Is Water-Only Fasting Safe?
Level 4 - case-series / case-control
Uncontrolled single-arm before-after study / case series
PubMed 34414015 · doi:10.1177/21649561211031178
What was done
Twelve middle-aged men completed an 8-day water-only fast consuming exclusively mineral water. Investigators assessed perceived stress, body composition, serum and urine electrolytes (sodium, potassium, calcium, magnesium), renal clearance parameters, serum glucose, plasma β-hydroxybutyrate, urea, uric acid, and urine characteristics (pH, osmolality, specific gravity) before and after the intervention.
What was found
The abstract reports directions of effect without numerical values or confidence intervals. After 8 days, participants experienced weight loss, reduced perceived stress, dehydration, increased ketogenesis, hyperuricemia, decreased serum glucose, and hyponatremia. Serum calcium and magnesium concentrations decreased. Renal compensatory responses included sodium, potassium, and protein sparing, accompanied by reduced daily volume of more acidic urine with increased specific gravity. All participants completed the fast safely with reported well-being, though authors noted adverse metabolic markers.
Why it matters
This study outlines the metabolic and renal adaptations to prolonged multi-day fasting, highlighting electrolyte and uric acid imbalances that may limit the safe duration of water-only fasts.
Limits
The sample was extremely small (n = 12) and limited entirely to middle-aged men. The study lacked a control group, the abstract reports no numerical values or variance metrics, and refeeding safety and longer-term recovery were not described.
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