Cramping in a marathon, should I take a magnesium supplement?

Cramping in a marathon, should I take a magnesium supplement?


Just when you feel you have done all the work and you are on track for a P.B., BANG, the dreaded calf cramp at the 32km mark of a marathon.  So will taking magnesium prior to the marathon prevent this?  To answer this question we will look into what a muscle cramp is and why it happens.


An Exercise Associated Muscle Cramp (EAMC) is a sudden, involuntary painful spasm or contraction of a muscle.  Usually it will happen in one of the active muscles of the activity you are doing, for example, when running; a calf cramp.


As much as the supplement companies would like you to believe that taking a magnesium supplement will stop you from cramping, scientific evidence to date suggests there is not a strong link between EAMC and magnesium supplementation.  The most agreed upon theory is that EAMC is fatigue of the exercising muscle.

What does cause a cramp then?


The common risk factors are:

-       Having a history of cramps (you are a cramper!)

-       Increased exercise intensity and duration (and unfortunately in a marathon, you never quite replicate those racing conditions for the same duration in training)

-       Inadequate conditioning for the race or event (you haven’t trained enough!)

-       A family history of cramping may also make you more at risk

-       Glycogen depletion, which leads to fatigue of the muscle


A balanced diet should provide you with adequate magnesium and therefore a magnesium supplement should not be needed.   Foods which are high in magnesium include nuts, seeds, fish, legumes and wholegrains.  Those who are doing very high levels of training or are eating a restricted energy intake could be at risk of a magnesium deficiency (or other deficiencies!) but before taking any supplementation, it is important to discuss with your doctor.


So how can I best avoid a cramp?

-       Have a hydration plan.  Know your hydrated weight and make sure you are consuming enough fluids around and in training to keep yourself hydrated.  Prior to an event, you should make an effort in the days leading up to concentrate on your hydration and make sure you carry a drink bottle with you and sip!  Your hydration needs during the event differ from person to person but between 600-1000ml / hour is a good start.  You also should be consuming a source of sodium, which easiest way done is through a sports drink.

-       Train!  Leading up to a marathon or any endurance event, you have to have done an appropriate program to make sure your muscles can cope with the load of the event.  You need to have had at least a 12 week lead up and a prior endurance base.

-       Make sure you carbo-load and have a fuelling plan during the event.  Anywhere from 30-60g of carbohydrate per hour is recommended in order to prevent glycogen depletion and to keep your muscles fuelled.  Not only will you prevent a cramp, but you will have more energy to finish!

If you need help fuelling for your marathon or with your hydration plan or worried about your intake of magnesium, see one of Eat Smarts Accredited Practicing Dietitians!



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