You skip dinner, then breakfast, then lunch. About how many hours without food before your liver's sugar reserve runs out?

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After about 24 hours without food, the liver's glycogen, the stored sugar it releases to keep your blood glucose steady, is gone. From there the liver builds new glucose out of protein and starts turning fat into ketones, but it takes about three days of fasting before ketones rise enough to become a major fuel for the brain.

Skip food for 24 hours and your liver's sugar reserve is gone. Then it builds new sugar from protein pulled out of your muscle.

Source: StatPearls (NCBI Bookshelf, Physiology: Fasting); International Journal of Obesity review · checked Jul 2023