MagnesiumSerum magnesium · Mg
A mineral needed by hundreds of enzymes, of which blood holds only a fraction.
Magnesium is a cofactor for several hundred enzyme reactions, including those that make and spend ATP, and it is involved in muscle and nerve function. Most of the body's magnesium is in bone and inside cells; well under one per cent circulates in blood.
It matters because that distribution limits what the blood test can tell you. Serum magnesium can sit comfortably in range while tissue stores are depleted, since the body defends the blood level first.
In practice it is measured to detect clear deficiency or excess, often alongside calcium and potassium, which are difficult to correct while magnesium is low. It is a poor guide to marginal intake.