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Biomarker library / Vitamins and minerals

Magnesium (Mg)

Also called serum magnesium, hypomagnesemia test

Serum magnesium measures the roughly 1% of body magnesium that circulates in blood. Because bone and soft tissue hold the other 99%, a normal serum value does not rule out low total-body magnesium; the test is best at catching frank deficiency.

Written by the VitalDecades editorial team Reviewed by the editorial review process Updated

01  /  Reference ranges

What counts as normal

Scale in mmol/L. Cut points from the sources listed below.

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Population Adults
Category Hypomagnesemia
Range under 0.75 mmol/L ODS definition; symptoms and low potassium or calcium make it urgent.
Source NIH Office of Dietary Supplements, Magnesium Fact Sheet for Health Professionals

Effective

Population Adults
Category Normal serum magnesium
Range 0.75 to 0.95 mmol/L About 1.82 to 2.31 mg/dL. Serum level understates total-body stores.
Source NIH Office of Dietary Supplements, Magnesium Fact Sheet for Health Professionals

Effective

Normal vs optimal

No optimal zone above the ODS normal range is established. High serum magnesium is rare with normal kidneys and mostly a kidney-failure or supplement-overuse finding.

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02  /  Why it matters

What the number tells you

Magnesium participates in hundreds of enzyme reactions, and frank hypomagnesemia (under 0.75 mmol/L per the NIH ODS) causes muscle cramps, arrhythmias, and low calcium and potassium that will not correct until magnesium is replaced. Chronic low intake tracks with hypertension and type 2 diabetes risk in observational data. The supplement aisle runs far ahead of the evidence, though: for people with normal levels and normal kidneys, benefits of supplementation beyond the RDA are mostly unproven, sleep included.

03  /  In practice

Testing and what moves it

When it is tested

  • With unexplained low potassium or calcium, arrhythmias, muscle cramps or seizures, chronic diarrhea, alcohol use disorder, or long-term proton-pump inhibitor or diuretic use.
  • RDA: 400 to 420 mg/day for men, 310 to 320 mg/day for women (ODS); food sources are nuts, seeds, legumes, whole grains and leafy greens.

What raises it

  • Kidney failure, high-dose supplements and magnesium-containing antacids or laxatives (mostly with impaired kidneys).

What lowers it

  • Chronic diarrhea and malabsorption, alcohol use disorder, poorly controlled diabetes, diuretics, long-term proton-pump inhibitors, low dietary intake.

04  /  Related

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Frequently asked questions

What is a normal magnesium level?

0.75 to 0.95 mmol/L in serum, per the NIH ODS, with hypomagnesemia defined below 0.75. Because only about 1% of body magnesium is in blood, a normal serum value does not exclude low stores.

Does magnesium help sleep?

The evidence is thin: small trials in older adults with insomnia show modest effects, and people with genuinely low magnesium may sleep better when it is corrected. For someone replete, sleep benefits beyond placebo are not established; the durable fixes are in our sleep guide.

Which magnesium supplement form is best?

Absorption differs modestly (citrate and glycinate absorb better than oxide, which is more laxative), but no form has outcome evidence behind the marketing claims. Food sources carry the strongest data. Stay under the 350 mg/day supplemental upper limit unless a clinician directs otherwise.

One document sets the numbers on this page, published in 2026.

We last re-read these sources on . How we verify.