Short-Term Magnesium Supplementation Has Modest Detrimental Effects on Cycle Ergometer Exercise Performance and Skeletal Muscle Mitochondria and Negligible Effects on the Gut Microbiota: A Randomized Crossover Clinical Trial

<b>Background/Objectives:</b> Although the importance of magnesium for overall health and physiological function is well established, its influence on exercise performance is less clear. The primary study objective was to determine the influence of short-term magnesium supplementation on cycle ergometer exercise performance. The hypothesis was that magnesium would elicit an ergogenic effect. <b>Methods:</b> A randomized, double-blind, placebo-controlled, two-period crossover design was used to study men and women who were regular exercisers. Fifteen participants ingested either a placebo or magnesium chloride (MgCl₂ 300 mg) twice per day, for 9 days, separated by a 3-week washout. During days 8 and 9, participants completed a battery of cycle ergometer exercise tests, and whole blood, vastus lateralis, and stools were sampled. The primary outcomes were the maximal oxygen uptake (VO_2max), a simulated 10 km time trial, and the sprint exercise performance. Additional outcomes included skeletal muscle mitochondrial respiration, and, on account of the known laxative effects of magnesium, the gut microbiota diversity. <b>Results:</b> Compared with a placebo, MgCl₂ supplementation increased the circulating ionized Mg concentration (<i>p</i> < 0.03), decreased the VO_2max (44.4 ± 7.7 vs. 41.3 ± 8.0 mL/kg/min; <i>p</i> = 0.005), and decreased the mean power output during a 30 s sprint (439 ± 88 vs. 415 ± 88 W; <i>p</i> = 0.03). The 10 km time trial was unaffected (1282 ± 126 vs. 1281 ± 97 s; <i>p</i> = 0.89). In skeletal muscle, MgCl₂ decreased mitochondrial respiration in the presence of fatty acids at complex II (<i>p</i> = 0.04). There were no significant impacts on the gut microbiota richness (CHAO1; <i>p</i> = 0.68), Shannon's Diversity (<i>p</i> = 0.23), or the beta-diversity (Bray-Curtis distances; <i>p</i> = 0.74). <b>Conclusions:</b> In summary, magnesium supplementation had modest ergolytic effects on cycle ergometer exercise performance and mitochondrial respiration. We recommend that regular exercisers, free from hypomagnesemia, should not supplement their diet with magnesium.