Magnesium (serum)

Serum magnesium measures only the small slice of your body's magnesium that is in your blood. About 99% is locked away in bone, muscle, and soft tissue. So a blood test is a poor gauge of magnesium inside your cells. Magnesium matters a lot. It is an essential helper for hundreds of enzyme reactions, including making ATP energy and copying and repairing DNA. It also acts as a natural calcium-channel blocker, important for nerve signaling and heart rhythm. Low magnesium (usually under 0.75 mmol/L) is tied to irregular heartbeats, twitchy nerves and muscles, insulin resistance, and more vascular calcification. It is common in type 2 diabetes, heavy alcohol use, and with long-term acid-blockers (PPIs) or loop diuretics. And population data link lower magnesium intake and lower blood levels to higher death rates and faster biological aging.

Last reviewed: June 22, 2026

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