Magnesium

A cofactor in several hundred enzyme reactions, including every one that uses ATP — which is to say, most of them. Around half of it sits in bone, and only about one per cent is in blood, which is what makes it awkward to measure.

A cofactor in more than three hundred enzyme systems, including energy production, protein synthesis, muscle and nerve function, and blood glucose control.

• Pumpkin seeds
• Almonds and cashews
• Spinach
• Black beans
• Dark chocolate
• Whole grains

Citrate and glycinate forms are absorbed better than oxide, which is mostly laxative.

400–420 mg (men) · 310–320 mg (women) for most adults. The upper limit, past which harm becomes likely, is 350 mg from supplements only — food is not limited.

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

Most at risk: Type 2 diabetes, heavy alcohol use, long-term diuretics or proton pump inhibitors, and older adults.

Low intake from a diet built on refined grains; losses through the kidney with alcohol, diuretics and poorly controlled diabetes; losses through the gut with prolonged diarrhoea or malabsorption; and long-term proton pump inhibitor use.

Muscle cramps and twitching, fatigue, irritability, and in severe cases abnormal heart rhythms. Mild shortage is common and easily missed, because blood levels stay normal while tissue stores fall.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

Serum magnesium is the usual test and is a poor reflection of body stores — it can sit in the normal range while tissue levels are low. There is no straightforward routine test that does better, which is worth knowing before drawing conclusions from a normal result.

Nuts, seeds, legumes, whole grains, dark chocolate and leafy greens. Refining grains removes most of it, so wholemeal over white makes a real difference here.

Forms differ mainly in how much reaches you and how much stays in the bowel. Oxide is poorly absorbed and acts largely as a laxative; citrate, glycinate and malate are absorbed better. Loose stools are the usual sign of too much.

Supplements cause diarrhoea long before anything worse — magnesium is a laxative, which is what milk of magnesia is. Dangerous accumulation happens only with kidney impairment.

Interactions: Reduces absorption of some antibiotics and bisphosphonates — separate them.