During fasting, the body excretes more sodium, and potassium with it. Controlled studies put the sodium shortfall at roughly 30-40 mmol a day over the first few days, when the drop in insulin is steepest, and insulin is what drives both. Magnesium is the least settled of the three: the claim that it is lost too comes from low-carb diet guidance. Symptoms people blame on “not eating” are often put down to low electrolytes, though no study has tied them to measured loss. Depletion accumulates over days, so a short fast costs a fraction of what a multi-day one does; for longer fasts, adding sodium, potassium and magnesium is the standard remedy, and a 2025 review finds a physiological rationale for it and no clinical studies showing it works.
This companion covers what happens during fasting, depletion symptoms, when and how to supplement. (5 min read)
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