Carbohydrates, which includes starches as well as sugars, are the major energy source for the body. All of them are converted to glucose (blood sugar) which is the major source of fuel for your brain, but pretty nasty to the rest of you in quantity, binding with various proteins and wreaking havoc along the way. Therefore, the body works very hard to keep blood glucose levels within a very narrow range. Here’s how it works. When blood glucose rises, insulin is secreted and the glucose is put first into muscles and the liver, then into fat cells. Insulin’s main purpose is to clear the blood of sugar, and it is needed for growth (though it is not a growth hormone). What’s more, a certain amount (not much!) of insulin is still required for body cells to use blood sugar (remember this fact for later). All the cells of the body, except the brain, can run indefinitely on either glucose or fatty acids. The brain only runs on glucose (about 120 g/day) and in extreme circumstances, ketones which are usually only produced during starvation. The liver releases stored glucose as needed. In other words, you can normally and probably should be using up fat. (This is why those running machines with a “fat burning” setting on them annoy me so much!)
A bit of physiology is in order too. Your blood cannot supply your muscles with enough energy for athletic activities, since it would have to have the chemical composition of cooking oil. (At peak, I burn off roughly 1200 – 1500 calories per hour.) Instead, the muscles store sugar and release it as needed. You have enough for roughly two plus hours of high-level activities or about 2,000 calories. Highly conditioned athletes have more. Long distance runners are well aware of this limit and refer to it as “hitting the wall” or “bonking” meaning that quite suddenly they are in an impaired state and have a hard time continuing their run. The cycle then is that as soon as you eat, muscles soak up the sugars. This has a very high priority in the body as a simple matter of survival: If muscle glycogen gets depleted, you cannot outrun that sabre-toothed kitty at all. This is so important that if you fail to eat after a heavy activity, you will tend to get very sleepy and probably take a nap, like it or not. This is actually almost, but not quite, passing out from sport-induced hypoglycemia (low blood sugar) where the muscles have such a priority recharging that you shut down until it happens.
Training tip. Since sugar is stored in the muscles, sports performance is mostly independent of initial blood sugar levels. Most athletes know that feeling a bit pokey, as long as they have adequately watched over their nutrition, will almost miraculously abate within a few minutes once they start training. Putting yourself on a training schedule so that you know you are able to perform well is a crucial step in consistency. If you must rely on motivation to get you to the gym you will tend to miss more training.