Magnesium doesn't get the attention sodium does. It's not what you taste in sweat, and it's not the electrolyte people ask about when they cramp mid-session. But it's a cofactor in more than 300 enzyme systems — energy production, glycolysis, protein synthesis, muscle and nerve function, blood glucose control — and unlike sodium, many people are already running short of it before they've done a single rep [1].
That combination — high physiological demand and common deficiency— is why magnesium is worth understanding properly.
Sweat losses: real, but not the main event
Magnesium concentration in sweat is low relative to sodium and potassium. Older calorimetry work under heat stress put typical sweat magnesium losses at around 3–4 mg per litre [2], and more recent reviews using ion chromatography report a similarly narrow band, in the region of roughly 2–12 mg/L [3]. Compare that to sodium losses of 350-2000 mg/L and it's clear magnesium isn't leaving the body through the skin in meaningful quantities through sweat.
The drop in plasma magnesium seen after strenuous exercise — commonly around 10% — is mostly not a sweat-loss issue. It's better explained by magnesium shifting between fluid compartments (extracellular fluid, plasma, red blood cells) as intensity rises, rather than by magnesium actually leaving the body [2]. Some of that magnesium is also being taken up by working muscle, which needs it for ATP turnover.
So sweat isn't where the loss occurs. Urine is.
Urinary losses: the part that actually adds up
A 2023 systematic review and meta-analysis pooling 14 studies (855 athletes vs. 521 untrained controls) found that athletes have significantly higher 24-hour urinary magnesium excretion than non-athletes — despite also eating significantly more dietary magnesium [4]. Athletes in the pooled data consumed roughly 52 mg/day more magnesium than controls, yet still ran lower serum magnesium concentrations. Higher intake but lower blood levels? The training itself is driving the difference.
This tracks with what's known about renal magnesium handling. Exercise increases catecholamine output and alters aldosterone and cortisol signalling, both of which affect how much magnesium the kidneys reabsorb versus excrete. More exercise leads to greater urinary excretion of magnesium. Add faster muscle turnover and higher energy needs, both of which pull on magnesium as a cofactor, and you get a mineral that's being lost faster in athletes than non-athletes.
Population-wide mild deficiency: most people start deficient
Here's the part that matters most for anyone training seriously: many people are deficient in magnesium.
An analysis of the UK National Diet and Nutrition Survey (n=3,238 adults) found that 19% of adults in their twenties had magnesium intakes below the Lower Reference Nutrient Intake — the threshold below which deficiency risk is considered likely — and that women across the 20–59 age range had significantly lower magnesium intake than men as a proportion of the Reference Nutrient Intake [5]. This isn't a one off finding; a broader 2025 review of global magnesium status describes suboptimal intake as widespread across essentially all world regions, flagged consistently by NHANES in the US and the NDNS in the UK, with women and older adults disproportionately affected [6].
None of this is overt deficiency causing pronounced symptoms— the kind that shows up as tetany or cardiac arrhythmias. It's the grey zone: intake below what's needed for optimal function, but above the level that produces obvious clinical signs. The practical effect is a population with low magnesium levels before any training stress is added, who then lose more of it through urine the harder they train [4].
For anyone in combat sports, Hyrox, or CrossFit with high training volume and intensity, that marginal starting position and elevated exercise-driven losses stack.
What supplementation actually does — and doesn't do
It's important to be clear about the evidence here rather than overselling it, because the research doesn't all point in the same direction.
Muscle soreness and recovery: the best current evidence is a 2024 systematic review of magnesium supplementation and muscle soreness in physically active populations. It's a small evidence base — four qualifying studies — but all four reported a positive effect on soreness, recovery, or exercise-induced muscle damage markers [7]. One trial using 350 mg/day of magnesium glycinate found significantly reduced soreness ratings at 24, 36, and 48 hours after an eccentric loading session compared to control [7].
Cramps: this is where it's important not to overstate the benefits. A Cochrane review of magnesium for skeletal muscle cramps found no randomised controlled trials at all specifically evaluating magnesium for exercise-associated cramping — the trial evidence that exists is almost entirely in non-exercise populations (older adults, pregnancy-associated cramps), and even there the effect was judged unlikely to be clinically meaningful [8]. Magnesium's role in neuromuscular excitability and calcium handling at the sarcoplasmic reticulum is well established mechanistically, but the direct evidence for "magnesium stops exercise cramps" isn't there yet. Anyone selling that as settled is ahead of the data.
Energy metabolism and glucose handling: Magnesium is required for glycolysis, oxidative phosphorylation, and ATP production directly, and it's a cofactor for enzymes involved in blood glucose regulation [1]. For anyone training, those are useful benefits.
Broader health context: low magnesium status is associated with worse markers across cardiometabolic and neuromuscular health at a population level [6].
How this applies to training
Most adults, and UK women in particular, are already under-consuming magnesium relative to reference intakes before they train at all [5][6]. Training then increases how much is excreted, even when dietary intake goes up to compensate [4]. The performance and recovery evidence is real but should be stated at the strength it actually has — solid for soreness and recovery, mechanistically sound for energy metabolism, genuinely unproven for cramp prevention specifically.
References
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National Institutes of Health, Office of Dietary Supplements. Magnesium — Fact Sheet for Health Professionals. ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/
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Institute of Medicine (US) Committee on Military Nutrition Research. The Effect of Exercise and Heat on Mineral Metabolism and Requirements, in Nutritional Needs in Hot Environments. Washington (DC): National Academies Press, 1993. ncbi.nlm.nih.gov/books/NBK236242/
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Jeukendrup, A. & Baker, L.B. Do you need potassium and magnesium in your sports drink? Summary of sweat electrolyte concentration data. trainingpeaks.com/blog/do-you-need-potassium-and-magnesium-in-your-sports-drink/
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Zhang, H., Wang, R., Guo, S., et al. (2023). Lower serum magnesium concentration and higher 24-h urinary magnesium excretion despite higher dietary magnesium intake in athletes: a systematic review and meta-analysis. Food Science and Human Wellness, 12(5), 1471–1480. https://doi.org/10.1016/j.fshw.2023.02.015
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Derbyshire, E. (2018). Micronutrient Intakes of British Adults Across Mid-Life: A Secondary Analysis of the UK National Diet and Nutrition Survey. Frontiers in Nutrition, 5:55. https://doi.org/10.3389/fnut.2018.00055
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Matek Sarić, M., Sorić, T., Juko Kasap, Ž., Lisica Šikić, N., Mavar, M., Andruškienė, J., & Sarić, A. (2025). Magnesium: Health Effects, Deficiency Burden, and Future Public Health Directions. Nutrients, 17(22), 3626. https://doi.org/10.3390/nu17223626
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Tarsitano, M.G., et al. (2024). Effects of magnesium supplementation on muscle soreness in different types of physical activities: a systematic review. Journal of Translational Medicine. https://doi.org/10.1186/s12967-024-05434-x
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Garrison, S.R., Korownyk, C.S., Kolber, M.R., et al. (2020). Magnesium for skeletal muscle cramps. Cochrane Database of Systematic Reviews. https://doi.org/10.1002/14651858.CD009402.pub3