Fasting, hour by hour
What happens after 24 hours of fasting?
Around 24 hours, your body reaches Ketosis: fat is the main fuel and ketones rise
- 0 hBlood Sugar RisingYour body is digesting your last meal and using that energy. Insulin is elevated.
- 4 hBlood Sugar FallingBlood sugar settles back to normal and insulin starts to drop.
- 12 hFuel SwitchYour liver's glycogen is getting low, so your body starts turning to fat for fuel.
- 16 hFat BurningYour body starts burning stored fat for energy. Stay hydrated!
- 24 hKetosisFat is now your main fuel, and your liver turns more of it into ketones.
What's happening in your body
During the first day your body uses up much of the sugar stored in your liver. It keeps your blood sugar steady by making new glucose 3 and turns more fat into ketones, which spare your muscles from being broken down to make sugar for your brain 1. Growth hormone rises during longer fasts 4, and resting metabolism rises slightly rather than slowing 5.
| Stage | Ketosis (24 to 36 h) |
|---|---|
| Main fuel | Fat and ketones |
| Insulin | Very low |
How you might feel
Some people feel clear and steady; others feel tired or get a headache. Listen to your body. Your hunger hormone still rises at your usual meal times and falls again on its own 6.
What helps
- Electrolytes matter now: a pinch of salt or a sugar-free electrolyte drink can help 7.
- End the fast gently if you feel unwell.
- Plan a nourishing meal with protein to break your fast.
Sources
- Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi.org/10.1146/annurev.nutr.26.061505.111258
- 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
- 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
- Ho KY, Veldhuis JD, Johnson ML, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988;81(4):968-75. doi.org/10.1172/JCI113450
- 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
- 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
- Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
- Laffel L. Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes. Diabetes Metab Res Rev. 1999;15(6):412-26. doi.org/10.1002/(sici)1520-7560(199911/12)15:6<412::aid-dmrr72>3.0.co;2-8
Keep reading
- The 24-hour fastDinner to dinner: a full day of fasting, for experienced fasters, once in a while
- What happens after 36 hours of fasting?Around 36 hours, Fasting Tracker marks Autophagy. Here's what that means, and what nobody knows yet
Extended fasts (24–72 h): doing them safelyWhat happens in your body after the first day, who should avoid them, and how to start and finish well
A note on health
Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.