Magnesium for leg cramps and performance: does it work?

Magnesium is the go-to remedy for muscle cramps, and it is often sold as a recovery and performance mineral for runners. It is genuinely essential for muscle and energy metabolism. But the trials testing magnesium tablets for cramps, and for performance in people who already get enough, are disappointing.

What it is

Magnesium is a mineral involved in hundreds of enzyme reactions, including making ATP (the cell's energy currency), muscle contraction and relaxation, nerve signalling and heart rhythm. It is found in leafy greens, nuts, seeds, legumes, whole grains and dark chocolate. A small amount is lost in sweat. Supplements come in several forms, such as citrate, glycinate and oxide, which differ in how well they are absorbed and how likely they are to upset the stomach.

Does it raise VO2max?

There is no good evidence that magnesium supplements raise VO2max. A review by Zhang and colleagues (2017) in Nutrients found that magnesium requirements increase as physical activity rises, and that magnesium status is associated with several strength measures. Intervention studies suggested supplements might improve functional measures such as quadriceps torque and mobility in elderly women. Those are strength and function outcomes, not aerobic capacity.

VO2max depends mainly on how much oxygen the heart and blood can deliver (see what limits VO2max). Magnesium is necessary for that system to work, but adding more than you need does not expand it.

Does it improve endurance performance?

For people with adequate magnesium intake, there is little evidence of an endurance benefit. The most likely scenario for a benefit is correcting a genuine shortfall — for example in athletes who restrict food intake, make heavy weight cuts or eat few plant foods. The IOC consensus on supplements (Maughan 2018) recommends a full nutritional assessment before supplementing and lists only a few products, such as caffeine, creatine, buffering agents and nitrate, as having good evidence for direct performance benefits. Magnesium is not one of them.

Magnesium for leg cramps and muscle cramps

This is the most common reason people buy magnesium, so the evidence deserves a closer look.

Night-time leg cramps. A Cochrane review by Garrison and colleagues (2020) pooled randomised trials of magnesium for skeletal muscle cramps. For idiopathic cramps, mostly in older adults presumed to have nocturnal leg cramps, magnesium made little difference to cramp frequency, intensity or duration compared with placebo. The authors concluded that magnesium is unlikely to provide clinically meaningful cramp prevention in older adults. For cramps in pregnancy, the studies were conflicting and could not be combined.

Cramps during exercise. A prospective study by Schwellnus and colleagues (2011) of 210 Ironman triathletes found that faster racing and a history of cramping predicted exercise-associated muscle cramps, while changes in body weight and blood electrolytes did not. That points to neuromuscular fatigue as the main driver, which is why the most practical fixes are pacing, training specifically for race intensity and stretching the cramping muscle. See strength training for runners for building fatigue resistance.

How it is used in studies

Trials use a wide range of doses and forms, often for several weeks. Because absorption and tolerance vary by form, and because needs depend on diet, there is no single dose worth copying. Most people can meet their needs with food: a handful of nuts or seeds, beans and leafy greens go a long way.

Side effects, safety and anti-doping

• Common side effects: diarrhoea, stomach cramps and nausea, especially with magnesium oxide and citrate at higher doses.
• Kidney disease: the kidneys clear excess magnesium. People with reduced kidney function can build up dangerous levels and should not supplement without medical advice.
• Interactions: magnesium can reduce absorption of some antibiotics and osteoporosis drugs, and some diuretics and acid-reducing medicines affect magnesium levels. Ask a doctor or pharmacist if you take regular medication.
• Anti-doping: magnesium is not prohibited, but competitive athletes should use third-party tested products to reduce the risk of contamination.

This page is not medical advice. Talk to a doctor if you have a medical condition, take medication or have frequent, severe cramps, which can occasionally signal an underlying problem.

Who might benefit

• People whose diet is low in plant foods, nuts and whole grains
• Athletes restricting energy intake or making weight
• People with a deficiency confirmed by a doctor, sometimes linked to certain medications or digestive conditions

Bottom line

Magnesium matters, but supplements are unlikely to fix cramps or raise VO2max if your diet is reasonable. For exercise cramps, look at pacing, fatigue and race-specific training first; for sodium and fluids in the heat, see electrolytes. Compare other options on the supplements page, build fitness with how to improve VO2max, and track progress with the VO2max calculator.

Sources

• Garrison SR, Korownyk CS, Kolber MR, et al. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev. 2020;9:CD009402.
• Zhang Y, Xun P, Wang R, Mao L, He K. Can magnesium enhance exercise performance? Nutrients. 2017;9(9):946. PMID 28846654.
• Schwellnus MP, Drew N, Collins M. Increased running speed and previous cramps rather than dehydration or serum sodium changes predict exercise-associated muscle cramping: a prospective cohort study in 210 Ironman triathletes. Br J Sports Med. 2011. PMID 21148567.
• Maughan RJ, Burke LM, Dvorak J, et al. IOC consensus statement: dietary supplements and the high-performance athlete. Int J Sport Nutr Exerc Metab. 2018. PMID 29589768.