Magnesium (Mg)

Magnesium is a mineral involved in more than 300 enzyme reactions — making energy, building bone and DNA, steadying nerves and muscle, and keeping your heartbeat regular. About 99% of your body's magnesium sits inside cells and bone; only a tiny fraction circulates in the blood, which is what the standard test measures.

That's the catch worth understanding upfront: because serum holds less than 1% of your total magnesium, and your body defends that blood level by pulling magnesium from bone and tissue, the serum number can read normal while your true stores run low — especially with long-term acid-reflux medication, diabetes, or alcohol use. A normal result is reassuring, not conclusive.

Magnesium is a quiet steadying force. It calms over-excitable nerves and muscle, it's essential for a regular heartbeat, and — the part most people don't know — it's the gatekeeper for potassium and calcium: when magnesium is low, those two often fall too, and won't come back up until the magnesium is fixed. So a magnesium problem can hide behind a stubborn potassium or calcium problem that supplementing those alone will never solve.

Low magnesium (hypomagnesaemia) is common — and often silent until it isn't. Mild shortfalls may cause cramps, tremor, fatigue, or twitching; larger drops bring stronger spasms, numbness, and — the dangerous part — heart-rhythm disturbances, including a life-threatening arrhythmia called torsades de pointes. Its signature is that it drags potassium and calcium down with it, making both resist treatment until magnesium is replaced. Common causes: long-term acid-reflux drugs (PPIs), diuretics, poorly controlled diabetes, alcohol, and gut losses — often not diet.

• Acid-reflux medication (PPIs) — long-term omeprazole and similar drugs are a leading, under-recognised cause of low magnesium; they impair how the gut absorbs it.
• Diabetes — poorly controlled blood sugar spills magnesium into the urine, which is why low magnesium is far more common in type 2 diabetes.
• Diuretics and alcohol — both increase magnesium loss through the kidneys, a frequent cause of a low level.
• Kidney function — the kidneys control magnesium excretion, so kidney disease is the main driver of high magnesium.
• Gut losses — diarrhoea, malabsorption, and vomiting deplete magnesium regardless of diet.
• Supplements and antacids in kidney disease — magnesium-containing products can push levels dangerously high when the kidneys can't clear them.

If magnesium is genuinely low, the fix is usually diet plus removing the cause. Magnesium-rich foods — beans and lentils (ful), whole grains, leafy greens, nuts and seeds — are the foundation; a supplement helps when diet isn't enough. Just as important is the why: reviewing long-term acid-reflux drugs with a doctor, controlling blood sugar, and cutting excess alcohol. If magnesium is high, the priority is the kidneys and stopping any magnesium-containing antacids or laxatives. Retest after any change.

Not all magnesium supplements are equal. Magnesium oxide is cheap but poorly absorbed and mostly acts as a laxative; citrate and glycinate absorb far better and are gentler on the stomach. Supplements are generally safe for people with healthy kidneys — the body excretes the excess — staying near the usual limit, but they can be dangerous in kidney disease, where magnesium builds up. Popular claims about magnesium for sleep and anxiety are weaker than the marketing suggests; the solid uses are correcting a real deficiency and specific medical situations. Check with a doctor first if your kidneys are impaired.

Magnesium is best understood as the gatekeeper of an electrolyte trio. Potassium leaks out through the kidneys when magnesium is low, so a stubborn low potassium often won't fix until magnesium does. Calcium depends on it too — severe magnesium deficiency shuts down parathyroid hormone (PTH), causing a low calcium that resists treatment until magnesium is replaced. Vitamin D needs magnesium as a cofactor to be activated, so low magnesium can blunt vitamin D's effect. Phosphorus tends to fall alongside magnesium in the same settings (refeeding, alcohol, critical illness). And kidney function (creatinine) sets the ceiling — the kidneys are what clear magnesium, so they decide the high end. The rule clinicians use: if potassium or calcium is low and won't correct, check magnesium.