What happens after 20 hours of fasting?

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!

What's happening in your body

Insulin has been low for most of the fast, and your fat cells are releasing fatty acids for your muscles and heart ^1,2. Much of your liver's glycogen has been used, and it keeps your blood sugar steady mostly by making new glucose ³. Ketone levels keep climbing from here, and over the next day they become a bigger share of your brain's fuel ⁴.

Stage	Fat Burning (16 to 24 h)
Main fuel	Mostly stored fat
Insulin	Low

How you might feel

Many people feel lighter and more stable once hunger settles, and energy often comes back. Others feel tired or a little cold by now. Hunger still comes in waves around your usual meal times and passes on its own: in people who fasted for a full day, the hunger hormone rose before each missed meal and then fell again ⁵.

20 hours and the 20:4 plan

A 20-hour fast with a 4-hour eating window is the 20:4 plan, called Warrior in Fasting Tracker: one big meal and a snack. In adults with obesity, a 4-hour window led to about 3% weight loss in 8 weeks, mostly because people ate less, but it didn't do clearly better than a 6-hour window ⁶. So the extra hours cost more effort than they add. See the 20:4 plan for sample schedules.

Water, salt and when to stop

While insulin stays low, your kidneys let go of more sodium and water ⁷, which is one reason long fasts can bring a headache or lightheadedness. A pinch of salt in a glass of water often helps, and a headache from too little water usually eases within half an hour of drinking ⁸.

• Stop and eat if you feel dizzy, shaky, confused or unusually weak, or have palpitations.
• Stand up slowly, and avoid hard training or long drives late in the fast.

Breaking a 20-hour fast

You don't need a special meal after 20 hours. Make it a balanced one with plenty of protein: your body keeps using a large serving for many hours ⁹, which matters when your window has room for only one or two meals. Aim for about 1.6 g of protein per kilo of body weight a day if you train ¹⁰.

Sources

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2. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. doi.org/10.1056/NEJMra1905136
3. 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
4. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi.org/10.1146/annurev.nutr.26.061505.111258
5. 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
6. Cienfuegos S, Gabel K, Kalam F, et al. Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity. Cell Metab. 2020;32(3):366-378.e3. doi.org/10.1016/j.cmet.2020.06.018
7. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
8. Blau JN, Kell CA, Sperling JM. Water-deprivation headache: a new headache with two variants. Headache. 2004;44(1):79-83. doi.org/10.1111/j.1526-4610.2004.04014.x
9. Trommelen J, van Lieshout GAA, Nyakayiru J, et al. The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans. Cell Rep Med. 2023;4(12):101324. doi.org/10.1016/j.xcrm.2023.101324
10. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376-384. doi.org/10.1136/bjsports-2017-097608
11. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873