MECHANISM ENGINE

What Is Glycogen?

Took me a while to get my head round this one. I assumed all stored energy was the same thing, just fat, in different amounts. It isn’t. There are two separate stores, and they behave completely differently.

The Glove Box

Glycogen is the glove box. Small, quick to reach, holds maybe a day’s worth of go-to essentials. It’s the first thing your body reaches for when it needs fuel in a hurry, first few hours of a fast, first few minutes of exercise. Body fat is the boot, or further back, the garage. Much bigger capacity, harder to get into quickly, but there’s weeks of reserve sitting in there. Most people spend their entire life topping up the glove box and never once open the garage door.

Glycogen is glucose, chained together and stored mainly in the liver and muscles. Liver glycogen keeps blood sugar steady between meals and overnight. Muscle glycogen fuels the muscle it’s stored in. Total glycogen stores are small, roughly a day’s worth of typical energy use. Once glycogen starts running low, usually somewhere in the first 12 to 24 hours without food, the body shifts toward drawing on fat stores instead. That shift is the switch fasting is really about.

NHS and NICE materials on carbohydrate metabolism describe glycogen as the body’s short-term energy reserve, distinct from longer-term fat storage, with depletion timing varying by activity level and prior diet.

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Related mechanisms

Educational only, not medical advice. Speak to your GP before changing your fasting pattern, diet, or medication.

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What this is based on

NHS and NICE guidance, cross-referenced against Neil’s own NHS blood test record going back to April 2012. See the full timeline and current numbers.