Magnesium: 34 Trials on Dose, Timing and Status
The finding: Across 34 randomised double-blind placebo-controlled trials, magnesium supplementation reliably raised magnesium status, and 300 mg a day for one month was enough to do it.
What they did
The team searched MEDLINE and EMBASE for every randomised, double-blind, placebo-controlled trial of oral magnesium supplementation in adults published up to February 2016. Thirty-four trials qualified, covering 2,028 participants. The primary question was magnesium's effect on blood pressure, and along the way the analysis pinned down the dose and duration needed to shift magnesium status at all, which is the part most useful when you are standing in front of a shelf.
They pooled the results using random-effects meta-analysis and modelled the dose-response relationship with a restricted cubic spline curve. Restricting to double-blind placebo-controlled trials only is a much higher bar than most supplement meta-analyses set, and it is why 34 trials is the number rather than several hundred. It was published in Hypertension, an American Heart Association journal.
What they found
- The median dose across trials was 368 mg a day, taken for a median of three months.
- Serum magnesium rose by 0.05 mmol/L against placebo, 95% confidence interval 0.03 to 0.07.
- The dose-response modelling showed 300 mg a day, or one month of supplementation, was sufficient to raise serum magnesium.
- On the primary outcome, systolic blood pressure fell by 2.00 mmHg (95% confidence interval 0.43 to 3.58) and diastolic by 1.78 mmHg (95% confidence interval 0.73 to 2.82).
- Greater blood pressure reductions tended to appear in the trials rated higher quality and those with lower dropout rates.
What this means for you
This is the paper to reach for when someone asks how much magnesium and for how long, because it answers both with numbers rather than opinion.
Three hundred milligrams a day is the threshold the dose-response curve identified, and one month is roughly how long it took to register. Most trials used around 368 mg. If you are taking a magnesium supplement that delivers well under 300 mg of elemental magnesium, this analysis suggests you are below the dose the evidence is built on.
Read the elemental figure, not the compound figure. A 1,000 mg magnesium glycinate capsule does not contain 1,000 mg of magnesium. Glycinate is roughly 14% elemental magnesium by weight, citrate around 16%, oxide around 60%. A good label states the elemental amount; a vague one states the compound and lets you assume.
Form changes tolerance more than anything else. Oxide carries the most magnesium per gram and is also the most likely to loosen your bowels, because the magnesium it does not absorb stays in the gut and draws in water. Glycinate, citrate and malate are generally easier to take at a useful dose.
The study
Zhang X, Li Y, Del Gobbo LC, et al. Effects of magnesium supplementation on blood pressure: a meta-analysis of randomized double-blind placebo-controlled trials. Hypertension. 2016 Aug;68(2):324-333.
General information only. This is a summary of published research, not a therapeutic claim and not medical advice. A result found in a trial population is not the same as a product doing that for you. Blood pressure is something to manage with your doctor. Always read the label and follow the directions for use. If you have a health condition or take medication, check with your health practitioner before use.
