Glycinate or citrate? 200 mg or 400? Does it really help you sleep? This independent UK guide reads the trials, grades the evidence for each claim and shows you how much magnesium is actually in the tub, so you can decide whether you need it at all.
Each common claim, graded by the quality of the human trials behind it. Magnesium does more than most supplements, but far less than the labels suggest, and it helps most when you were short of it to begin with.
Magnesium salts are long-established osmotic laxatives, and placebo-controlled trials of magnesium oxide show clear benefit.
Several placebo-controlled trials and a meta-analysis found fewer attacks at around 600 mg a day, but one trial found nothing.
A meta-analysis of 34 double-blind trials found a small fall of about 2 mmHg, larger in people with insulin resistance.
Several small placebo-controlled trials found people fell asleep a little faster or rated their sleep better. The effect is modest.
Blood sugar and type 2 diabetes
Meta-analyses of double-blind trials found slightly lower fasting glucose in type 2 diabetes but no clear change in HbA1c. Gains were largest in people low in magnesium.
A systematic review found hints of benefit in anxious people and those with premenstrual symptoms, but rated the studies as poor quality.
One single-blind trial of 75 people found magnesium oxide beat placebo after two months; a systematic review could not reach a conclusion.
Low mood and depression
One trial found a large improvement, but it had no placebo group, so expectation could explain much of it.
Premenstrual mood changes
One small trial of 32 women found better mood scores; it has not been well replicated.
A Cochrane review found magnesium is unlikely to help older adults with ordinary night cramps. Evidence in pregnancy is conflicting.
Menopausal hot flushes
A trial of 289 women found magnesium oxide worked no better than placebo.
Energy and tiredness
We found no placebo-controlled trial showing that magnesium reduces tiredness in people whose levels are normal.
A supplement is magnesium bound to something else, and the partner makes up most of the weight. This is why two capsules both labelled “500 mg” can differ fivefold. See every form compared.
Most adults can meet their magnesium needs from food. Seeds, nuts, beans, wholegrains and leafy greens do the heavy lifting, and a small handful of pumpkin seeds covers about half a day. Supplements make most sense when your diet falls short or a doctor has found a low level.
Foods high in magnesiumSix guides, in the order most people need them: what magnesium does, which form to buy, what it helps, how to take it, safety and buying.
Photographs from Wikimedia Commons: Tiia Monto (CC BY-SA 4.0), John Brandauer (CC BY-SA 3.0), Dmitry Makeev (CC BY-SA 4.0), Father.Jack from Coventry, UK (CC BY 2.0), Chandrajit Karmakar (CC BY-SA 4.0), Mark Fergus (CC BY 3.0).
Short answers, each linked to the guide that explains it in full.
Most adults don't, if they eat a varied diet with wholegrains, nuts, seeds, beans and green vegetables. The Department of Health and Social Care says you should be able to get all the magnesium you need from food. A supplement can make sense if your diet is limited, you take medicines that lower magnesium, or a doctor finds a low level.
There is no single best form. Citrate and glycinate are good all-rounders. Oxide is best kept for constipation. If a form upsets your stomach, switch to glycinate or take smaller doses with food.
The best-supported uses are as a laxative for constipation, to help prevent migraine at fairly high doses, and to lower blood pressure by a small amount. It may modestly shorten the time it takes to fall asleep. It is not an energy supplement for people whose levels are normal.
It makes little difference for most purposes. If you are taking it for sleep, trials gave it in the evening. If it upsets your stomach, take it with a meal.
From food, almost never. From supplements, the usual problem is diarrhoea. Dangerous overdose is rare and mostly happens with very high doses, magnesium laxatives or antacids, or in people whose kidneys can't clear magnesium.
The amount of magnesium per serving in the nutrition table, the number of capsules in a serving, the form or forms used, and whether the product contains other active ingredients. Ignore large front-of-pack numbers until you have checked what they refer to.