Like calcium and phosphorous, magnesium supports bone health. It is also involved in many enzyme systems, nerve impulse transmission, immune function, normal muscle contraction, and critical to normal heart function. Magnesium acts in all the cells of the soft tissues where it forms part of the protein-making machinery and is necessary for energy metabolism. Recent studies show a link to sudden cardiac death and magnesium deficiency (less than 260 mg/day). Magnesium also proves to help protect against hypertension. It has been noted that people living in areas where "hard water" which contains high concentrations of calcium and magnesium, tend to have lower rates of heart disease (34%). Most magnesium deficiencies are the result of disease, alcohol abuse, diuretic use, kidney disorders, and prolonged diarrhea & vomiting. Athletes exposed to extreme heat resulting in dehydration are at risk for symptoms, such as dizzines and muscle fatigue. Toxicity (overdose) is rare though deadly, and usually results from supplements. The RDA of magnesium is 350 to 400 mg./day.
Significant dietary sources of magnesium are nuts, legumes, whole grains, dark leafy vegetables, seafood (especially halibut), chocolate, and cocoa.
~American Journal of Clinical Nutrition, 2011