategy to boost gains, support long-term health, and achieve peak performance.

Curious about optimizing your fueling strategy? Book a one-on-one session to customize your nutrition plan and get the most out of your training. Together, we’ll design a balanced approach to keep you energized, strong, and ready to perform at your best!

Sources:

Devrim-Lanpir, A., Hill, L., & Knechtle, B. (2021). Efficacy of Popular Diets Applied by Endurance Athletes on Sports Performance: Beneficial or Detrimental? A Narrative Review. Nutrients, 13(2), 491. https://doi.org/10.3390/nu13020491

Zouhal, H., Saeidi, A., Salhi, A., Li, H., Essop, M. F., Laher, I., Rhibi, F., Amani-Shalamzari, S., & Ben Abderrahman, A. (2020). Exercise Training and Fasting: Current Insights. Open access journal of sports medicine, 11, 1–28. https://doi.org/10.2147/OAJSM.S224919