Prolonged water-only fasting

What it is

Eating little or no food or caloric drinks for several days to weeks, usually 5 to 21 days, followed by a staged refeeding period. Most published data come from residential clinics where the fast is medically supervised, either water only or the Buchinger method that allows about 200 to 250 kcal a day of juice and soup.

What the research says

Evidence is limited and mostly observational, from clinics that run the programmes. Large supervised cohorts report falls in weight, waist and blood pressure and describe the fasts as safe, but a narrative review of trials found that about two-thirds of the weight lost is lean mass, weight can be fully regained within 3 months, and metabolic benefits disappeared 3 to 4 months later. Mild to moderate adverse events are very common (fatigue in about half of visits), and serious events including severe low sodium, heart attack in a man with known heart disease, and hospital admissions for dehydration occurred. Continuous glucose monitoring showed blood sugar below 70 mg/dL for two-thirds of the time by day 5 without symptoms. No randomised trial compared prolonged fasting with ordinary daily calorie restriction.

Evidence grade: Limited evidence.

The schedule

• Pattern: periodic
• Calorie limit on fast days: water only in TrueNorth and Kolnes studies; 200-250 kcal a day of juice, broth and optional honey in Buchinger programmes

Why people choose it

People choose a prolonged fast for rapid weight loss or blood pressure reduction in a supervised clinic; fasting for 5 to 20 days produced 2 to 10% weight loss. Much of the weight lost is lean mass, and one study saw all the weight regained within 3 months. Metabolic benefits were no longer seen 3 to 4 months later in the trials reviewed. Adverse events such as fatigue, insomnia, headache and nausea were common, and rare serious events included dangerously low sodium.

Cautions

• Anyone ending a fast of more than 5 to 10 days, especially with low body weight or low potassium, phosphate or magnesium: Refeeding syndrome is a potentially fatal shift in fluids and electrolytes when a malnourished person starts eating again; NICE advises a slow restart for people at high risk. (Source 1)
• People with low body weight (BMI under 18.5) or recent unintended weight loss: NICE's high-risk criteria for refeeding problems start at a BMI under 16, or under 18.5 combined with another risk factor such as more than 5 days of little intake. (Source 2)
• Older adults and people with heart disease: The serious events in supervised fasting cohorts occurred in men aged 70 to 75, including a heart attack in a man with known coronary disease and severe low sodium. (Source 3)
• People with gout or high uric acid: A gout attack occurred during fasting in a patient already treated for frequent gout, and older data link fasting to urate swings and acute attacks. (Source 4)
• People taking insulin or sulfonylureas: Blood sugar fell well below 70 mg/dL in healthy people during a week of water fasting; for people on these medicines hypoglycaemia is the most immediate risk named by an expert review. (Source 5)
• Pregnant or breastfeeding women, frail older adults, people with an eating disorder: The expert review says these groups should be dissuaded from fasting or not fast at all. (Source 5)
• Anyone considering fasting without medical supervision: The large cohort's authors say their results may not apply to people fasting without medical supervision or outside specialised facilities. (Source 4)

What goes wrong

In the same cohort, two people were admitted to hospital: a 75-year-old man with known coronary disease had a heart attack on fasting day 9, and a 67-year-old woman had vomiting, dizziness and diarrhoea; transient heart rhythm problems, low blood sugar and a gout attack also occurred. (Source 4)

• Cohort study, Certainty not rated.
• Size: 1,422.
• Who: Adults at a supervised fasting clinic.
• How long: 4-21 days.
• Result: 2 hospitalisations; sleep disturbance 14.94% (most frequent mild symptom); adverse effects reported in less than 1% overall.
• Funding: Industry-funded (clinic's research company)

Limit of this finding: Adverse effects were recorded by clinic staff; the authors note results may not apply outside specialised facilities.

A 75-year old man with known coronary artery disease had on the 9th fasting day a non-ST segment elevation myocardial infarction and received uncomplicated percutaneous coronary intervention.

In a chart review of 768 medically supervised water-only fasts, fatigue, nausea, insomnia and headache each occurred in roughly a third to half of visits. (Source 3)

• Case series, Certainty not rated.
• Size: 768 visits.
• Who: Adults 21 and older who water-fasted 2 days or more at a residential centre (TrueNorth Health Center, California), 2006-2011.
• How long: water-only fasts of 2 days or longer plus refeeding.
• Result: Fatigue 48.2%, insomnia 33.5%, nausea 32.2%, headache 30.1%, hypertension 29.2%, presyncope 28.3%; grade 3 event in 212 visits; 75% of events mild.
• Funding: not stated in the text retrieved; single-site review of the centre's own charts.

Limit of this finding: Retrospective; notes were not written to capture adverse events, so events may be under-recorded.

During the majority of visits, subjects experienced AEs that were classified as fatigue (n = 370, 48.2%), nausea (n = 247, 32.2%), insomnia (n = 257, 33.5%), headache (n = 231, 30.1%)

Two serious adverse events required hospital care: severe low sodium with speech difficulty on day 9 in a 70-year-old man, and dehydration on day 3 in a 73-year-old man. (Source 3)

• Case series, Certainty not rated.
• Size: 768 visits.
• Who: Adults at a supervised water-fasting centre.
• How long: water-only fasts.
• Result: 2 serious adverse events (grade 4 hyponatraemia; grade 3 dehydration); no deaths.
• Funding: not stated in the text retrieved.

The other was a grade 4 hyponatremia event that occurred on fasting day 9 in a 70-year-old, male patient.

In 12 healthy adults on a 7-day water-only fast, continuous glucose monitors showed glucose below 70 mg/dL for two-thirds of the time by day 5, with no symptoms reported. (Source 6)

• Cohort study, Certainty not rated.
• Size: 12 (7 men)
• Who: Healthy adults, mean age 29.7, BMI 25.
• How long: 7 days.
• Result: Time below 70 mg/dL rose from 3.0% to 66.0% by day 5 (P < .001); daily minimum glucose fell from 76 to 50 mg/dL.
• Funding: not stated in the text retrieved.

Limit of this finding: Single-arm study in young healthy adults; the lack of symptoms means low glucose may go unnoticed.

Time <70 mg/dL increased from 3.0% ± 7.1% at baseline to 66.0% ± 25.7% by day 5

A narrative review of human trials found that fasting for 5 to 20 days produced 2 to 10% weight loss, but about two-thirds of the weight lost was lean mass. (Source 7)

• Expert review, not systematic, Certainty not rated.
• Size: human trials of prolonged fasting (number not captured)
• Who: Adults in prolonged fasting trials.
• How long: 5-20 day fasts.
• Result: Weight loss 2-10%; about two-thirds lean mass, one-third fat mass.
• Funding: not stated in the text retrieved.

approximately two-thirds of the weight lost is lean mass, and one-third is fat mass

In a 10-day fast with light exercise in 16 healthy men, lean soft tissue accounted for about 60% of the 7% weight loss, though protein loss slowed as ketone production increased. (Source 8)

• Cohort study, Certainty not rated.
• Size: 16.
• Who: Healthy men.
• How long: 10 days.
• Result: Body weight -7% (-5.9 kg); lean soft tissue about 60% and fat mass about 40% of loss.
• Funding: not stated in the text retrieved.

Limit of this finding: Buchinger-type fast with minimal calories rather than water only; the authors concluded it appears safe in healthy men with physical activity.

fat mass and lean soft tissues (LST) accounted for about 40% and 60% of weight loss

What the evidence supports

In a one-year cohort of 1,422 people at a German fasting clinic, 4- to 21-day Buchinger fasts were associated with significant reductions in weight, waist circumference and blood pressure. (Source 4)

• Cohort study, Certainty not rated.
• Size: 1,422.
• Who: Adults attending Buchinger Wilhelmi Clinic, including non-obese people.
• How long: Fasts of 4 to 21 days (grouped 5, 10, 15, 20 days)
• Result: Weight, abdominal circumference and blood pressure reduced (each p<0.001); 84.4% of 404 people with health complaints reported improvement.
• Funding: Industry-funded: financed by Amplius GmbH, the research company of the Buchinger Wilhelmi clinics; lead author is on the clinic's board.

Limit of this finding: Not water only: 200-250 kcal a day of juice and soup. No control group; data entry was not blinded and was done by clinic staff.

Significant reductions in weight, abdominal circumference and blood pressure were observed in the whole group (each p<0.001).

In the same group, weight, BMI, waist, blood pressure, total and LDL cholesterol and hsCRP were still significantly lower than baseline six weeks after the fast. (Source 9)

• Cohort study, Certainty not rated.
• Size: 33 completers.
• Who: Adults with overweight or obesity without diabetes.
• How long: about 6 weeks after refeeding.
• Result: Significant decreases from baseline at follow-up in body weight, BMI, abdominal circumference, systolic and diastolic blood pressure, total and LDL cholesterol, hsCRP and fatty liver index.
• Funding: not stated in the text retrieved.

Limit of this finding: No control group; 6 weeks is a short follow-up compared with the 3-4 month data in the review above.

At the FU visit, body weight (BW), body mass index (BMI), abdominal circumference (AC), systolic blood pressure (SBP), diastolic blood pressure (DBP), total cholesterol (TC), low-density lipoprotein (LDL), hsCRP, and FLI were significantly decreased from baseline.

What the evidence does not support

The same review found that metabolic benefits were no longer seen 3 to 4 months after the fast, even when weight loss was kept off, and one trial saw all weight regained within 3 months. (Source 7)

• Expert review, not systematic, Certainty not rated.
• Size: human trials of prolonged fasting.
• Who: Adults in prolonged fasting trials.
• How long: follow-up of 3-4 months.
• Result: 6% weight lost after 5 days of water fasting was fully regained after 3 months in one study; metabolic benefits gone at 3-4 months.
• Funding: not stated in the text retrieved.

3–4 months after the fast was completed, all metabolic benefits were no longer observed, even when weight loss was maintained

In 33 adults with overweight or obesity who completed a median 14-day water-only fast and 6-day refeed, insulin resistance (HOMA-IR) and triglycerides rose by the end of refeeding and were back at baseline six weeks later. (Source 9)

• Cohort study, Certainty not rated.
• Size: 40 enrolled, 33 completed.
• Who: Adults with overweight or obesity without diabetes at a single residential centre.
• How long: median 14-day fast, 6-day refeed, 45-day follow-up.
• Result: HOMA-IR and triglycerides increased at end of refeed and returned to baseline at follow-up; no lasting improvement in insulin resistance.
• Funding: not stated in the text retrieved.

Limit of this finding: Single-arm, open-label, no control group.

Triglycerides (TG) and HOMA-IR scores, which had increased at EOR, returned to baseline values at the FU visit.

What official bodies say

NICE guidance classes people who have had little or no nutritional intake for more than 10 days as high risk of refeeding problems, and more than 5 days as a risk factor when combined with another. (Source 2)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: Adults receiving nutrition support in the UK National Health Service.
• How long: not applicable.
• Result: Recommends starting at no more than 10 kcal/kg/day and increasing slowly over 4 to 7 days for high-risk people.
• Funding: Government body (NICE, England)

Limit of this finding: Written for malnourished patients receiving nutrition support, not specifically for voluntary fasting.

little or no nutritional intake for more than 10 days

Where the research disagrees

Whether prolonged fasting is safe and worthwhile

• Buchinger Wilhelmi Clinic researchers (clinic-funded), cohort: 4- to 21-day fasting is safe and well tolerated and improves cardiovascular risk factors (Source 4)
• TrueNorth Health Center chart review, case-series: Most adverse events were mild to moderate; the protocol can be safely used in a medical setting with minimal risk of a serious event (Source 3)
• Ezpeleta, Varady and colleagues (University of Illinois Chicago), narrative-review: Weight loss is mostly lean mass and metabolic benefits do not last 3 to 4 months (Source 7)

A common belief, and what the research shows

The belief: A long water fast burns mostly fat and the health benefits last.

What the research shows: A review of human trials found about two-thirds of the weight lost is lean mass and metabolic benefits were no longer seen 3 to 4 months after the fast; one study saw all the weight regained within 3 months.

References

1. BMJ. Refeeding syndrome: what it is, and how to prevent and treat it. 2008. PMID 18583681, DOI 10.1136/bmj.a301. Read the source
2. National Institute for Health and Care Excellence (NICE). Nutrition support for adults: oral nutrition support, enteral tube feeding and parenteral nutrition (CG32), Recommendations. 2006. Read the source
3. BMC Complementary and Alternative Medicine. Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. 2018. PMID 29458369, DOI 10.1186/s12906-018-2136-6. Read the source
4. PLoS One. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. 2019. PMID 30601864, DOI 10.1371/journal.pone.0209353. Read the source
5. Nutrients. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. 2019. PMID 31003482, DOI 10.3390/nu11040873. Read the source
6. Journal of Diabetes Science and Technology. Marked Increases in Continuous Glucose Monitor-Detected Hypoglycemia During a Seven-Day Water-Only Fast in Healthy Men and Women. 2026. PMID 41724658, DOI 10.1177/19322968261421956. Read the source
7. Nutrition Reviews. Efficacy and safety of prolonged water fasting: a narrative review of human trials. 2024. DOI 10.1093/nutrit/nuad081. Read the source
8. Journal of Cachexia, Sarcopenia and Muscle. Is muscle and protein loss relevant in long-term fasting in healthy men? A prospective trial on physiological adaptations. 2021. DOI 10.1002/jcsm.12766. Read the source
9. Nutrients. A Six-Week Follow-Up Study on the Sustained Effects of Prolonged Water-Only Fasting and Refeeding on Markers of Cardiometabolic Risk. 2022. PMID 36296997, DOI 10.3390/nu14204313. Read the source