Optimal fueling is vital for performance, especially if you’re tackling longer distances like a half marathon , marathon , or even an ultramarathon. But while there is now a plethora of energy gels, drinks, and chews on the market, it can be hard to figure out exactly when and how much fuel you need to take. That’s on top of knowing what to eat before and after a training session, too. “Glucose is the currency our body uses for carbohydrates,” says David Lipman, a health and human performance specialist. “It’s stored in the muscles and the liver as glycogen, and then we mobilize it to use it as glucose.” During moderate to intense activity, glucose is the body’s preferred fuel—and there’s plenty of evidence to show that taking in carbs during exercise improves endurance. Now, I’ve run a lot of marathons, and fueling is still one of the things I find difficult to nail—particularly as someone whose early 20s were dominated by diet culture, which advocated for eating less, rather than more.…