Magnesium
Magnesium is a cofactor in over 300 reactions, and most athletes run low. The deficiency is quiet but expensive.
Why athletes run chronically low
Magnesium deficiency in athletes is quiet, common, and expensive in ways that rarely get attributed to their actual cause. Sweat carries between 3 and 8 milligrams of magnesium per litre. An athlete losing one to two litres of sweat in a hard session can shed well over ten milligrams of magnesium in a single workout, and the losses climb further in heat and at higher training intensities — exactly the conditions under which the body most needs magnesium for energy production and nervous-system regulation. Train hard several times a week and the cumulative loss steadily outpaces what a typical diet replaces. a review of magnesium status in athletes reviews magnesium status in athletes and finds suboptimal levels common even in those eating well.
Diet often fails to close the gap, for reasons that have nothing to do with nutritional negligence. Magnesium is concentrated in dark leafy greens, legumes, nuts, seeds and dark chocolate — foods that many athletes eat inconsistently. Compounding this, decades of intensive agriculture have steadily depleted magnesium from farmed soils, reducing the magnesium content of these foods by an estimated twenty to thirty percent compared with mid-twentieth-century levels. The result is that a hard-training athlete eating what looks like a reasonable, balanced diet is frequently running a persistent magnesium deficit without any obvious dietary failing.
What the deficiency actually costs
Magnesium is a cofactor in over three hundred enzymatic reactions, which is why a deficiency produces such a scattered, easily-misattributed set of symptoms. Most fundamentally, every molecule of ATP — the body's energy currency — must be bound to a magnesium ion to be biologically usable. When magnesium is low, cellular energy production runs below its potential capacity, which is felt as an earlier fatigue ceiling at the same training effort. The athlete plateaus or tires sooner and blames overtraining, poor sleep, or simply a bad day, when the underlying issue is a mineral deficit.
Magnesium also governs the calcium-magnesium exchange that controls muscle contraction and relaxation. Calcium drives contraction; magnesium is required for the muscle to relax afterward. Insufficient magnesium shows up as cramping and as muscles that struggle to fully release — particularly the nocturnal cramps that many athletes wrongly accept as a normal part of training hard. And magnesium is central to sleep: it acts as a natural antagonist at the NMDA receptor, applying a brake to the excitatory glutamate signalling that keeps the nervous system aroused. Depleted athletes frequently describe difficulty switching off at night and shallow, unrefreshing sleep — a pattern that often reflects magnesium status more than lifestyle or stress.
Forms that work and forms that do not
Oral magnesium supplementation is effective but constrained by two factors: gut absorption, which the body downregulates as stores fill, and a laxative effect that appears at higher doses. Within those constraints, the form matters enormously — and the cheapest, most widely sold form is among the worst absorbed.
| Form | Absorption | Notes |
|---|---|---|
| Glycinate | High | Best for sleep, gentle on the gut |
| Malate | High | Good for daytime use and energy |
| Citrate | Moderate | Effective but can loosen stool |
| Oxide | Very low | Avoid — cheap and poorly absorbed |
Magnesium glycinate, where the mineral is bound to the amino acid glycine, is well absorbed, gentle on the digestive system, and the glycine itself has a mild calming effect that makes it particularly suitable for evening use. Magnesium malate, bound to malic acid, is also well absorbed and tends to suit daytime supplementation. Magnesium oxide, by contrast, is poorly absorbed and largely passes through — it is cheap and common precisely because the magnesium content per tablet looks high on paper, even though little of it reaches the bloodstream.
Transdermal delivery and the heated soak
Because oral dosing is capped by gut absorption, transdermal delivery offers a genuinely useful alternative route. Magnesium absorbed through the skin during a warm soak bypasses the gut's absorption ceiling entirely, delivering the mineral directly to tissue and circulation. Heat enhances this effect by dilating surface vasculature and opening pores, and the extended contact time of a proper soak accomplishes considerably more than a quick topical spray or lotion. A heated magnesium spa combines this delivery route with the parasympathetic benefit of sustained heat — which is why heated magnesium spa sessions have become a popular evening recovery tool among Melbourne athletes who are managing both the chronic mineral deficit and the nervous-system load that comes with hard training. The combination addresses the magnesium status and the sleep-disrupting sympathetic arousal in a single session.
A practical approach
- Use oral glycinate or malate daily as the foundation — glycinate in the evening for its sleep benefit
- Avoid magnesium oxide regardless of its low price and high label figures
- Prioritise magnesium-rich whole foods: leafy greens, nuts, seeds, legumes
- Add a transdermal soak as an evening top-up, especially after the heaviest training days
- Expect the clearest early sign of repletion to be improved sleep quality rather than any acute performance jump
How much, and how to tell it is working
The recommended dietary intake sits around 400–420mg daily for adult men and 310–320mg for adult women, but hard-training athletes sit at the higher end of requirement because of their sweat losses. A common practical approach is to cover the dietary baseline through food and add a supplemental 200–400mg of elemental magnesium from a well-absorbed form, adjusting down if any loosening of the stool appears. Because blood magnesium levels are tightly regulated and reflect only about one percent of total body magnesium, a standard blood test is a poor guide to actual tissue status — which is part of why the deficiency goes undetected so often. The most reliable practical signal is symptomatic: improved sleep, fewer cramps, and a sense of recovering more completely between sessions usually appear within a couple of weeks of genuine repletion. That subjective improvement, rather than a lab number, is what most people end up using to confirm they have addressed the gap.
Most athletes have never been properly magnesium-replete. The first time tissue levels normalise, the sleep improvement alone is noticeable.
