Carbohydrates are your body’s main source of fuel. But some carbs are high in sugar and starch, and low in fiber. That means they can cause a sudden spike in your blood sugar and energy, but just as big a drop. High-fiber carbs, like whole grain cereals and fresh fruit are digested more slowly and tend to raise your blood sugar in a slow, steady way. That helps keep your energy on an even keel throughout the day.