magnesium is the fourth most abundant cation in the body, and is about one-sixth as plentiful in cell as potassium. About 50 to 60 percent of the body's magnesium is in bone , with the remainder distributed equally between muscles and other soft tissue. The magnesium in bone provides a large reservoir in case deficiencies in soft tissue magnesium occur. Most magnesium resides in cell, with only 1 percent in extracellular fluid.