Magnesium is an essential mineral, playing a key role in over 300 enzymatic reactions in metabolism. The reactions include those involved in the Krebs cycle (one of the body's energy production systems), energy storage, the breakdown of fatty acids, protein synthesis, DNA metabolism, the relaxation  of voluntary and involuntary muscle tissue, neurotransmitter activity, and hormone regulation. Magnesium is stored primarily in the bones and plays a role in the absorption of calcium by the bones.