Background: High blood pressure is associated with over 7 million deaths each year, about 13% of all deaths worldwide. Magnesium, potassium and calcium dietary salt supplements have been tried as alternatives to drug therapy. Functional and cellular concentrations of these salts in the body are inter-related. Raised levels of sodium are associated with high blood pressure and other electrolytes may affect blood pressure directly or through their effects on sodium levels. Some epidemiological studies report an inverse association between levels of magnesium, potassium and calcium intake and blood pressure but others do not. Such associations are difficult to interpret as the levels of these salts in the diet is correlated with levels of other nutrients. Recent meta-analyses have reported small, short-term reductions in blood pressure consequent to increased levels of magnesium, potassium and calcium separately [1,2,3]. However, these meta-analyses included short-term studies and normotensive patients, which may not be relevant to the long-term treatment of hypertensive patients. Objectives: To evaluate the effect on blood pressure of dietary supplementation with magnesium, potassium, or calcium.

Methods: We included randomised controlled trials of a parallel or crossover design with an intervention period of at least 8 weeks. We considered patients with a minimum systolic blood pressure (SBP) 140mmHg or minimum diastolic blood pressure (DBP) 85mmHg. We excluded studies of pregnant women, people with diabetes and studies including patients on antihypertensive medication which varied during the study. Relevant studies were identified by searching the Cochrane Library, MEDLINE, EMBASE, ISI Proceedings, web-based trials registers and the reference lists from existing systematic reviews. Meta-analyses estimated mean differences in blood pressure at endpoint using a random effects model.

Results: We found 11, 12 and 5 studies of magnesium, potassium and calcium respectively, enrolling 593, 410 and 414 participants. Trials were of poor quality, only 3 reporting adequate concealment of allocation. Findings were heterogeneous and the only statistically significant findings were small reductions in DBP and SBP with magnesium and calcium supplementation respectively.

Conclusions: The current, limited evidence does not support a consistent or clinically important effect on blood pressure due to dietary supplementation with magnesium, potassium, calcium or combined salts. Our findings differed from those of other systematic reviews as we excluded short-term studies and studies of normotensive patients. Further, our conclusions take account of the heterogeneity between trials and of the quality of included studies.

References: 1. Griffith LE, Guyatt GH, Cook RJ, Bucher HC, Cook DJ. The influence of dietary and non-dietary calcium supplementation on blood pressure: an updated meta analysis of randomized controlled trials. Am J Hypertens. 1999; 12:84-92. 2. Jee SH, Miller ER, III, Guallar E, Singh VK, Appel LJ, Klag MJ. The effect of magnesium supplementation on blood pressure: a meta-analysis of randomized clinical trials. Am J Hypertens. 2002;15:691-696. 3. Whelton PK, He J, Cutler JA, Brancati FI, Appel LJ, Follmann D. Effects of oral potassium on blood pressure: Meta-analysis of randomized controlled clinical trials. JAMA. 1997;277:1624-1632