Oral magnesium supplementation for high blood pressure in adults

This review examined whether taking magnesium supplements could be recommended for treating adults with high blood pressure from no known cause. It reviewed 12 trials enrolling 545 people, which compared magnesium supplementation with a dummy drug (placebo) or no treatment, and measured blood pressure 8 weeks to 6 months later. The results of trials varied a lot: some trials found magnesium lowered blood pressure much more than placebo, while others found little difference between magnesium and placebo. On average, people receiving extra magnesium achieved slightly lower diastolic blood pressure at the end of trials. None of the studies reported any serious side effects of taking magnesium supplements.

However, most included trials were of poor quality, so their results may not be reliable. The trials were not long enough or large enough to measure whether extra magnesium can reduce possible consequences of high blood pressure: death, heart attack or stroke.

The review did not find robust evidence that oral magnesium supplementation reduces high blood pressure in adults. Larger, longer duration, better quality trials are needed to clarify whether magnesium supplementation can lower high blood pressure.

Authors' conclusions: 

In view of the poor quality of included trials and the heterogeneity between trials, the evidence in favour of a causal association between magnesium supplementation and blood pressure reduction is weak and is probably due to bias. This is because poor quality studies generally tend to over-estimate the effects of treatment. Larger, longer duration and better quality double-blind placebo controlled trials are needed to assess the effect of magnesium supplementation on blood pressure and cardiovascular outcomes.

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Background: 

Epidemiological evidence on the effects of magnesium on blood pressure is inconsistent. Metabolic and experimental studies suggest that magnesium may have a role in the regulation of blood pressure.

Objectives: 

To evaluate the effects of magnesium supplementation as treatment for primary hypertension in adults.

Search strategy: 

We searched the Cochrane Library, MEDLINE, EMBASE, Science Citation Index, ISI Proceedings, ClinicalTrials.gov, Current Controlled Trials, CAB abstracts, and reference lists of systematic reviews, meta-analyses and randomised controlled trials (RCTs) included in the review.

Selection criteria: 

Inclusion criteria were: 1) RCTs of a parallel or crossover design comparing oral magnesium supplementation with placebo, no treatment, or usual care; 2) treatment and follow-up ≥8 weeks; 3) participants over 18 years old, with raised systolic blood pressure (SBP) ≥140 mmHg or diastolic blood pressure (DBP) ≥85 mmHg; 4) SBP and DBP reported at end of follow-up. We excluded trials where: participants were pregnant; received antihypertensive medication which changed during the study; or magnesium supplementation was combined with other interventions.

Data collection and analysis: 

Two reviewers independently abstracted data and assessed trial quality. Disagreements were resolved by discussion or a third reviewer. Random effects meta-analyses and sensitivity analyses were conducted.

Main results: 

Twelve RCTs (n=545) with eight to 26 weeks follow-up met our inclusion criteria. The results of the individual trials were heterogeneous. Combining all trials, participants receiving magnesium supplements as compared to control did not significantly reduce SBP (mean difference: -1.3 mmHg, 95% CI: -4.0 to 1.5, I2=67%), but did statistically significantly reduce DBP (mean difference: -2.2 mmHg, 95% CI: -3.4 to -0.9, I2=47%). Sensitivity analyses excluding poor quality trials yielded similar results. Sub-group analyses and meta-regression indicated that heterogeneity between trials could not be explained by dose of magnesium, baseline blood pressure or the proportion of males among the participants.

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