48-Hour Fast
Two-Day Water Fast
Evidence grade 2 of 5, Thin. Read how the Atlas grades evidence.
1 human trial, about 18 studies, ~700 participants. The written rationale for this grade is still being prepared.
- FDA
- Caution / supervision advised
- WADA
- Not prohibited
Reference only. Not medical advice.
What can go wrong
Eating behavior is sensitive. This entry is informational and not a how-to; anyone considering extended fasting, and especially anyone with a current or past eating disorder, should work with a qualified medical provider. Restrictive practices including extended fasting can trigger or worsen disordered eating in susceptible people, including restrict-then-binge cycles. Documented short-term effects of a two-day fast include orthostatic hypotension and dizziness, fatigue, headache, and shifts in sodium, potassium, and magnesium. A two-day fast is generally tolerated by healthy adults, but it is not appropriate for people taking glucose-lowering or blood-pressure medication, those with cardiovascular disease, those who are pregnant or breastfeeding, or those underweight or malnourished. Refeeding syndrome is primarily a risk after longer or more severe deprivation rather than a single 48-hour fast, but a gradual, careful first meal is still the safer approach, and unusual symptoms warrant medical attention.
A two-day water fast, often framed as the practical midpoint of extended fasting. By 48 hours the body has shifted decisively to fat and ketone fuel, with blood beta-hydroxybutyrate commonly in the low-millimolar range and rising, free fatty acids and glucagon elevated, and insulin low. Some effects often attributed to 48 hours, including a large growth-hormone rise and robust autophagy, are better documented at adjacent durations (24 hours for growth hormone) or in animals (autophagy) than measured at exactly 48 hours in people. Controlled human data at the 48-hour mark is mostly small mechanistic physiology; larger safety evidence comes from observational multi-day fasting cohorts. Generally tolerated by healthy adults with attention to hydration, sodium, potassium, and magnesium and a careful first meal, but not appropriate for everyone.
The midpoint of extended fasting. Deep ketosis, strong autophagy, manageable for many.
What people take it for
These are the reasons people give for taking it. They are not effects it has been shown to have.
What people get wrong
Often saidA 48-hour fast switches on a large growth-hormone surge that 24 hours does not
ActuallyGrowth hormone rises during fasting, but the best controlled human measurement of that rise was made at 24 hours, not 48. A randomized crossover (Horne 2025) found marked growth-hormone increases after a single 24-hour water-only fast, independent of weight loss. Attributing a distinct 48-hour growth-hormone threshold to controlled human data overstates what has actually been measured at that duration.
Often saidAt 48 hours autophagy is robustly switched on, proven in humans
ActuallyAutophagy increases with fasting in animal models, and is biologically plausible in people, but it has not been directly demonstrated in humans at the 48-hour mark. Human autophagy studies have been small and short, often measured indirectly in blood cells, and the field openly calls the evidence preliminary. This is a contested-mechanism claim, not a measured 48-hour human finding.
Often said48 hours improves insulin sensitivity
ActuallyIn the short term the opposite is the better-documented effect. A clamp crossover (Bergman/Klein 2007) found peripheral glucose uptake was significantly reduced after a 48-hour fast in both lean and obese adults, and prolonged fasting can produce an abnormal glucose tolerance test. The liver's insulin response is relatively preserved, but whole-body insulin action dips acutely during the fast and recovers after refeeding. Any longer-term metabolic benefit is a separate question from what happens at hour 48.
Often said48 hours is simply twice as good as 24 hours
ActuallyThe relationship is biological, not linear. Fuel and hormonal shifts continue across the 24-to-48-hour window, so two days is metabolically deeper than one, but 'twice as good' is not a meaningful claim and is not supported by head-to-head outcome data.
Who profits, and who pushes back
Eating-disorder clinicians
Extended fasting is a form of dietary restriction that can trigger or entrench disordered eating in vulnerable people, and is not advised for anyone with a current or past eating disorder; for them, structured fasting carries real psychological risk regardless of metabolic appeal.
Metabolic physiology researchers
At 48 hours the cleaner controlled finding is reduced peripheral insulin action and transiently impaired glucose tolerance, which complicates any simple 'improves metabolism' framing of a single two-day fast.
Evidence-appraisal skeptics
Much of what is asserted about exactly 48 hours, the growth-hormone surge, robust autophagy, is borrowed from other durations or from animals; the dedicated 48-hour controlled-trial base is small and several key trials are still in progress.
Extended-fasting community
48h is a popular monthly practice; strong electrolyte and refeeding norms.
Dr. Mindy Pelz
Sells this. Has a commercial interest in a product sold as this compound, as of July 2026.
Promotes periodic longer fasts; sells related programs. D.C., not M.D.
Where the research comes from
| Place | Share of published research |
|---|---|
| USA | 48% |
| Europe | 28% |
| Other | 24% |
Research published from: USA, Global.
How we got here
Therapeutic and ritual fasting of two days or more long predates any clinical study. Multi-day water fasting appears across cultures: in Jain practice in South Asia, in Orthodox Christian and Islamic and Jewish fasting traditions, and in Indigenous North American fasting used for vision and rites of passage. As a measured physiological object, the 48-hour fast entered modern science through mid-twentieth-century starvation and fuel-metabolism research, which mapped how glucose, free fatty acids, ketones, and counter-regulatory hormones change over the first days without food. European fasting clinics, most prominently the Buchinger tradition in Germany founded in the 1920s, built supervised multi-day fasting into a clinical service and later contributed large observational safety datasets. The specific 48-hour framing as a discrete, schedulable practice is recent and largely a product of online fasting communities in the 2010s, which positioned two days as the step between an accessible 24-hour fast and a more demanding 72-hour fast.
- antiquityMulti-day fasting practiced in Jain, Christian, Islamic, Jewish, and Indigenous North American traditions, well before clinical measurement
- 1920sOtto Buchinger establishes a supervised multi-day therapeutic fasting practice in Germany, seeding a European fasting-clinic tradition
- 1960s-1980sStarvation and fuel-metabolism research maps glucose, free fatty acid, ketone, and counter-regulatory hormone shifts across the first days of fasting
- 2001Controlled human work shows prolonged fasting alters nutrient oxidation and glucose tolerance after a normal mixed meal (J Appl Physiol)
- 2007Bergman/Klein clamp crossover directly compares a 12-hour and a 48-hour fast in lean and obese adults, finding reduced peripheral insulin action after 48 hours (AJP-Endo)
- 2010sOnline fasting communities popularize the 48-hour fast as the schedulable midpoint between 24 and 72 hours
- 2019Buchinger Wilhelmi observational study of 1,422 subjects reports multi-day fasting (4 to 21 days) was safe and well tolerated, adverse effects in under 1 percent (PLOS ONE)
- 2024-2025Dedicated controlled 48-hour fasting trials begin (e.g. NCT06737224, recruiting); a 24-hour growth-hormone crossover RCT publishes (Horne 2025); a multi-day fasting study reports transient inflammation and platelet activation (Fontana 2025)
At a glance
- Duration
- 48 hours
- Ketosis
- Deep (BHB 1-3 mmol/L)
- Autophagy
- High
- Frequency
- Monthly typical
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about 48-Hour Fast. A short entry is not a safe entry.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
Sources
Bergman BC, Klein S, et al.
American Journal of Physiology - Endocrinology and Metabolism (PubMed abstract)
Soeters MR / prolonged fasting and glucose tolerance group
Journal of Applied Physiology (American Physiological Society)
Horne BD, et al.
Masedunskas A, et al.
ClinicalTrials.gov (FASTEX, NCT06737224)
Fontana L, et al.
Wilhelmi de Toledo F, Grundler F, et al.
Frontiers in Nutrition (ketogenic metabolic therapy review)
StatPearls / NCBI Bookshelf
Equip Health (eating-disorder treatment provider)