Magnesium
Evidence: weak
Magnesium matters physiologically and a real deficiency impairs performance, but in runners who already eat enough, supplementing does little: no endurance benefit, no reliable help for cramps, and only weak, mixed evidence for sleep. The marketing runs far ahead of the data. Fix a genuine shortfall with food first; a supplement is for a measured deficiency, not a default.
Magnesium is one of the most heavily promoted supplements in running, sold for cramps, sleep, recovery and performance, usually on the claim that “most people are deficient”. It is a real and important mineral, involved in hundreds of reactions including energy metabolism, nerve conduction and muscle contraction. The gap is between that physiology and what a supplement does for a well-fed runner, which is the same pattern the bundle applies to any supplement claim.
Who is actually at risk
A genuine magnesium deficiency does impair exercise, so the mineral is not irrelevant. The question is who is short of it. Frank deficiency is uncommon in people eating a varied diet, but some runners are more exposed: those on restricted or low-energy diets, those in low energy availability or RED-S, and, to a smaller degree, heavy trainers, because exercise raises both urinary and sweat losses (Heffernan et al. 2019). The risk tracks poor or restricted intake more than the act of running itself. This is the real lever: identifying and correcting a shortfall, ideally through food.
Performance: no endurance benefit
In runners who are not deficient, supplementing magnesium does not improve endurance. A systematic review found no evidence that magnesium enhances endurance capacity, and only modest, inconsistent and status-dependent signals for some strength and power measures, mostly in untrained people (Heffernan et al. 2019). It is not among the few supplements with consistent performance evidence in the IOC consensus. For a runner eating adequately, more magnesium does not buy more performance.
Cramps: the marketing’s weakest claim
The cramp claim is the one magnesium is sold on hardest and supported on least. A Cochrane review concluded that magnesium is unlikely to be effective for ordinary muscle cramps at any dose, and found no trials at all in people who cramp while exercising (Garrison et al. 2020). This fits the wider picture that exercise-associated cramp is driven by fatigue-related neuromuscular control, not electrolyte depletion (Schwellnus). Taking magnesium to stop running cramps is treating the wrong mechanism.
Sleep: a thin signal
The sleep claim is the most defensible of the marketed uses, but only just. A systematic review found mostly positive but inconsistent results across small, heterogeneous trials of varied dose and form, and judged magnesium likely useful mainly in people with low baseline status (Rawji et al. 2024). The observational links are stronger than the trial evidence. If sleep is the goal, the high-return habits there matter far more than a magnesium tablet.
Practical guidance
- Food first. Adults need roughly 300 to 420 mg a day, well covered by leafy greens, nuts, seeds, wholegrains, legumes and dark chocolate. The higher food volume that training appetite drives makes a shortfall less likely, not more.
- Supplement only a likely deficiency, ideally one flagged by a restricted diet, low energy availability or a blood test rather than guessed at, and bear in mind serum magnesium is an imperfect marker of body stores.
- Mind the form and the dose. Magnesium oxide is poorly absorbed; glycinate and citrate are better tolerated. High doses, especially oxide and citrate, cause diarrhoea and nausea (Garrison et al. 2020), and routinely megadosing is pointless if you are already replete.
Keep it in proportion
Magnesium is cheap, generally safe at sensible doses, and worth correcting if your diet is genuinely short. But it is not a performance aid, a cramp cure or a substitute for sleep and adequate fuelling, and the confident marketing to that effect is not backed by the evidence.