Fueling Strategies for Endurance: Why It’s Not Just About Calories

When it comes to endurance sports like running and cycling, athletes often wonder: is the goal to maintain glucose levels or replace calories? The answer is both simple and nuanced. While replacing calories sounds logical, the primary objective during endurance activities is to maintain stable blood glucose levels—not to break even on calories.

Why You Can't Fully Replace Calories During Exercise

Endurance activities burn calories at a rate much faster than your body can digest and absorb fuel. For example, a marathon runner might burn 2,500-3,000 calories during a race but can realistically only consume about 800-1,000 calories through gels and sports drinks. Trying to replace all calories during exercise isn’t just impractical—it can lead to gastrointestinal distress, slowing you down more than helping.

Instead, the focus is on providing enough carbohydrates to keep energy levels stable and prevent “bonking” or hitting the wall. Post-exercise recovery is the time to worry about full calorie replenishment.

Is Fueling Necessary if Glucose Levels Feel Stable?

You might think that if your glucose levels feel steady, there’s no need to fuel. However, by the time you feel a dip in energy, it’s often too late. Your muscles may have already started depleting glycogen, making it harder to recover mid-effort. Regular fueling helps spare muscle glycogen, reduce perceived exertion, and delay fatigue—even if you feel strong early on.

Skipping fueling can work for short, low-intensity efforts, but for anything longer or more intense, proactive fueling keeps you ahead of the energy crash.

What About Hard, 1-Hour Efforts?

For hard, 1-hour tempo or threshold efforts, you might not need to fuel if you’ve eaten a high-carb meal beforehand. Your body has enough glycogen to sustain high-intensity exercise for about 60-90 minutes.

However, taking in 20-30 grams of carbs mid-effort can boost performance, particularly toward the end. Even a simple carb mouth rinse can activate brain pathways that reduce perceived exertion and help you maintain output.

How Long Can Your Liver Produce Glucose?

The liver stores about 80-100 grams of glycogen and plays a key role in maintaining blood glucose. At moderate intensity, liver glycogen can last 90-120 minutes, but at higher intensities, it depletes faster—within 60-90 minutes. When liver glycogen runs out, blood glucose drops, leading to fatigue and a sharp performance decline.

To extend liver glucose production, eat a high-carb meal 2-3 hours before exercise and consume 30-60 grams of carbs per hour during longer efforts.

Why the 2-Hour Mark is a Tipping Point

Most athletes hit a significant drop-off around the 2-hour mark because both liver and muscle glycogen stores are running low. If athletes haven’t fueled consistently from the start, performance will suffer.

To push past this wall, start fueling 15-30 minutes into the workout, even if you feel good. Aim for 30-60 grams of carbs per hour (up to 90 grams if your gut is trained to handle it). This proactive approach can make the difference between fading out or finishing strong.

Strategic Fueling

Endurance fueling isn’t about matching every calorie burned. It’s about strategic carbohydrate intake to maintain energy, prevent fatigue, and optimize performance. Whether you’re training or racing, the right fueling strategy can help you stay strong from start to finish.