Magnesium in Athletes: What the Evidence Actually Shows About Deficiency, Muscle Cramps, and Supplementation
Table of Contents
Introduction
Magnesium in athletes has long been surrounded by a strong sports nutrition myth: that taking it as a supplement will reliably reduce muscle cramps. For years, many people have used magnesium with the belief that it prevents cramping, and it remains a common supplement among athletes who hope it will accelerate muscle recovery or improve muscle function. The evidence, however, is considerably more nuanced than the packaging suggests.
I wrote this article to clarify what is actually known about magnesium in athletes: how magnesium status behaves during training, whether supplementation is genuinely useful, whether magnesium deficiency is a meaningful cause of exercise-associated muscle cramps, and whether magnesium improves performance or recovery in real-world athletic settings.
Why Magnesium in Athletes Matters for Performance
Magnesium is the fourth most common mineral in the human body, with 53% stored in bone, 27% in muscle, 19% in soft tissues, and less than 1% in the serum [1]. Its functional reach is enormous: magnesium is involved as a cofactor in more than 300 enzyme systems and is required for fundamental processes including energy production and nucleic acid synthesis [1]. In skeletal muscle specifically, magnesium in athletes is integral to ATP synthesis and utilisation, plays a role in oxygen uptake, and is involved in electrolyte balance — all of which sit at the core of athletic performance [1].
Magnesium also plays a key role in the active transport of calcium and potassium ions across cell membranes, a process that is important for nerve impulse conduction, muscle contraction, and normal heart rhythm [1]. Given this physiological centrality, the question of whether athletes are getting enough magnesium is clinically meaningful.
True hypomagnesaemia, when clinically significant, does not usually present as a clean and specific “magnesium deficiency syndrome.” Mild abnormalities are often asymptomatic, while more severe deficiency is associated with neuromuscular irritability and may contribute to cardiac arrhythmias. Hypomagnesaemia frequently coexists with hypokalaemia and hypocalcaemia, which can further modify the clinical presentation [5].
These symptoms are not specific to magnesium deficiency. In practice, they often overlap with the effects of other electrolyte abnormalities, nutritional deficiencies, medication effects, or broader illness. This is why symptoms such as fatigue, weakness, cramps or poor recovery should not be assumed to reflect magnesium deficiency without considering the wider clinical context.
In routine clinical practice, magnesium is rarely at the centre of the assessment. It is not included in many standard laboratory panels, and clinically obvious magnesium deficiency appears to be uncommon in otherwise healthy people, including athletes with a reasonably varied diet. In most cases, dietary intake is sufficient when overall energy intake and food variety are adequate.
When magnesium deficiency does occur, it is usually worth looking for a broader explanation rather than assuming that training alone is the cause. Clinically relevant deficiency is more likely in the setting of persistent diarrhoea or other gastrointestinal losses, poor nutritional intake, alcohol misuse, certain medications, or kidney-related disturbances in magnesium handling.
Magnesium Deficiency in Athletes: What the Evidence Shows
Evidence suggests that some athlete cohorts show lower serum magnesium and higher urinary magnesium excretion than untrained controls, suggesting that exercise-related losses may contribute to lower magnesium status alongside dietary factors.
An 8-year longitudinal study of 192 Olympic and Paralympic athletes on the British Athletics world class performance plan found that on at least one blood test during the study period, 22% of athletes were identified as clinically deficient (<1.19 mmol/L red cell magnesium) [2]. Magnesium was significantly lower in female athletes and those with Black or Mixed-Race ethnicity, and athletes with a history of Achilles or patella tendon pain had significantly lower magnesium levels than average [2].
Beyond this single elite cohort, a systematic review and meta-analysis incorporating 14 cross-sectional studies with 855 athletes and 521 control subjects found that serum magnesium concentration was significantly lower in athletes (mean difference −0.04 mmol/L; 95% CI −0.06 to −0.01; p = 0.02) despite significantly higher dietary magnesium intake (mean difference 51.72 mg/day; 95% CI 14.62 to 88.83; p = 0.006) [3]. Crucially, 24-hour urinary magnesium excretion in athletes was significantly higher than in the untrained population (mean difference 0.76 mmol/day; 95% CI 0.11 to 1.41; p = 0.02) [3].
In practical terms: athletes eat more magnesium, yet show lower serum levels and excrete more. The specific mechanisms driving this pattern remain an area of active research.
In my clinical experience, the lower magnesium values reported in some athlete studies should not automatically be treated as clinically significant deficiency. When I see otherwise healthy athletes, training alone is not usually enough to explain severe hypomagnesaemia, and a slightly lower magnesium result should not be interpreted in isolation.
Magnesium supplementation is also a relatively uncommon sight on a patient’s medication list in routine clinical work, at least compared with calcium or iron supplementation. That contrast is worth noting: magnesium is highly visible in sports nutrition marketing, but it is much less often a central issue in everyday clinical assessment.
I would be more concerned when a low or borderline magnesium result appears alongside another risk factor: restricted energy intake, persistent gastrointestinal losses, medication effects, alcohol misuse, or an underlying medical condition. In that situation, magnesium becomes part of a broader clinical picture rather than a stand-alone explanation.
For that reason, I do not see magnesium supplementation as something athletes should start automatically just because they train hard or because one magnesium marker is slightly low. I would rather interpret the result in context: diet, symptoms, training load, gastrointestinal health, medications, and the specific laboratory marker used.
Why Athletes Lose More Magnesium During Exercise
Strenuous exercise apparently increases urinary and sweat losses that may increase magnesium requirements by 10–20% [4]. This research has shown that exercise induces a redistribution of magnesium in the body to accommodate metabolic needs [4]. Based on dietary surveys and recent human experiments, a magnesium intake less than 260 mg/day for male and 220 mg/day for female athletes may result in a magnesium-deficient status [4].
Athletes participating in sports requiring weight control, such as wrestling and gymnastics, are apparently especially vulnerable to an inadequate magnesium status [4]. The RED-S population — athletes with low energy availability — may be at compounded nutritional risk more broadly, and assessing magnesium status as part of a wider micronutrient evaluation may be clinically reasonable when dietary restriction or symptoms are present.
In practice, however, magnesium deficiency rarely turns out to be an athlete’s main problem purely because of training-related loss or increased use. When I do encounter concern about low magnesium status in an athlete, the more relevant issue is often overall intake: restricted eating, low energy availability, poor dietary variety, or a broader pattern of under-fuelling.
In other words, diet-related magnesium insufficiency is usually more plausible than the idea that athletic training alone has created a clinically meaningful deficiency. I would still emphasise that magnesium deficiency remains an atypical finding in otherwise healthy patients without another underlying pathology or clear nutritional risk factor.
The Testing Problem: Why Standard Blood Tests Miss Magnesium Deficiency in Athletes
Here is where clinical practice diverges sharply from popular understanding. Serum total magnesium — the standard blood test — reflects less than 1% of total body magnesium [1] and does not accurately reflect intracellular stores [5]. This creates a meaningful interpretive challenge: magnesium status should not be judged from serum magnesium alone, just as other intracellular markers require careful contextual interpretation.
An athlete can present with serum magnesium within the reference range while still having low total body or tissue magnesium status. Red cell magnesium measurement — which is what Pollock et al. used in their 8-year British Athletics study [2] — is one alternative measure used in athlete research, though no single magnesium test is considered fully satisfactory [5]. The magnesium-loading or tolerance test is one proposed method for assessing magnesium retention and status, but practical limitations restrict routine clinical use.
For practical purposes: a normal serum magnesium does not necessarily exclude low total body or tissue magnesium status. This is also why magnesium can be considered as part of a structured athlete blood work panel when clinically relevant, and why preparation before testing — particularly training timing — influences results.
Total body magnesium cannot be measured accurately with a single routine blood test. Serum magnesium is useful for detecting clear abnormalities, but it represents only a small fraction of body magnesium and may miss low tissue stores. A more detailed assessment of magnesium status is generally limited to research settings or selected specialist care, where tests such as red cell magnesium, ionised magnesium, 24-hour urinary magnesium, or magnesium-loading protocols may be available.
In everyday primary care, it is therefore uncommon to reach a point where true magnesium deficiency can be confidently demonstrated, especially in an otherwise healthy athlete. When patients say they “have magnesium deficiency,” this is often a personal interpretation based on symptoms, supplement marketing, or a nonspecific laboratory result rather than a confirmed diagnosis. That does not mean magnesium status is never clinically relevant, but it does mean the label should be used carefully.
The Muscle Cramp Myth: Does Magnesium Deficiency Cause Exercise-Associated Muscle Cramps?
This is probably the most persistent myth in sports nutrition surrounding magnesium in athletes, and the evidence does not support it.
Exercise-associated muscle cramps (EAMC) — sudden, involuntary contractions occurring during or after intense exercise — are a distinct clinical phenomenon from cramps related to systemic metabolic disorders such as severe hypomagnesaemia. A review of the EAMC evidence base concluded that research data increasingly support altered neuromuscular excitability as the principal pathophysiological mechanism, while prospective cohort studies found that serum electrolyte concentrations were not significantly different between athletes who cramped and those who did not [7].
A Cochrane systematic review (2020 update) examined all available randomised controlled trial evidence for magnesium supplementation in skeletal muscle cramps [6]. The review found no RCTs evaluating magnesium for exercise-associated muscle cramps or disease-state-associated muscle cramps, other than a single small inconclusive study in people with liver cirrhosis, only some of whom suffered cramps [6]. For idiopathic rest cramps in older adults, the pooled analysis showed a statistically nonsignificant difference of −9.59% (95% CI −23.14 to 3.97) in the percentage change in cramp frequency from baseline at four weeks, and the 95% confidence interval excluded a clinically meaningful 25% reduction beyond placebo [6]. The authors concluded that it is unlikely that magnesium supplementation provides clinically meaningful cramp prophylaxis to older adults experiencing skeletal muscle cramps [6].
If you are treating EAMC, the appropriate clinical approach involves addressing sodium balance and sweat losses, training load management, hydration, and neuromuscular fatigue — before attributing the problem to magnesium. The electrolyte picture in athletes extends across sodium, potassium, calcium, and chloride — magnesium is one piece of a broader puzzle.
In clinical practice, patients often suggest magnesium deficiency as a possible explanation for muscle cramps, and some specifically ask whether their magnesium level should be tested. In these situations, I usually explain that the available evidence does not support magnesium deficiency as a common explanation for exercise-associated muscle cramps. The same caution applies to other electrolytes: cramping is often attributed to electrolyte deficiency, but this is more commonly a belief than a confirmed diagnosis.
The current understanding of exercise-associated muscle cramps points more toward altered neuromuscular excitability and fatigue-related dysregulation than a simple lack of magnesium or another electrolyte. That does not mean hydration, sodium balance or broader nutrition are irrelevant, but it does mean that cramps should not automatically be reduced to “low magnesium.”
At the same time, modest magnesium supplementation is unlikely to cause harm in most healthy adults with normal kidney function when used within recommended intake limits. If a patient feels that it helps, the benefit may sometimes come through expectation, placebo effect, improved routine, or correction of a mild dietary insufficiency. I do not object to reasonable supplementation in a low-risk patient, but I try to separate that from the claim that magnesium deficiency is the usual cause of athletic cramping.
Conclusion
Magnesium is physiologically important for athletes, but its role is often overstated in supplement marketing. Some athlete studies show lower serum magnesium and higher urinary magnesium excretion than untrained controls, yet this does not automatically mean clinically significant magnesium deficiency. In otherwise healthy athletes, true magnesium deficiency is uncommon and is more likely to reflect restricted intake, gastrointestinal losses, medication effects, alcohol misuse, kidney-related disturbances, or broader under-fuelling than training alone.
Serum magnesium is also an imperfect marker, so results must be interpreted in clinical context rather than treated as a stand-alone diagnosis. The evidence does not support magnesium deficiency as a common cause of exercise-associated muscle cramps, which appear to relate more strongly to altered neuromuscular excitability and fatigue. Modest magnesium supplementation is unlikely to harm most healthy adults with normal kidney function when used within recommended limits, but it should not be presented as a proven solution for cramps, recovery, or performance in athletes without documented or strongly suspected insufficiency.
References
[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5637834/
[2] https://pubmed.ncbi.nlm.nih.gov/31829845/
[3] https://www.sciopen.com/article/10.1016/j.fshw.2023.02.015
[4] https://pubmed.ncbi.nlm.nih.gov/17172008/
[5] https://pmc.ncbi.nlm.nih.gov/articles/PMC6163803/
[6] https://pmc.ncbi.nlm.nih.gov/articles/PMC8094171/
