Consuming BCAAs during your workout replenishes your muscular energy stores and prevents catabolism (the breakdown of muscle for energy). Leucine is metabolized in your muscles for energy after the glycogen is depleted and it also signals the protein synthesis process to start. This allows longer exercise without running out of gas and feeling worn down. Combining BCAAs with a high glycemic index sugar spikes your insulin. The purpose of insulin is to transport nutrients from the bloodstream into the cells. If you eat sugar and you're not working out, guess where it goes? Fat cells. Insulin is a double edged sword, so use it appropriately.