Magnesium: Supplementation, absorption and effect on blood pressure and exercise

Introduction Magnesium is required by the human body in modest amounts for the maintenance of health and optimal functioning. Objectives This portfolio of work sets out to investigate whether magnesium supplementation has hypotensive effects and to determine if habitual dietary magnesium intake or loading strategies modulate the effects of magnesium supplementation. The habitual dietary magnesium intake of hypertensive patients was also examined to ascertain adequacy of dietary magnesium in this cohort. A meta-analysis was performed on the effect of magnesium supplementation on blood pressure. Other variables such as dosage, duration and study design were considered and findings from the meta-analysis used to influence future work. A further objective was to examine the effect of supplementation on aerobic and resistance exercise and subsequent recovery. Finally, the efficacy of an alternative means of magnesium delivery in the form of a transdermal magnesium cream was investigated. Methods A 300 mg.day-1 elemental magnesium aspartate or magnesium citrate was used as a supplementation in studies 1,2,4 and 5. Participants were instructed to continue with their normal diet and for study 6 participants were required to eat the same foods for the 24 hours prior to both laboratory blood taking sessions. With the exception of the meta-analysis, food diaries were kept for various lengths of time, detailed in the publications. Aerobic and resistance exercise protocols were carried out in studies 1,2 and 4, with both performance and cardiovascular parameters investigated for any effect from supplementation. Where supplementation was in the form of a transdermal cream, this was applied to the torso and absorption of the cream was determined by investigating changes in serum and urinary magnesium levels. Summary of results Blood pressure decreased with magnesium supplementation of 300 mg.day-1 for 7 days with greater reductions in systolic versus diastolic blood pressure consistently evident. Magnesium supplementation of 300 mg.day-1 for 7 and 14 days increased power during resistance exercise but no changes in aerobic exercise performance were observed. A high habitual dietary magnesium intake attenuated the hypotensive effect derived from magnesium supplementation when compared to those on a low habitual dietary intake. The meta-analysis supported these results. A habitually low dietary magnesium intake was observed in a cohort of clinically diagnosed primary hypertensives. Conclusion These studies show that there is a link between low habitual dietary magnesium intake and elevated blood pressure and that magnesium supplementation appears to be associated with blood pressure. An improvement in resistance exercise performance with magnesium supplementation was also observed. Finally, a transdermal magnesium cream was shown to increase serum magnesium levels and may provide an alternative to oral supplementation.