Magnesium: Supplementation, Absorption and Effect on Blood Pressure and Exercise
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Magnesium: supplementation, absorption and effect on blood pressure and exercise. Submitted by Lindsy Kass to the University of Exeter as a thesis for the degree of PhD by Publication in Sport and Health Sciences August 2017 This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material has previously been submitted and approved for the award of a degree by this or any other University. Signature: ………………………………………………………….. 1 Abstract 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. 2 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. 3 Contents Abstract ................................................................................................................................................. 2 List of tables .......................................................................................................................................... 5 List of Appendices .................................................................................................................................. 5 Author’s declaration .............................................................................................................................. 6 Definitions and abbreviations ................................................................................................................. 7 Section 1 Introduction ............................................................................................................................ 8 Literature Review ........................................................................................................................................... 8 1.1 Background to magnesium ....................................................................................................................... 8 1.2 Magnesium absorption, excretion and markers of flux and homeostasis ............................................... 8 1.3 Methods of Assessment of Magnesium absorption and concentration ................................................ 12 1.3.1 Serum Magnesium ........................................................................................................................... 12 1.3.2 24-hour Urinary Mg2+ Excretion ..................................................................................................... 13 1.4 Magnesium and blood pressure ............................................................................................................. 13 1.5 Magnesium and energy metabolism ...................................................................................................... 14 1.6 Magnesium and exercise ........................................................................................................................ 14 1.6.1 Magnesium, exercise and inflammation ......................................................................................... 16 1.7 Magnesium consumption ....................................................................................................................... 17 1.8 Magnesium supplementation – dosage and type .................................................................................. 18 Section 2. Aims, objectives, methodology and overview of findings ...................................................... 21 2.1 List of contribution, by publication, from Lindsy Kass:- ......................................................................... 21 2.2 Aims and objectives ................................................................................................................................ 22 2.2.1 General aim ..................................................................................................................................... 22 2.2.2 Specific aims .................................................................................................................................... 22 2.3 General Methods .................................................................................................................................... 22 2.3.1 Ethical approval, consent forms and health and safety considerations.......................................... 22 2.3.2 Participants ...................................................................................................................................... 23 2.3.3 Measurements ................................................................................................................................. 23 2.4 Overview, findings and results ............................................................................................................... 24 2.4.1 The effect of magnesium on blood pressure, power and peak torque with short term high intensity exercise ...................................................................................................................................... 24 2.4.2 Dietary stratification of high and low magnesium intake and effect on blood pressure and performance ............................................................................................................................................. 27 2.4.3 Effect of magnesium supplementation on blood pressure – a meta-analysis ................................ 30 2.4.4 Further effects of magnesium on blood pressure with acute and chronic dosing strategies ....... 34 2.4.5 Habitual dietary magnesium intake in the hypertensive population; observations on general population dietary intake ......................................................................................................................... 38 2.4.6 Transdermal magnesium absorption ............................................................................................. 43 2.4.7 Limitations to studies ...................................................................................................................... 46 4 2.4.8 Summary of findings, future work and contribution made by the papers in the context of the approved field of study............................................................................................................................. 48 Section 3 Publications .......................................................................................................................... 53 3.1 Pulford, R & Kass, L,. The effects of acute magnesium supplementation on maximal intensity short term exercise and subsequent effect on blood pressure and isokinetic knee extension during recovery. Proceedings of the Nutrition Society