You can be magnesium deficient by not taking in enough magnesium.

Along with not taking enough magnesium in, you can also become magnesium deficient by using your muscles too much.

The body requires Calcium to fire, or contract, a muscle fiber. Magnesium is required to release, or turn off that contraction.

Firing a muscle over an over obviously increases the amount of calcium used up to contract a muscle fiber, and thus increases the amount of magnesium used to to turn each of those firing muscle fibers 'off'.

So basically, there are two primary ways to create a magnesium deficiency. You either don't eat enough sources of magnesium, or for a variety of reasons, your body uses a lot of magnesium and that leaves you deficient in magnesium.

Which comes right back down to not having enough magnesium intake in your diet.