Nutrition: Fuelling for Endurance *Tips for female cyclists and runners

When it comes to endurance sports, there’s often confusion about how the body uses fat and carbohydrates for energy. A common myth you may have heard from commentators is that athletes “switch to fat metabolism” during long-duration exercise. However, the reality is that both fat and carbohydrates are used simultaneously, with their contribution to energy production varying depending on exercise intensity.

Carbs vs. Fat: How the Body Really Works

At lower exercise intensities, the body relies more on fat for energy, while carbohydrates become the primary energy source as intensity increases. For female endurance athletes, this dynamic is even more fascinating. Research has consistently shown that females rely more on fat stores—such as intramuscular lipid (IMCL)—to fuel their energy needs during exercise compared to males.

This reliance on fat allows women to spare precious carbohydrate stores, including liver and muscle glycogen. Why is this important? Glycogen depletion is directly linked to fatigue, and the ability to conserve glycogen enables female athletes to maintain energy levels for longer. This makes women naturally more fatigue-resistant and better equipped to sustain performance during long-duration events like marathons and cycling races.

While this physiological advantage highlights the importance of fat oxidation, carbohydrates still play a crucial role, especially during high-intensity efforts. Even with slower glycogen depletion, female athletes still need to prioritize carbohydrate intake to support bursts of intensity and avoid running out of energy reserves in the latter stages of an event.

There’s no actual “switch” that flips from carbs to fat metabolism, as many might think. Both processes are active all the time, but the proportion of energy coming from each changes. So, while fat oxidation peaks at around 65% of your VO2 max (which could be a moderate effort on a long ride), carbohydrate use increases significantly as you ramp up the intensity.

What This Means for Your Nutrition Strategy

As a female endurance athlete, it’s crucial to recognize that:

• Carbs Are Essential for High-Intensity Efforts: Especially in the later stages of a marathon or during sprints on a bike, muscle glycogen—your body’s stored form of carbohydrate—will be your primary fuel source. Ensuring your glycogen stores are topped up before training and races can help you avoid hitting the dreaded wall.

• Fat Is Still Important for Long, Low-Intensity Training: During longer, slower efforts, your body will rely more on fat for energy. Including healthy fats in your diet can support this, but remember that fat alone won’t sustain high-intensity efforts.

• No Need to ‘Switch’ to Fat Burning: Since your body never truly switches off carbohydrate metabolism, strategies aimed at entirely switching to fat burning—such as low-carb diets—aren’t always ideal for female athletes looking to perform their best. Instead, balance is key.