Fasting, hour by hour
What happens after 48 hours of fasting?
Around 48 hours, Fasting Tracker marks Deep Renewal. Only fast this long with medical guidance
- 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.
- 36 hAutophagyCells may start clearing out old components, a natural recycling process.
- 48 hDeep RenewalGrowth hormone levels may rise, helping to preserve muscle.
What's happening in your body
Ketones made from fat are now a major fuel, sparing your muscles from being broken down for sugar 2. Growth hormone, which helps release fat for fuel and protect muscle, rises during long fasts: in healthy men its total level over 24 hours was about three times higher on the fifth day of a fast 1. Resting metabolism doesn't crash; it rose by about 14% by day 3 in one study 3.
| Stage | Deep Renewal (48 to 72 h) |
|---|---|
| Main fuel | Fat and ketones |
| Insulin | Very low |
How you might feel
You might feel calm and clear, or weak and cold. Both are common at this length. Stop and eat if you feel faint, confused or very weak, have palpitations, or can't keep fluids down.
What helps
- Only fast this long with medical guidance.
- Avoid intense exercise.
- Ease back into eating over a day, with small meals.
The largest study of long fasts followed 1,422 people fasting for 4 to 21 days on around 200 to 250 calories a day in a specialised clinic, checked every day. Fewer than 1% reported adverse effects 4. That's encouraging, but it's not the same as a long fast alone at home.
Sources
- 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
- Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi.org/10.1146/annurev.nutr.26.061505.111258
- 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
- Wilhelmi de Toledo F, Grundler F, Bergouignan A, et al. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019;14(1):e0209353. doi.org/10.1371/journal.pone.0209353
Keep reading
- What happens after 72 hours of fasting?Around 72 hours, Fasting Tracker marks Immune Reset: extended fasting territory, for medical supervision only
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- The 36-hour fastFrom dinner to breakfast two days later: an occasional fast for experienced fasters only
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.