Glycogen is an important energy source that your body draws on during exercise. It can be topped off (aka: carb loading) and is replenished with carbohydrates in the form of food and/or supplements. A study published in the Journal of the International Society of Sports Nutrition examined whether high or low glycemic index (GI) carbs are better 30 minutes before training.
High GI carbohydrates digest rapidly and spike blood sugars while lower GI foods digest more slowly to provide a steady stream of glycogen replenishing carbohydrate support. When subjects consumed 1.5 grams of either high or low GI carbs per kg of body weight 30 minutes before cycling for an hour at 65% of capacity then kicking the effort up to 90%, differences in the time to exhaustion were statistically insignificant.
True Strength Moment: It should be noted that this study used healthy but untrained men. Since they hadn’t built up a lot of endurance, just about any type of workout like this would probably tire them out quickly. Still, if there’s little if any difference between the types of carbs you eat before training, go with slower burning low GI selections. They’re generally packing more dietary fiber and might help you avoid a post-workout energy crash, too.