Although magnesium is in many foods, research suggests that about half of Americans don't consume enough of it in their diet. In other words, you may be unknowingly magnesium-deficient, which could be problematic, since a magnesium deficiency could cause issues with muscle contraction, sleep, brain function, energy production, and may even increase your risk of death by heart disease.But don't panic just yet. With a few small diet tweaks, magnesium-rich foods could easily find their way into your meal plan.What Is Magnesium? Magnesium is an electrolyte that is essential for cellular and bodily function, explains Brooke Alpert, M.S., R.D., C.D.N and Cure nutritional advisor. Why Do Runners Need Magnesium?Unfortunately, runners may run the risk of losing magnesium when they sweat, which means that if their levels were low to begin with, they may be further depleting their body's magnesium resources while they run.…