An umbrella meta-analysis of randomized and controlled trials was published in the journal Current Therapeutic Research, Clinical and Experimental, describing the effect of magnesium supplementation on systolic and diastolic blood pressure. This meta-analysis included over 8600 participants and 10 previously published meta-analyses on this topic (over a nearly 20-year period), which have had conflicting results.
Overall, pooling the data in these analyses suggests a significant but small effect of magnesium on systolic blood pressure (-1.25 mmHg), but several variables influenced the magnitude of the reduction, such as duration of supplementation and dose. For example, at doses above 400 mg per day, a larger and more clinically meaningful effect was observed (-6.38 mmHg) in systolic pressure. Similar effects were observed for diastolic pressure, with a small reduction overall (-1.4 mmHg), but at doses above 400 mg, a larger effect was observed (-3.71 mmHg). As the authors point out, magnesium is unlikely to sufficiently control hypertension when used as a monotherapy, but it may have clinical significance when used in combination with other approaches. It’s worth noting that a 5 mmHg reduction in systolic blood pressure has been associated with a 10% reduction in the risk for major cardiovascular events, with benefit even among normotensive individuals.
Magnesium acts via several mechanisms to help reduce blood pressure; it acts as a calcium channel blocker, competes with sodium binding on vascular smooth muscle cells, improves endothelial function, etc. (additional mechanisms discussed in the umbrella review). A somewhat recently developed marker, the magnesium depletion score (MDS, a calculated value based upon risk factors for magnesium depletion), has been associated with a higher overall mortality rate in a large cohort (NHANES) of people with hypertension.