Training
Magnesium and muscle
It is a cofactor in several hundred enzyme reactions, roughly half of adults get less than the reference intake, and the marketing has attached it to the one outcome the trials are least kind to.
What most people believe
Magnesium is the recovery mineral. Take it after training or before bed and muscles relax, sleep improves and cramp stops.
What the evidence actually says
The actual job
Magnesium does not do anything by itself. It is what several hundred enzymes need in order to do their work — including the ones that build protein, copy DNA, and turn food into usable energy. Every molecule of ATP is functionally magnesium-bound. That is why a shortfall shows up as diffuse tiredness rather than as one specific complaint: it is not one system failing, it is everything running slightly worse.
In muscle it sits opposite calcium. Calcium tells a fibre to contract; magnesium is part of what lets it release. That pairing is the seed of the recovery marketing, and the biology is real — it is the leap from the biology to the tub that does not hold.
Why training raises the requirement
Some is lost in sweat, in quantities that matter over long sessions in heat. Some is lost in urine, and hard exercise increases that loss. And magnesium redistributes during exercise — it moves between compartments — which is one reason blood levels are such a poor guide to status.
How much extra an athlete needs is genuinely unsettled, and the table says so. The figure usually quoted is modest, and it is smaller than the gap most people already have from eating refined grain.
Where a blood test misleads
About sixty per cent of the body’s magnesium is in bone and most of the rest is inside cells. Under one per cent is in blood, and the body defends that fraction by pulling magnesium out of bone.
So a normal serum magnesium is compatible with a depleted store, in exactly the way a normal blood calcium is compatible with a skeleton that has been paying for it for years. There is no cheap, routine test that sees the store, which is why the sensible approach is to look at what you eat rather than to chase a number.
The gap between fixing a shortfall and buying an effect
If your intake is low, raising it is worth doing — for the enzymes, the blood pressure evidence, and the bone. That is a real and common situation, and milling is why: refining a grain removes the germ and bran and about four-fifths of its magnesium.
If your intake is already adequate, adding more has not been shown to do much of anything, including for cramp and including for sleep. The trials that show benefit are largely trials in people who were short.
There is also a ceiling that surprises people: the upper limit applies to supplemental magnesium only, not to food. You cannot overdo it from almonds. You can from a tub, and the first sign is diarrhoea, because the forms that absorb worst are precisely the ones sold as laxatives.
The numbers, and how firmly they are held
| What it describes | Figure | Evidence |
|---|---|---|
| Magnesium lost in sweat | ≈ 15 mg/L | Probable |
| Additional requirement in athletes Genuinely unsettled; smaller than most dietary gaps | 10–20 % above the RDA | Contested |
| Benefit from supplementing Trials showing effect are mostly in people who were short | only if intake is low | Probable |
| Upper limit, supplements only Does not apply to magnesium from food | 350 mg | Established |
Established means the position stands agree and you can act on it. Probable means the weight of evidence points one way with real dissent. Contested means it is genuinely unsettled, and anyone selling you certainty about it is selling something.
What follows is easier when you know what you are already eating.
What to actually do
Change the grain before you buy the tub
Milling removes about eighty per cent of the magnesium. Wholegrain instead of refined moves more than a supplement does, and brings everything else that went with it.
Do not read serum magnesium as reassurance
Under one per cent of your magnesium is in blood, and the body defends that fraction from bone. A normal result does not rule out a depleted store.
If you do supplement, mind the form
Oxide is poorly absorbed and is what most cheap tubs contain. Citrate and glycinate absorb better. And the upper limit applies to supplements, not food.
Sources
- NIH Office of Dietary Supplements — Magnesium
- Magnesium status and exercise — a review
- Magnesium supplementation and outcomes — systematic review
This page is education, not medical advice. It does not diagnose anything and it is not a substitute for a clinician who knows your history. Sports nutrition in particular is a field where the evidence moves, and where a figure that suits a trained twenty-five-year-old may be wrong for you.
Every nutrient named above, measured — in the app.