Magnesium

USE: Magnesium is an essential mineral required by the body for maintaining normal muscle and nerve function, keeping a healthy immune system, maintaining heart rhythm, and building strong bones. Magnesium is also involved in at least 300 biochemical reactions in the body. A deficiency in magnesium can lead to muscle spasms, cardiovascular disease, diabetes, high blood pressure, anxiety disorders, migraines, osteoporosis, and cerebral infarction. Consuming too much magnesium typically causes diarrhea as the body attempts to excrete the excess. 
About 50/60% of a person's magnesium is stored in the bone, and for that reason it plays a key role in bone structure. Researchers have found that even a mild ongoing Magnesium deficiency can lead to a significant amount of bone loss.
Part of the reason that this occurs is that when magnesium intake goes too low, levels of parathyroid hormone go down. This leads to a reduced absorption of calcium in the intestines, as well as increased loss of calcium and magnesium in the urine.
The body also always thrives to maintain balanced levels of calcium and magnesium, so when one is low, the other is simply excreted out of the body to maintain the level between the two. 
SOURCE: Great sources of Magnesium include spinach and swiss chard, collard greens, mustard greens, turnip greens, green beans and sea vegetables.
Legumes, nuts, and seeds are good sources especially tempeh (fermented soybeans), pinto beans, lima beans, and kidney beans. The top nuts and seeds for magnesium are pumpkin seeds, sesame seeds, sunflower seeds, cashews, and almonds. Amongst the grains, spelt, buckwheat, brown rice, quinoa, and millet rank high. All you chocolate lovers out there will be happy to know that dark chocolate is a great source too!