The 24-hour fast

What is a 24-hour fast?

A 24-hour fast skips a full day of meals, usually from dinner to dinner, so you still eat every calendar day. Fasting Tracker calls it Full Day. It's an expert plan, meant for occasional use once 18- to 20-hour fasts feel comfortable, not every day.

What happens in your body

During the first day your body uses up much of the sugar stored in your liver and relies more on fat. The liver keeps your blood sugar steady, mostly by making new glucose ⁴, and turns more fat into ketones, which your brain and muscles can use ⁵.

1. 0 hBlood Sugar RisingYour body is digesting your last meal and using that energy. Insulin is elevated.
2. 4 hBlood Sugar FallingBlood sugar settles back to normal and insulin starts to drop.
3. 12 hFuel SwitchYour liver's glycogen is getting low, so your body starts turning to fat for fuel.
4. 16 hFat BurningYour body starts burning stored fat for energy. Stay hydrated!
5. 24 hKetosisFat is now your main fuel, and your liver turns more of it into ketones.

Hunger behaves differently than you'd expect. When healthy volunteers fasted for 24 hours, their hunger hormone still rose at their usual meal times and then fell again without food, drifting lower over the day ².

MYTH

After a day without food, your metabolism crashes.

FACT

In the first days of a fast, resting metabolism actually rose slightly, by about 14% by day 3, driven by noradrenaline ³.

How to do it safely

• Stop and eat if you feel faint, confused or very weak, have palpitations, or can't keep fluids down.
• Mild headache, tiredness and feeling cold are common.

Sources

1. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254-268. doi.org/10.1002/oby.22065
2. Natalucci G, Riedl S, Gleiss A, et al. Spontaneous 24-h ghrelin secretion pattern in fasting subjects: maintenance of a meal-related pattern. Eur J Endocrinol. 2005;152(6):845-50. doi.org/10.1530/eje.1.01919
3. Zauner C, Schneeweiss B, Kranz A, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000;71(6):1511-5. doi.org/10.1093/ajcn/71.6.1511
4. Rothman DL, Magnusson I, Katz LD, et al. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR. Science. 1991;254(5031):573-6. doi.org/10.1126/science.1948033
5. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi.org/10.1146/annurev.nutr.26.061505.111258
6. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941