Fueling Calculator

During endurance exercise, the body primarily relies on glycogen (stored carbohydrates) for energy. As exercise continues, glycogen levels gradually deplete. Fueling with carbohydrates allows you to replenish glycogen stores so you can exercise longer and harder. 

The fueling calculator provides personalized carbohydrate intake recommendations based on science-backed principles. It primarily considers these three factors:

• Duration: The longer you exercise, the more your glycogen stores will become depleted. The calculator estimates how much glycogen you’ll burn based on the duration of your session.
• Intensity: Higher-intensity exercise depletes glycogen faster. The calculator adjusts recommendations depending on how hard you plan to exercise.
• Absorption Limits: Your body can only absorb so much fuel at a time. Research shows that people can absorb roughly 60 g of glucose plus 30 g of fructose per hour, for a maximum of around 90 g of carbohydrates per hour.

Fueling during endurance exercise primarily is limited by how many carbohydrates the gut can absorb per hour. However, bodyweight still plays a role. 

Heavier or larger athletes typically need more energy to produce the same power output or speed because they have more mass to move. Even though their gut absorption limits may be similar, bigger athletes may need more fuel than smaller athletes to meet their energy demands.

There is also some evidence showing that larger athletes oxidize more carbohydrates per hour than smaller athletes. 

Yes, the fueling calculator provides personalized guidelines for all endurance sports including cycling, running, ultra-trail, and triathlon. However, adjustments may be needed depending on the sport. For example, triathletes cannot consume fuel during the swim portion, so intake should be planned for before or after the swim.

Fueling is typically not needed for sessions lasting less than 90 minutes because your body’s stored glycogen provides enough energy. However, there are some exceptions where fueling for shorter, easier sessions is beneficial — such as if you have a very hard session the next day or it’s a recovery session. Learn more here. 

When you begin exercising (especially if you've eaten a carb-rich meal beforehand), your muscle glycogen and liver glycogen stores are still relatively full. These are your body's preferred fuel source at the beginning of endurance activities, especially at moderate to high intensities.

So during the first hour, your body can mostly rely on internal fuel, and you don’t need to ingest a large amount of fuel immediately. As the session goes on and glycogen stores start to decline, your body begins to rely more on ingested carbohydrates to maintain blood glucose levels and energy output. That’s why the fueling calculator recommends increasing amounts after the first hour. 

Further, fueling with smaller amounts of carbohydrates in the first hour helps your gut adapt. If you fuel with large amounts immediately, you might experience some GI distress such as bloating or nausea. 

While it’s possible that some athletes can absorb more than 90 grams of carbohydrates per hour, there is currently no evidence showing that fueling with amounts higher than this improves performance. Because of this, the fueling calculator only recommends up to 90 grams per hour, even during long, intense sessions.