Magnesium is involved in several hundred enzyme reactions in the body, which is the sort of sentence that gets printed on packaging and means very little on its own. What is more useful is knowing where it comes from and how close an ordinary week of eating gets to the recommended amount.
In the United States, the recommended dietary allowance is 320 mg a day for adult women and 420 mg for adult men. National dietary surveys have consistently found that a substantial share of adults take in less than that.
The foods that carry the most
- Pumpkin seeds — around 150 mg in a small handful, which is the densest common source by a wide margin.
- Almonds — roughly 80 mg per ounce.
- Spinach, cooked — roughly 78 mg per half cup.
- Black beans — roughly 60 mg per half cup.
- Dark chocolate — roughly 65 mg per ounce, at 70 per cent cocoa or above.
- Wholemeal bread — roughly 23 mg per slice.
The pattern is worth noticing: magnesium lives in seeds, nuts, pulses and leaves. It is largely stripped out when grains are refined — white flour holds a small fraction of what wholemeal does. A diet built on refined carbohydrates can be perfectly adequate in calories and still run short here.
On supplements
Food first is the sensible order, and for many people a handful of pumpkin seeds is a cheaper and more pleasant fix than a bottle. Where a supplement is used, the form on the label matters a great deal — the various magnesium compounds differ substantially in how much the body absorbs, and the cheapest are not generally the best absorbed.
Magnesium supplements can interact with certain antibiotics, diuretics and medications for osteoporosis. Anyone taking prescription medicine should raise it with a pharmacist before starting one — a two-minute conversation at the counter, free of charge.