What is magnesium?
Magnesium is a mineral involved in over 300 enzymatic reactions in your body. It's essential for muscle function, nerve signaling, energy production, blood sugar regulation, bone health, and DNA repair. Despite being one of the most important minerals you need, most people don't get enough.
When doctors order a standard "magnesium" blood test, they usually measure serum magnesium — the amount floating freely in your blood. The problem is that only about 1% of your body's magnesium is in your blood. You can have a "normal" serum level while being significantly depleted inside your cells. That's why the RBC (red blood cell) magnesium test is better — it measures magnesium inside the cells, giving a more accurate picture of your true status.
Why it matters for longevity
Magnesium deficiency is linked to a long list of health problems: poor sleep, chronic stress, muscle cramps, headaches, high blood pressure, insulin resistance, and weakened bones. Research estimates that 50–80% of Americans don't meet the recommended daily intake for magnesium.
Part of the problem is that modern soils are depleted of minerals compared to a few decades ago. Even if you eat a whole-food diet, your food simply contains less magnesium than it used to. Add stress to the equation — stress literally burns through magnesium — and it's easy to see why deficiency is so widespread.
For women, magnesium is especially important. It helps with PMS symptoms, menstrual cramps, sleep quality during hormonal shifts, and bone density as estrogen declines with age.
What the numbers mean
The conventional reference range for RBC magnesium is 4.2–6.8 mg/dL. Many integrative practitioners recommend aiming for the upper end:
- Low: below 4.2 mg/dL (clear deficiency)
- Suboptimal: 4.2–5.0 mg/dL
- Optimal: 5.0–6.5 mg/dL
If you're only getting the standard serum magnesium test, a "normal" result (1.7–2.2 mg/dL) doesn't rule out deficiency. Ask specifically for RBC magnesium to get a more accurate picture.
What affects it
- Diet: Magnesium-rich foods include dark leafy greens, pumpkin seeds, almonds, dark chocolate, avocado, and black beans. But even a healthy diet may fall short due to soil depletion.
- Stress: increases magnesium excretion through your kidneys. Chronic stress is one of the fastest ways to deplete your stores.
- Exercise: Physical activity increases magnesium needs. Athletes and regular exercisers are at higher risk of deficiency.
- Medications: Proton pump inhibitors (PPIs), diuretics, and certain antibiotics can reduce magnesium absorption or increase excretion.
- Alcohol and caffeine: Both increase magnesium loss through urine.
- Age: Absorption decreases with age, while excretion increases — a double hit.
How to get tested
Ask your doctor specifically for an RBC magnesium test (not just "magnesium" or "serum magnesium"). It's a simple blood draw that does not require fasting. You can also order it through online lab services. Since magnesium stores change slowly, testing once or twice a year is typically sufficient.
How to improve it
- Choose the right form of magnesium. Not all supplements are equal. Magnesium glycinate is excellent for sleep and relaxation. Magnesium citrate supports bowel regularity. Magnesium threonate (also called Magtein) is the form studied for brain health and cognitive function. Avoid magnesium oxide — it's cheap but poorly absorbed.
- Start low, increase gradually. Begin with 200 mg per day and work up to 300–400 mg. Taking too much at once can cause loose stools (especially citrate). Splitting doses between morning and evening works well.
- Take it at night. Magnesium glycinate taken 30–60 minutes before bed can noticeably improve sleep quality. Use our Sleep Calculator to find your ideal bedtime and pair it with a magnesium routine.
- Eat magnesium-rich foods daily. Pumpkin seeds (one ounce provides about 150 mg), dark chocolate, spinach, almonds, and avocado are all excellent sources.
- Manage stress. Since stress depletes magnesium and low magnesium worsens stress, breaking this cycle is important. Consistent sleep, movement, and relaxation practices all help.