Magnesium: The Most Common Deficiency You Have Probably Never Tested For

Ask your doctor in Portland or Manchester about your magnesium level and you’ll likely get a serum magnesium test. The problem is that serum magnesium reflects less than 1% of your total body magnesium. The rest sits inside your cells and bones, where a standard blood draw can’t reach it.

Why Magnesium Matters

Magnesium is involved in more than 300 enzymatic reactions. It’s needed for muscle contraction, nerve signaling, blood sugar regulation, blood pressure management, and protein synthesis. Research links low magnesium intake to muscle cramps, disrupted sleep, anxiety, headaches, and irregular heart rhythm.

The National Institutes of Health estimates that roughly half of Americans consume less magnesium than recommended. Soil depletion, water filtration, and decades of shifting toward processed foods have all pulled average intake down.

How to Test More Accurately

RBC magnesium (red blood cell magnesium) measures the mineral inside your red blood cells rather than floating in your plasma. It gives a closer approximation of cellular stores. An optimal RBC magnesium result is generally considered to be above 5.0 mg/dL, though many standard labs set their lower reference limit around 4.2. The same logic applies to other commonly undertested nutrients; vitamin D is another example where standard reference ranges often differ from levels researchers consider optimal.

Not every lab offers RBC magnesium as a standard order. You may need to ask for it specifically, so it’s worth raising the question at your next appointment.

Getting More Magnesium From Food

Dark leafy greens, pumpkin seeds, almonds, black beans, and dark chocolate are among the best dietary sources. If your intake is consistently low, a healthcare provider can help you figure out whether a supplement might make sense and which form is appropriate for you. Forms of magnesium vary considerably in how well they’re absorbed, and what works for one person may not suit another. It’s also worth noting that magnesium is one of several electrolytes that work together; if you’re evaluating overall mineral status, sodium and potassium levels are worth reviewing alongside it.

Sources

• National Institutes of Health, Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. Updated 2025.
• DiNicolantonio JJ, et al. Subclinical magnesium deficiency: a principal driver of cardiovascular disease and a public health crisis. Open Heart. 2018;5(1):e000668.

This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider before making any health decisions.