The glycemic index (GI) is a scale that rates carbohydrate-carrying foods from 0 to 100 based on their direct effect on blood-sugar levels. Foods that are high on the GI scale are digested quickly and spike sugar levels in the blood; low-GI foods take longer to digest and allow sugar to leak slowly into the bloodstream, providing a more constant and even source of energy. Most veggies, fruits, nuts, legumes, and healthy grains are low to medium on the scale, while white bread, white rice, and sweetened foods (like candy or soda) are much higher. Typically, high-GI foods are given a bad rap because of the energy ups and downs, but for an athlete, there are times when you need that rush. They can be lifesavers for endurance athletes, including climbers. The rule of thumb is to eat foods with low to medium numbers before exercise, and then medium to high foods during exercise, varying from a chocolate chip cookie to a low-sugar sports drink. The most important rule is to eat something high on the scale 15 to 30 minutes after your workout because this is when the enzymes that help the body replenish muscle glycogen are most active, and the longer you wait, the longer it will take those muscles to recover. Keep a bagel and almond butter (or some leftover chicken fried rice) on hand so you can give your body what it needs on the drive home from the crag or the gym. Keep in mind that it’s ill-advised, if not impossible, to base your entire diet on GI numbers. For example, eating a steady stream of foods like pizza, beer, and ice cream every day, while fun and a good mix on the GI, won’t likely help your performance—or harness size. Use these numbers as guidelines, especially when you know you’ll be needing an extra boost during a long day in the mountains or after a hard workout.