Fat-Adaptation vs Carb-Loading - The Great Debate

Today’s newsletter is written by Letty from Marathon Media. Letty hosts both, the Marathon Running Podcast and LettyRuns YouTube Channel.

In the world of running, there are two distinct fueling philosophies, and they often stand in direct opposition. On one side, the traditional advice advocates for a carbohydrate-centric approach, consuming carbs before every run to maximize energy stores. On the other, a growing number of runners are exploring the practice of becoming "fat-adapted" to train their bodies to use a more abundant fuel source.

For a deep dive into this topic, you can listen to our full conversation on the podcast or watch the video on YouTube, where we're joined by renowned exercise physiologist Dr. Patrick M. Davitt. But for now, here is an educational breakdown of this complex subject.

Defining the Terms: Fat-Adapted vs. Fat Loss

A common misconception is that being "fat-adapted" is simply another term for a keto diet or a weight-loss strategy. In reality, fat-adaptation is a physiological state where the body increases its ability to burn fat for fuel at higher exercise intensities. For a marathoner, this means running at a faster pace while still relying on a greater percentage of fat for energy.

Here is a key distinction from fat loss: The goal of becoming fat-adapted is not to lose weight, but to improve metabolic flexibility and athletic performance.

The Two Fueling Philosophies

The core difference between the two fueling philosophies lies in their approach to energy supply:

• The Carb-Centric Philosophy: This model is built on the belief that a continuous supply of carbohydrates is necessary for all runs. The theory is that by consuming dietary carbs, you "spare" your stored muscle glycogen, saving it for when you need it most. However, research indicates this is a physiological misconception. Muscle glycogen breakdown is primarily dictated by the intensity of the muscle contraction, and consuming dietary carbs does not significantly spare those stored reserves.

• The Fat-Adapted Philosophy: This approach is a training strategy. By performing certain runs in a low-carb or fasted state, you force your body to become more efficient at utilizing its nearly limitless fat reserves. This metabolic training can lead to beneficial adaptations, such as increased mitochondrial density, which improve your body's overall ability to burn fat.

Strategic Fueling for Different Types of Runs

The fueling strategy should be periodized to match the demands of the workout.

• Easy/Recovery Runs: It is not necessary to eat before every single workout, especially for runs lasting an hour or less. Running these low-intensity sessions in a fasted or low-carb state can be a powerful tool to enhance metabolic flexibility and improve your body's ability to burn fat.

• Long Runs: While a fat-adapted runner can go further on fat alone, a strategic amount of carbohydrate intake is still recommended during long runs. By using fat as the primary fuel for the bulk of the run, you preserve your body's limited glycogen stores. This allows you to have a reserve of energy for the final, higher-intensity miles, delaying fatigue and preventing the proverbial "bonk."

• High-Intensity Workouts (Tempo runs, Intervals): These are the workouts where carbohydrates are critical. High-intensity efforts require a fast-burning fuel source that only carbs can provide. It is crucial to be well-fueled for these sessions to hit your target paces and get the most out of the training stimulus. Consuming carbohydrates before and during these key workouts is also an excellent way to practice your race-day fueling strategy.

Race Day and the Myth of Sparing Glycogen

On race day, even a highly fat-adapted runner will still consume carbohydrates. The goal of fat-adaptation is not to eliminate carbs but to decrease the body's reliance on them. This means a fat-adapted runner may be able to perform at the same pace on 30 grams of carbohydrates per hour, whereas a traditional carb-loader might require 60 grams per hour to maintain that same performance.

The main benefits of this reduced reliance include:

• Decreased GI Distress: Consuming less sugar during a race can significantly reduce the risk of stomach issues.

• More Stable Energy: By relying on a blend of both fat and carbs, you avoid the extreme highs and lows that can come from a sugar-dependent fueling strategy.

Potential Dangers and Final Takeaway

While the benefits are significant, there are potential pitfalls when trying to become fat-adapted. A runner may feel sluggish or "bonk" during the adaptation phase. It is essential to listen to your body and introduce these strategies gradually.

The ultimate advice is that fat-adaptation is a valuable tool, but it's not for everyone. It is a powerful option for runners who are looking for a new way to break through plateaus, improve their metabolic health, or find an alternative to feeling dependent on constant fueling. If you are happy with your current performance, there may be no need to change. However, for those who want to unlock their full potential, experimenting with fat-adapted training can be a game-changer.