Glycogen
The body's stored form of carbohydrate, held in muscle and liver.
Glycogen is stored carbohydrate: branched chains of glucose held mainly in muscle and liver, ready to be broken back down when energy is needed. The two stores do different jobs. Muscle glycogen fuels the muscle it sits in and cannot be shared. Liver glycogen is released into the bloodstream and is what keeps blood sugar steady between meals and overnight.
It matters because it is the fuel that hard exercise runs on. Fat is the dominant fuel at easy intensities and can supply energy almost indefinitely, but as intensity rises the body leans harder on carbohydrate, and the tank is finite. Running the muscle store low is a large part of what hitting the wall feels like.
The store is also the reason for some confusing scale readings. Each gram of stored glycogen is held with roughly three grams of water, so a full tank carries a few kilograms of water with it. Start a low carbohydrate diet and several kilograms leave in the first week; go back to eating carbohydrate and they return. Neither event is fat.
It explains the flat look too. Muscles hold their shape partly through that stored water, which is why a depleted lifter looks smaller in the mirror after a hard week or a low carbohydrate stretch, and fuller within a day or two of eating normally again.
Refilling it is straightforward: eat carbohydrate. Replenishment is fastest in the hours after training, which matters if you are training twice in a day or racing tomorrow, and matters very little if your next session is on Thursday. Total daily carbohydrate does most of the work.
Fuelling long efforts is a related but separate problem, because you cannot store enough for a marathon. That is where taking carbohydrate in during exercise comes in, capped near 60 g an hour from a single sugar source and closer to 90 g an hour from a glucose and fructose mix.
In practice, if your training is hard and your sessions feel flat, look at total carbohydrate before looking at anything else. Our macro calculator sets the target once protein is accounted for.
Sources
- Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med 2014;44(Suppl 1):S25-33
- Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: nutrition and athletic performance. J Acad Nutr Diet 2016;116:501-528