Eating high glycemic index foods will spike blood sugar and insulin levels allowing insulin to convert CHO (glucose) into fatty acid (fat). Futhermore, high CHO intake causes high increases in blood sugar and then an insulin “rush” which then lowers your blood sugar. Because the brain uses glucose as its only fuel, this “drop” in blood sugar causes mental fatigue, a decrease in mental alertness and a “tired and sleepy feeling” associated with low blood sugar.
High insulin levels inhibit (blocks) the release of HGH (human growth hormone). Eating high amounts of CHO one hour before or immediately after a workout will inhibit the release of this needed HGH. Also high levels of insulin limits other things like IGF (insulin growth factor I & II) which interacts and reacts at the muscular level with testosterone and other local growth factors to stimulate muscle protein synthesis (growth).
That is from The Science of Hurdling and Speed, by Brent McFarlane
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