Extended fasts (24–72 h): doing them safely

Daily fasts of 12 to 18 hours are where most of the research is, and where most people get the benefits. Some experienced fasters also try occasional longer fasts of one to three days. These can be done safely by healthy adults, but they're a different challenge: your body moves deeper into fat burning, your hormones shift, and the risks grow with every day. This article explains what happens and how to approach a longer fast with care.

What happens after the first day

During the first day, your body uses up much of the sugar stored in your liver and starts relying more on fat. As the fast continues, the liver turns more of that fat into ketones. The physician-scientist George Cahill spent decades studying people during long fasts, and showed that ketones are what allow humans to survive long periods without food: they spare the body from having to break down muscle to make sugar for the brain ¹.

Your hormones adapt

Fasting is a stress on the body, and its hormones respond to protect you. One of the biggest changes is in growth hormone, which helps release fat for fuel and protect muscle. In healthy men, growth hormone was released in more frequent and larger pulses on the fifth day of a fast, and its total level over 24 hours was about three times higher than on a normal day ³.

What large studies show about safety

The largest study of longer fasts followed 1,422 people in a specialised fasting clinic who fasted for 4 to 21 days, taking around 200 to 250 calories a day under medical supervision. Adverse effects were reported by fewer than 1% of participants, and most said they didn't feel hungry ⁵. That's encouraging, but notice the setting: these people were checked every day in a clinic. It's not the same as a long fast alone at home.

Before, during and after

• 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 ⁷.
• Stand up slowly and keep activity gentle: walking is ideal.
• Break the fast with a small, easy meal, such as soup, eggs or yoghurt, and return to normal meals over the next day.

Breaking a fast of one to three days is usually simple. After longer periods with little or no food, though, restarting eating can cause dangerous shifts in the body's minerals, called refeeding syndrome. It's one of the main reasons why fasts of more than a few days belong under medical supervision ⁶.

Sources

1. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. doi.org/10.1146/annurev.nutr.26.061505.111258
2. 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
3. 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
4. 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
5. 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
6. 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
7. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941