What is glycogen?
Glycogen is the form in which your body stores sugar, packed in your liver and muscles as a quick-access energy reserve.
Think of glycogen as the cash in your wallet, ready to spend right now, while body fat is your savings account. You use the wallet first, and only dip into savings in a big way when the wallet runs low.
Why it matters for aging
Your liver's glycogen keeps blood sugar steady between meals, and it's the main reason you don't run out of energy overnight. Muscle glycogen fuels your muscles directly, especially during hard exercise.
How long liver glycogen lasts depends on activity and what you ate, but it typically runs low somewhere around 12 to 24 hours without food. That's one reason the deeper effects of fasting take longer than many people expect.
A common myth
Once you skip a meal, your body immediately starts burning fat.
First it spends stored glycogen, especially from the liver, to keep your blood sugar steady. Only as those stores run down, usually after many hours without food, does fat burning and ketone production really ramp up.
Go deeper
Deep dives
- Does intermittent fasting slow aging? What the trials say
- Does coffee break a fast? It depends what you're fasting for
Related terms
Updated Nov 9, 2026 · All terms