Fasting, hour by hour
What happens after 72 hours of fasting?
Around 72 hours, Fasting Tracker marks Immune Reset: extended fasting territory, for medical supervision only
- 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.
- 72 hImmune ResetExtended fasting territory. Only continue with medical guidance.
What's happening in your body
By three days, your body is running on fat and ketones, and ketones are on their way to becoming the brain's main fuel, as they do in fasts of weeks under close medical care 1,2. Your kidneys keep losing sodium and water as long as insulin stays low 3.
| Stage | Immune Reset (72 h and beyond) |
|---|---|
| Main fuel | Fat and ketones |
| Insulin | Very low |
How you might feel
Your body is under real stress by now. Stop if you feel dizzy, confused or unwell.
What helps
- Only continue under medical supervision.
- Stop right away if you feel faint or unwell.
- Refeeding after a long fast needs care. Plan it with a professional.
Before a 72-hour fast
- Talk to your doctor first, and plan the fast with them.
- Build up gradually: be comfortable with 18 to 20 hour fasts before trying 24 hours, and with 24 hours before going further.
- Choose a quiet few days without hard training, long drives or important events.
- Drink water through the day. Longer fasts increase the loss of sodium in urine, so a pinch of salt in water can help 3.
After: breaking a 72-hour fast
Start with a glass of water and a small, easy meal, such as soup, eggs or yoghurt, eaten slowly. Return to normal meals gradually, following the plan you made with your doctor.
After several days with little or no food, the body's minerals can shift dangerously when eating resumes, a condition called refeeding syndrome 4. It's one of the main reasons why fasts longer than a few days belong under medical supervision.
Sources
- Owen OE, Morgan AP, Kemp HG, et al. Brain metabolism during fasting. J Clin Invest. 1967;46(10):1589-95. doi.org/10.1172/JCI105650
- Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi.org/10.1146/annurev.nutr.26.061505.111258
- Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
- Mehanna HM, Moledina J, Travis J. Refeeding syndrome: what it is, and how to prevent and treat it. BMJ. 2008;336(7659):1495-8. doi.org/10.1136/bmj.a301
Keep reading
- What happens after 48 hours of fasting?Around 48 hours, Fasting Tracker marks Deep Renewal. Only fast this long with medical guidance
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- Fasting stages, hour by hourWhat your body may be doing from your last meal to 72 hours of fasting, and roughly when each stage begins
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.