From Fellrnr.com, Running tips
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Carbohydrates (carbs) are a key energy source for runners. The way carbs are used depends on the state you are in. If you are in the middle of exercise, these carbs tend to be used directly for energy. If you are at recovering from exercise, these carbs will go into quick access storage ([[Glycogen]]). If your [[Glycogen]] stores are full, then the carbs will tend to be stored as fat.
Some carbs are easily digested, with the fuel becoming ready for use quickly. These 'quick carbs' are great in the middle of a run, as the muscles (and brain) will start to burn them. If you are at rest, these quick carbs can raise the blood sugar levels quickly, causing a 'blood sugar spike'. The body reacts by producing insulin, which can overcompensate for the spike and result in a 'blood sugar crash'. Neither the spike nor the crash is good for you.
So what carbs are 'easily digested'? In some literature, quickly digested carbs are considered 'simple' and slow digesting carbs are 'complex', but this is not a useful division. The difference between simple and complex is based on the chemistry of the carb molecule - small molecules like sugar are 'simple' and big molecules like starch (bread, etc) are 'complex'. This division into simple and complex is unfortunately crap (biochemistry term meaning 'not useful').