Step 5 The economy · 6 min

Energy & storage

ATP, glycogen and body fat: where the fuel goes once it is inside you.

Food isn't energy your cells can use directly. It's converted into one universal currency, , a tiny rechargeable battery your body spends and refills all day long.

is the charged battery. Use it and it becomes , the flat version. Energy from food charges it back up. This happens billions of times a second.

Anything you don't burn right away gets stored in one of two places. Push the pace into hard exercise and watch which one runs out first.

Your energy stores, drawn to scale

FAT ≈ 100,000 kcal

That orange sliver on the left is all your carb storage (~1,800 kcal). Fat storage is on another scale entirely.

Glycogen

quick carbs

1,800kcal left

Body fat

deep reserve

100,000kcal left

Hours of hard exercise 0 h

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Working hard, you burn mostly glycogen. At this pace it runs out in about 3 hours, the point runners call hitting the wall. Then your body shifts to fat.