Extended Fasting (24–72 h).

In the first 12 to 24 hours, the body depletes liver glycogen and transitions to fat oxidation as the primary fuel source. By 24 to 36 hours, ketone production is measurable and rising. By 48 to 72 hours, the body is in sustained nutritional ketosis, autophagy markers are elevated in animal studies (human data is limited), and growth hormone levels are significantly increased. Insulin levels drop to baseline. These are the physiological changes that drive the research interest in extended fasting — and also the changes that create the medical risk profile.

Extended fasting carries real risks that scale with duration: electrolyte imbalances (sodium, potassium, magnesium), orthostatic hypotension, cardiac arrhythmia in susceptible individuals, refeeding syndrome when breaking the fast improperly, and dangerous interactions with medications — particularly insulin, sulfonylureas, blood pressure medications, and anticoagulants. Anyone fasting beyond 24 hours should inform their physician. Anyone fasting beyond 48 hours should be under active medical supervision.

Type 1 diabetics, type 2 diabetics on insulin or sulfonylureas without physician supervision, pregnant or breastfeeding women, anyone with a history of eating disorders, anyone underweight (BMI under 18.5), children and adolescents, anyone on medications that require food intake, anyone with active cardiac disease, and anyone with a history of refeeding syndrome. This is not an exhaustive list — the default position should be to consult a physician before any fast beyond 24 hours.

Refeeding after an extended fast is where the medical risk is highest. The standard protocol is to break with a small, easily digestible meal — bone broth, a small portion of cooked vegetables, or a light soup — and wait 60 to 90 minutes before eating more. Avoid large meals, high-sugar foods, and heavy starches in the first meal. For fasts beyond 48 hours, the refeeding window should extend over 24 to 48 hours of gradually increasing meal size and complexity.

Most evidence-informed practitioners recommend extended fasts no more frequently than once per month for 24-hour fasts, once per quarter for 48-hour fasts, and once or twice per year for 72-hour fasts. More frequent extended fasting risks lean mass loss, hormonal disruption (particularly in women), and disordered eating patterns. A practitioner who recommends weekly 72-hour fasts is operating outside the evidence base.

Have you consulted your physician about fasting beyond 24 hours? What is your refeeding protocol? Are you monitoring electrolytes during the fast? Do you have any medications that interact with fasting? What is your plan if you feel unwell during the fast — do you know when to break early? Are you tracking any biomarkers to assess the effects?