Complete withdrawal of food, water only, for days to a few weeks. In people with excess weight or raised blood pressure, supervised fasts of one to three weeks lower weight, waist size and blood pressure, often allowing blood-pressure drugs to be stopped. Much of the loss is water and lean tissue; gains fade within months unless the diet changes. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Seated and standing blood pressure | <120/80 mmHg seated; <20 mmHg systolic drop on standing | Primary efficacy marker and the main safety limit |
| Body weight | Loss of 0.3–0.7 kg/day after day 3 | Tracks fluid shift and energy deficit |
| Fat-free mass | Loss limited to <5% of baseline | The principal cost of the intervention |
| Serum potassium | 4.0–4.5 mmol/L | Prevents arrhythmia |
| Serum sodium | 138–142 mmol/L | Detects dilutional hyponatremia from water without sodium |
| Serum phosphate | 1.0–1.4 mmol/L | The defining marker of refeeding syndrome |
| Serum magnesium | 0.85–1.0 mmol/L | Supports potassium retention and cardiac stability |
| Uric acid | <6.0 mg/dL (357 µmol/L) | Predicts and tracks gout risk |
| Fasting glucose | 70–90 mg/dL (3.9–5.0 mmol/L) | Safety marker and efficacy marker |
| Glycated haemoglobin | <5.4% | Medium-term glycemic efficacy |
| Blood or urine ketones | 2–5 mmol/L blood β-hydroxybutyrate | Confirms the fast is metabolically real |
| Estimated glomerular filtration rate and creatinine | >90 mL/min/1.73 m² | Detects dehydration-related kidney injury |
| Lipid panel | Low-density lipoprotein cholesterol <100 mg/dL | Longer-term cardiometabolic efficacy |
| High-sensitivity C-reactive protein | <1.0 mg/L | Inflammatory response, which is disputed in this setting |
| Complete blood count | Haemoglobin and platelets stable from baseline | Detects haemoconcentration and platelet activation |
Cadence: Baseline in the two weeks before the fast; daily during a supervised fast for weight, seated and standing blood pressure, heart rate, ketones and symptoms; laboratory electrolytes at baseline, at least twice weekly for fasts beyond 7 days, on the first and third days of refeeding and again at 4 weeks; follow-up laboratory work at 6 weeks and at 3–6 months