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Originally Posted by kstreeter513  Simply put, the more glycogen you can store in your muscles, the longer it takes for fatigue to set in, meaning a longer workout, ... |
It is a bit more complicated than this. Anaerobic exercise (pushing weights, sprinting etc.) raises adrenalin levels, which in turn causes your alpha cells to spit out glucagon. In the absence of glucagon, glycogen stays put. So if you are running or walking, you won't be using glycogen. And the amount stored in your muscles won't determine your ability to keep going.
Maybe it will help to look at some numbers here. Your liver can store about 120 grams of glycogen and your muscles can store about twice this amount, or another 240 grams. That makes a total of 360g grams of glycogen, or 1,440 calories. This is not a lot. If you are using, 500 calories an hour, you would run out of energy from this source completely before three hours are up. This is why fat becomes the main source energy soon after you start exercising.
Every Kg of fat that you carry can be turned into 9,000 calories of energy. And the reality is that about 70% of the calories you expend during an aerobic workout come from fat. This may come as surprise to you (it certainly did to me). The remaining 30% comes from glycogen and glucose in the blood stream.
People typically have access to about 80 grams of glucose in the bloodstream, or about 240 calories. This what you use when you start exercising. When this runs out, runners often "hit the wall". At this point the body switches to keytones as the primary source of energy. The more accustomed to burning fat the metabolism is, the smoother this process is. But from then on, consumption of glucose drops right off, except during anaerobic spurts. As a T1, you may have noticed that your blood sugar drops fast during the first twenty minutes and then stabilises. That is because not nearly as much glucose is being used up.
It is hard to know how much of our glycogen we actually use. It depends on the nature and the intensity of the exercise. But if we assume that 75% of muscle glycogen is used during training, that is still only 180grams (240 x .75). So much of the 300 grams of carb you eat on a "hard day" gets turned into fat, simply because it has nowhere else to go.
Our metabolism adjusts to whatever source of energy we give it. Which is why you lack energy if you
don't eat lots of carbs. But it doesn't has to be that way if you don't want it to be.
