Originally Posted by MoAlpha

My semi-informed take is that you don't need to put the Mg or the K in a drink, since there are body stores of those elements, which can be repleted by diet except in rare cases of Mg deficiency. Na, by contrast, is not really stored and needs to be replaced in real time during prolonged exercise.

I think you're right that subtle Mg deficiency may be common in industrialized societies, but it's easy to eat foods that contain it. I practically live on legumes, nuts, dark chocolate and leafy greens.