48 hour fast stages: what happens hour by hour

The stages of a 48-hour fast, hour by hour: from ketosis through the rise of autophagy to the edge of the deep fast, what the evidence supports at hour 48, and the preparation it needs.

In brief: a 48-hour fast passes through the fed state, early fasting, lipolysis and ketosis, then spends a full day in the window where autophagy rises more clearly, and ends at the start of the deep fast. Insulin falls from around hour 4, fat release begins near hour 12, ketosis starts around hour 16, autophagy rises after hour 24, and by hour 48 growth hormone is rising and ketones supply a large share of the brain's energy. These hours are typical ranges, not thresholds.

A 48-hour fast is two full days without food. It goes a day further than a 24 hour fast and stops a day short of a 72 hour fast. It is demanding, and it is a fast to plan for: electrolytes, a quiet schedule and a gentle way to break it. The full 0 to 72 hour picture is in the fasting timeline.

What are the stages of a 48 hour fast?

A 48-hour fast spends time in five of the six phases and ends at the edge of the sixth.

Hour of the fast	Stage	What typically happens	Evidence in humans
0–4	Fed state	Insulin is elevated; glucose from the meal is used or stored (Cahill 2006)	Well established
4–12	Early fasting	Insulin falls, glucagon rises, and liver glycogen is drawn down (Cahill 2006; Anton 2018)	Well established
12–16	Lipolysis	Fat cells release free fatty acids; the liver begins making ketones (Cahill 2006)	Well established
16–24	Ketosis	Blood β-hydroxybutyrate rises past about 0.5 mmol/L and keeps climbing (Owen 1967; Newman & Verdin 2017)	Well established; levels vary
24–48	Autophagy rising	Sustained low insulin and mTOR suppression drive cellular recycling, clearer after about hour 24 in animal studies (Alirezaei 2010; Mizushima & Komatsu 2011)	Mechanism strong; human evidence limited
48	Edge of the deep fast	Growth hormone is rising over baseline, which helps preserve lean mass; ketones supply a large share of the brain's energy (Ho 1988; Owen 1967)	Established for ketones and growth hormone

What happens on the first day?

The first 24 hours are the same as a 24 hour fast: insulin falls from around hour 4, fat release begins near hour 12, ketosis starts around hour 16, and by hour 24 ketone levels are commonly around 0.5 to 1.5 mmol/L. Hunger comes in ghrelin waves at your usual mealtimes and passes if you do not eat (Cummings 2001).

What happens on the second day?

The second day is spent in the window where autophagy rises more clearly. With insulin and mTOR suppressed for a sustained stretch, cells break down damaged proteins and worn-out organelles for reuse (Mizushima & Komatsu 2011); in animal studies the signal is clearer after about hour 24 (Alirezaei 2010). In people it cannot be measured directly without a biopsy, so treat the hours as estimates; see autophagy, explained. Towards hour 48, growth hormone rises over baseline, which helps preserve lean mass against the pressure of going without food (Ho 1988), and ketones supply a growing share of the brain's energy (Owen 1967; Cahill 2006). Many people find hunger quieter on day two, while energy and electrolytes become the main challenge.

What should you drink and watch for?

Electrolytes are not optional at this length. Low insulin makes the kidneys excrete more sodium, and past 24 hours that loss drives the "fasting headache" and lightheadedness on standing; most people take roughly 2 to 3 g of sodium a day plus potassium and magnesium, as set out in the electrolyte handbook. Water, plain tea and black coffee are fine (what to drink while fasting). Faintness, confusion or palpitations are reasons to stop and eat. Break the fast gently: something small and low in sugar first, then wait one to two hours (how to break a long fast).

Who should be careful with a 48 hour fast?

It is for experienced fasters who have done several 24-hour fasts without problems. Do not attempt it if you are pregnant or breastfeeding, have type 1 diabetes, take insulin or other glucose-lowering medication, have a history of an eating disorder, are under 18, or are significantly underweight. Talk to your doctor first if you have type 2 diabetes, kidney, heart or thyroid disease, gout, a history of gallstones, or take regular medication. This page is educational, not medical advice.

Frequently asked questions

What happens during a 48 hour fast?

Over 48 hours the body moves from the fed state through early fasting and lipolysis into ketosis by around hour 16, spends the second day in the window where autophagy rises more clearly, and by hour 48 growth hormone is rising and ketones supply a large share of the brain's energy (Cahill 2006; Ho 1988).

Is a 48 hour fast good for autophagy?

It spends a full day past the 24-hour point where the clearer autophagy signal appears in animal studies, so it covers much of the window usually cited for autophagy. How much happens in a given person cannot be measured directly, and the human evidence for specific benefits is still preliminary (Alirezaei 2010; Mizushima & Komatsu 2011).

How much weight do you lose in a 48 hour fast?

Often one to two kilograms or more on the scale, but most of it is water and glycogen rather than fat, and much of it returns within days of eating normally. See fasting and weight loss for what the trials show.

Sources

1. Cahill GF Jr. "Fuel metabolism in starvation." Annu Rev Nutr, 2006. doi:10.1146/annurev.nutr.26.061505.111258
2. Owen OE, Morgan AP, Kemp HG, Sullivan JM, Herrera MG, Cahill GF Jr. "Brain metabolism during fasting." J Clin Invest, 1967. doi:10.1172/JCI105650
3. Newman JC, Verdin E. "β-hydroxybutyrate: a signaling metabolite." Annu Rev Nutr, 2017. doi:10.1146/annurev-nutr-071816-064916
4. Anton SD, Moehl K, Donahoo WT, et al. "Flipping the metabolic switch: Understanding and applying the health benefits of fasting." Obesity (Silver Spring), 2018. doi:10.1002/oby.22065
5. Mizushima N, Komatsu M. "Autophagy: renovation of cells and tissues." Cell, 2011. doi:10.1016/j.cell.2011.10.026
6. Alirezaei M, Kemball CC, Flynn CT, Wood MR, Whitton JL, Kiosses WB. "Short-term fasting induces profound neuronal autophagy." Autophagy, 2010. doi:10.4161/auto.6.6.12376
7. 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. doi:10.1172/JCI113450
8. Cummings DE, Purnell JQ, Frayo RS, Schmidova K, Wisse BE, Weigle DS. "A preprandial rise in plasma ghrelin levels suggests a role in meal initiation in humans." Diabetes, 2001. doi:10.2337/diabetes.50.8.1714
9. Related reading: the fasting timeline by hour, 24 hour fast stages, 36 hour fast stages, 72 hour fast stages, the electrolyte handbook, and how to break a long fast.