Blood magnesium levels are a poor barometer for true body (intracellular) magnesium.
Only 1% of the body’s magnesium is in the blood, the remaining 99% stored in various body tissues, particularly bone and muscle. If blood magnesium is low, cellular magnesium levels are indeed low—very low.
If blood magnesium is normal, cellular or tissue levels of magnesium may still be low. Unfortunately, tissue magnesium levels are not easy to obtain in living, breathing humans. In all practicality, a blood magnesium test only helps if it’s low, while normal levels don’t necessarily mean anything and may provide false reassurance.
Short of performing a biopsy to measure tissue magnesium levels, several signs provide a tip-off that magnesium may be low:
• Heart arrhythmias —Having any sort of heart rhythm disorder should cause you to question whether magnesium levels in your body are adequate, since low magnesium levels trigger abnormal heart rhythms. In fact, in the hospital we give intravenous magnesium to quiet down abnormal rhythms. • Low potassium — Low magnesium commonly accompanies low potassium. Potassium is another electrolyte depleted by diuretic use and is commonly deficient in many conditions (e.g., excessive alcohol use, hypertension, loss from malabsorption or diarrhea). Like magnesium, potassium may not be fully replenished by modern diets. • Muscle cramps — Magnesium regulates muscle contraction. Leg cramps, or “charlie-horses”, painful vise-like cramps in calves, fingers, or other muscles, are a common symptom of magnesium deficiency. (Leg cramps that occur with physical activity, such as walking, are usually due to atherosclerotic blockages in the leg or abdominal arteries, not low magnesium.) • Migraine headaches —Reflective of magnesium’s role in regulating blood vessel tone, low magnesium can trigger vascular spasm in the blood vessels of the brain. In some emergency rooms, they will actually administer intravenous magnesium to break a migraine. • Metabolic syndrome—Magnesium plays a fundamental role in regulating insulin responses. Metabolic syndrome (low HDL, high triglycerides, small LDL, high blood pressure, increased blood sugar, excessive abdominal fat, etc.) is triggered by insulin responses gone awry and is clearly linked to low magnesium levels.
The absence of any of these tell-tale signs does not necessarily mean that tissue levels of magnesium are normal.
Then how do you really know? There really is no easy, available method to gauge body magnesium. As a practical solution, we therefore have aimed for maintaining serum levels of >2.1 mg/dl or RBC magnesium (a surrogate for tissue levels) of >6.0 mg/dl. (Going too high is not good either, so occasional monitoring really helps. However, I've only seen this once in a psychotic woman who drank ungodly amounts of magnesium-containing antacids for no apparent reason; she almost ended up on a respirator due to respiratory suppression by the magnesium level of 11 mg/dl!)
In all practicality, because of magnesium’s crucial role in health, its widespread deficiency in Americans, and the growing depletion of magnesium in water, supplemental magnesium is necessary for nearly everyone to ensure healthy levels.