Water Fasting (Longo, Fung)

Water-only fasting means going without food or calories, taking only water, for a day or longer.

Evidence Score

52/100

Developer

Jason Fung, Valter Longo

Duration

Set only with a licensed clinician

Steps

6 phases

Used for

3 conditions

Protocol Overview

Water-only fasting means going without food or calories, taking only water, for a day or longer. This page describes what people do, what the research shows and the risks, so you can talk it through with a licensed clinician. It is not a home program, and it gives no fasting schedule.

Used For

ObesityType 2 diabetes (with prescriber oversight)Metabolic health

Obesity / Weight Loss Resistance →Type 2 Diabetes Reversal →Cancer (Integrative Support) →

Key Resources

The Complete Guide to Fasting by Jason Fung, The Longevity Diet by Valter Longo

Safety Profile

Contraindications & Risks

Do not water fast: pregnancy or breastfeeding; children and teens under 18; underweight (BMI under 18.5) or recent unintended weight loss; a current or past eating disorder; type 1 diabetes

Only with your prescriber's plan, never on your own: type 2 diabetes on insulin or sulfonylureas; any blood pressure medicine or diuretic; any medicine that must be taken with food

Only with a clinician, and often not at all: kidney disease, gout, heart rhythm problems or heart failure, low sodium or other electrolyte problems, liver disease, older age with frailty, heavy alcohol use

Refeeding syndrome risk rises with longer fasts and in anyone undernourished, underweight or drinking heavily

Cancer: fasting around treatment is early research; only with your oncology team, and never delay treatment

Important Notes

Evidence: low-moderate. The largest safety report, a 2018 chart review of 768 supervised water-only fasts of 2 days or more at one US center, found most side effects were mild to moderate and known reactions to fasting. Still, 212 visits had at least one severe (grade 3) event, and 2 people needed hospital care: one for dehydration on day 3 and one for dangerously low sodium on day 9. The authors are tied to that center. A small 2022 study of median 17-day supervised fasts found lower blood pressure, waist size, LDL and CRP at the end of the fast, but triglycerides and insulin resistance went up, and only 26 of 48 people finished. Autophagy research earned the 2016 Nobel Prize (Yoshinori Ohsumi, in yeast); how much fasting boosts it in people is not known. Long controlled trials of water fasting are lacking. If your goal is weight or blood sugar, time-restricted eating and supervised low-calorie programs have more research behind them; ask a licensed clinician which fits you.

Find a Practitioner

Gabriel can find and book a practitioner who works with this approach.

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EvidenceLow-Moderate

Evidence Score52/100

Categoryfasting

DurationSet only with a licensed clinician

About Evidence Score

The Evidence Score (0 to 100) rates the strength of evidence from studies in people for the uses listed: higher means stronger, more consistent research. It does not measure safety. It is not medical advice.

For education, not medical advice. Talk with a licensed clinician before you start, and never stop or change a prescribed medication without your prescriber.