Glucose & Glycogen 101

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Glucose is sugar in your blood. Glycogen is how your body stores that sugar in muscles and in the liver. Muscles burn glucose for fast energy and refill their tanks by storing glycogen between sessions. When you train, you drain those tanks. How you eat, hydrate, and recover determines how quickly you refill them and how well your muscles adapt.

Powering your training

Glucose circulates in your bloodstream and feeds working muscle fibers. Your body keeps it in a tight range with hormones like insulin and glucagon. During exercise your muscles can pull in glucose more easily through contraction driven pathways, which is one reason regular training improves blood sugar control across the day.

Energy reserves inside your muscles

Glycogen is chains of glucose packed inside muscle cells and in the liver. Muscle glycogen fuels muscle only. Liver glycogen keeps blood glucose steady for your brain and other tissues. Each gram of glycogen stored carries several grams of water with it, which is why you look and feel fuller when stores are topped up. 

Easy efforts lean on fat with a smaller share of carbohydrate. Moderate to hard efforts pull more from glycogen. Sprints and heavy sets depend mostly on carbohydrates because glycolysis can deliver ATP quickly. When glycogen is low, power output, total volume, bar speed, and repeat sprint ability tend to fade sooner. 

What training does to your stores

Training raises your capacity to store glycogen and to use it efficiently. Endurance work increases enzymes that burn carbohydrate and fat. Resistance training increases the number and size of glycogen granules in fast twitch fibers and can deplete glycogen meaningfully even in short sessions.

Signs of low glycogen

Work feels heavier than it should. First sets look normal but following sets collapse. You lose the pump and appear flatter. Morning scale weight drifts down quickly with a hard training block, which can reflect glycogen plus water loss rather than fat loss, given the water bound to stored glycogen.

A quick note on carb loading history

The idea that starting a session with higher glycogen extends endurance capacity goes back to the biopsy era. Diet manipulation after exhaustive exercise created large differences in muscle glycogen within the same person, and higher starting glycogen supported longer performance. That principle underpins today’s fueling strategies even for strength athletes who care about total quality reps.

CGMs, briefly

Continuous glucose monitors measure glucose in the fluid between cells and stream data to an app. Interest among athletes has grown, and early reviews describe potential uses such as spotting hypoglycemia in long sessions, personalizing fueling, and linking patterns to sleep or training load. The same reviews also note limits, like sensor lag during hard changes, variable responses between people, and a thin evidence base for performance gains in non diabetics. Governing bodies have even restricted race day use in some sports, which keeps the focus on training insights rather than in competition advantages. For now, treat CGMs as a learning tool if you choose to try it, not as a score to chase. We will dive deeper in a separate post.

Why you should care

Glucose and glycogen are part of the building blocks of hard training. Manage them well and you lift heavier, complete more quality reps, recover faster, and look fuller. Keep your eye on consistent fueling, smart hydration with sodium, and regular training that teaches your muscles to store and spend energy efficiently. Your muscles will repay that attention with better workouts and better results.

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