What it measures.
Magnesium activates ATP (your cellular energy currency), supports over 300 enzymatic reactions, regulates muscle and nerve function, blood pressure, and sleep quality. Serum magnesium reflects only 1% of body stores, making RBC magnesium a more reliable marker for intracellular status.
Serum magnesium measures blood levels; RBC magnesium measures intracellular stores. Serum levels are tightly regulated and may appear normal even with significant tissue deficiency.
Why it matters.
Approximately 50% of Americans don't get adequate magnesium. Deficiency often presents as muscle cramps, poor sleep, anxiety, or irregular heartbeat while serum tests appear normal. Optimal magnesium supports energy, relaxation, and cardiovascular health.
Physiology.
Magnesium is primarily intracellular, with only 1% in blood. It's a cofactor for ATP-dependent reactions, neurotransmitter release, and muscle contraction/relaxation. The kidneys regulate blood magnesium, maintaining narrow serum levels by adjusting excretion.
Testing & preparation.
How to prepare
- No special preparation needed
- RBC magnesium preferred for intracellular status
- Note any magnesium supplementation
When to test
Evaluating muscle cramps, sleep issues, anxiety, irregular heartbeat, or as part of comprehensive health assessment.
How often
Annually; as needed if supplementing or symptomatic.
Interpretation.
High magnesium
Common causes:
- Excessive supplementation
- Kidney failure (impaired excretion)
- Hypothyroidism
- Addison's disease
Implications:
- Rare unless kidney disease present
- Can cause muscle weakness, low blood pressure
- Severe: cardiac arrhythmias
Low magnesium
Common causes:
- Inadequate dietary intake
- Increased losses (diuretics, alcohol, sweating)
- GI losses (diarrhea, malabsorption)
- Diabetes (urinary losses)
- Medications (PPIs, diuretics)
- Chronic stress (increases magnesium demand)
Implications:
- Muscle cramps and spasms
- Poor sleep quality
- Anxiety and irritability
- Heart palpitations, arrhythmias
- Migraine headaches
- Insulin resistance
Optimization.
Diet
- Dark leafy greens (spinach, Swiss chard)
- Nuts and seeds (pumpkin seeds, almonds)
- Dark chocolate (70%+ cacao)
- Avocados, bananas
- Whole grains and legumes
Lifestyle
- Epsom salt baths for transdermal absorption
- Reduce alcohol (depletes magnesium)
- Manage stress (increases magnesium need)
- Avoid excessive caffeine
Supplements
- Magnesium glycinate (well-absorbed, calming)
- Magnesium threonate (crosses blood-brain barrier)
- Magnesium citrate (well-absorbed, may loosen stool)
- 200-400mg daily; take in evening for sleep support
- Avoid magnesium oxide (poorly absorbed)
FAQs.
Why might my serum magnesium be normal when I feel deficient?
Serum magnesium represents only 1% of total body magnesium. Your body tightly regulates blood levels, pulling from bone and tissue stores to maintain normal serum values. You can have significant intracellular deficiency while serum appears normal. RBC magnesium is more sensitive for detecting tissue deficiency. Symptoms (cramps, poor sleep, anxiety) may be the best indicator when serum is normal.
What form of magnesium should I take?
Different forms have different characteristics. Magnesium glycinate: well-absorbed, calming, good for sleep and anxiety. Magnesium threonate: crosses blood-brain barrier, best for cognitive support. Magnesium citrate: well-absorbed, can have mild laxative effect. Avoid magnesium oxide: cheap but poorly absorbed (4% bioavailability). For most people, glycinate or citrate 200-400mg before bed works well.