There is not a clinic day that goes by that I don’t take a supplement history from an athlete that includes a magnesium tablet or powder. It seems to be an almost universally accepted notion that magnesium supplementation will assist in preventing (exercise associated) muscle cramps. So let’s see if it is worthy of taking it’s place in your supplement cupboard?

The role of magnesium in muscle contraction

The activation of a contraction in skeletal muscle starts with the thought of movement occurring in the brain, which sends waves of electrical charge down the nerve to the muscle fibers. This electrical charge causes a release of calcium from storage areas in the muscle cells. Calcium binds onto specific sites and allows the muscle to contract. So where does magnesium come into this? In simple terms, magnesium competitively binds onto these same sites but exerts a relaxation effect instead. When muscle is relaxed these sites will be saturated with magnesium. Theoretically one could assume that when cramping occurs, there must be inadequate magnesium within the cells leaving calcium bound to these sites and the muscle unable to relax. If you were then to consume a magnesium supplement, exercise-induced muscle cramps would surely attenuate? Studies to date however, have not backed this statement up.

Magnesium requirements

On average, men require around 420mg of magnesium per day and women need around 300mg per day. Some great food sources of magnesium include leafy greens, nuts and seeds, fruits such as figs and avocado, seafood (salmon, mackerel, tuna), raw cacao, legumes, whole grains and tofu. A diet that is well balanced including plenty of fruits and vegetables, nuts, seeds and whole grains should provide more than enough magnesium to meet the average person’s daily needs. Athletes may require more magnesium than the average person, particularly if they are undertaking a chronically high training load, or are on an energy restricted diet. In a lot of cases due to higher energy requirements, with good food choices, athletes should also consume enough magnesium. If you suspect you are at risk of inadequate intake of magnesium, consult an Accredited Practising Dietitian or Sports Dietitian.

                                                                                    Magnesium (mg) per 100g
Green leafy vegetables (e.g. spinach and kale)    80
Dried figs                                                                     73
Almonds                                                                       260
Pumpkin seeds                                                            535
Tuna                                                                               63
Raw cacao                                                                     507
Tofu, firm                                                                      78
Nuttab 2010 Online Searchable Database

Magnesium deficiency

It is challenging to test magnesium deficiency because serum levels will not give information about magnesium stored within cells and bones. Referring back to the mechanism by which magnesium participates in muscle relaxation, the all important information would be how much exists within the muscle cell. Magnesium deficiency is rarely reported in developed countries and thought to be very low in prevalence, even in athletes. Supplements, although heavily marketed, are more often then not a waste of money particularly given we will vary our intestinal absorption of magnesium depending on our needs and content of the diet. Higher need = more absorption. Lower need= less absorption. This means that without a diagnosed deficiency, you are likely to be passing your magnesium (along with your money) down the toilet. The bottom line is that if you are in a risk group for having inadequate magnesium intake, or stores, then get this checked with your GP before commencing supplements.

Magnesium supplementation for prevention of muscle cramps

Cutting to the chase; a number of studies testing the effect of magnesium supplementation on muscle cramps (where no deficiency exists) have shown that it provides no greater benefit than placebo1,2 . There could be a call for more rigorous studies with larger sample sizes, looking at magnesium in exercise associated muscle cramps, but until then I will not be recommending it as a primary treatment or prevention modality without diagnosed deficiency.

1. Roffe C, Sills S, Crome P, Jones P. Randomised, cross-over, placebo controlled trial of magnesium citrate in the treatment of chronic persistent leg cramps. Med Sci Monit 2002;8:CR326–CR330.
2. Frusso R, Zarate M, Augustovski F, Rubinstein A. Magnesium for the treatment of nocturnal leg cramps: a crossover randomized trial. J Fam Pract 1999;48:868–871.