MagnesiumMost runners discover magnesium the same way — mid-run, mid-cramp, mid-curse. The calf locks. The hamstring spasms in the last mile of a long one. The toes splay at 3am after a hard session. Someone in a forum mentions magnesium. You shrug, buy a tub, take one when you remember, feel a bit better, forget about it.

That's the entry point. It's also the smallest possible version of what magnesium actually does for a runner.

mag is involved in over 300 reactions in the body — energy production, muscle contraction, nervous-system regulation, sleep architecture, electrolyte balance. Almost all of them matter more when you run regularly. And almost all of them are working harder, and emptying faster, the more you train.

Here's what's actually going on, and what to do about it.

Three things shift when you train consistently. Each one increases your magnesium demand. Together they create a chronic, low-grade deficit most runners never quite close.

You sweat more of it out. Sweat carries electrolytes — sodium most obviously, but also potassium, calcium and magnesium. The more you sweat, the more you lose. A long hot run can move more magnesium than a typical day's food intake replaces.

Your kidneys excrete more of it. Intense exercise temporarily increases urinary magnesium excretion for several hours afterwards. Train hard four or five times a week, and that loss compounds.

Your cellular demand goes up. Magnesium is required to generate ATP — the energy currency every muscle contraction runs on. The more contractions you produce in a week, the more magnesium is being pulled into the metabolic machinery.

Combined, regular running can increase magnesium requirements by around 20% above sedentary baseline. Most diets, even reasonably good ones, only just cover the sedentary requirement. The gap is small, persistent, and exactly what makes daily supplementation useful for runners.

Strip the marketing language back, and four functions matter most.

Muscle contraction and relaxation. Magnesium is half of the mineral pair (with calcium) that lets your muscles contract and then release. Low magnesium means muscles stay slightly contracted when they should let go — which is the underlying story of most exercise-associated cramps.

Energy production. ATP, the molecule that powers every step, can only function when bound to magnesium. No magnesium, no useful ATP. Run depleted and the body works harder to produce the same effort.

Nervous-system regulation. Magnesium quiets the sympathetic ("on") nervous system and supports the parasympathetic ("off") side. For runners, this matters at both ends of the day: lower resting tension, deeper sleep, easier wind-down after evening sessions.

Reduction of tiredness and fatigue. A recognised effect of magnesium, and one most runners feel viscerally when stores are low — that flat, dull, "legs feel heavy from kilometre one" sensation that can't be explained by training load alone.

Exercise-associated muscle cramps are messy, multi-causal, and not always fixed by a single nutrient. But magnesium is one of the most consistently useful inputs for runners who cramp regularly.

Three patterns are worth knowing.

End-of-run cramps, especially in the calves and hamstrings on long efforts. Often a combination of cumulative fatigue, electrolyte loss and a depleted magnesium baseline. Daily supplementation supports the baseline; race-day electrolyte intake supports the acute losses.

Nocturnal cramps, the ones that wake you at 2am after a hard day. These respond particularly well to consistent magnesium — especially glycinate, the form most studied for nervous-system relaxation.

Foot, toe and arch cramps, less obviously linked to running volume, often more linked to overall mineral status. Worth checking your diet first, then adding daily magnesium if the pattern persists.

What magnesium isn't is a miracle. If you cramp regularly, daily magnesium is one of the levers — alongside sodium intake, hydration, pacing and conditioning. It's the one most often missing.

The slowest improvement curves in running come from runners who train hard and recover poorly. The body adapts to training only during the rest between sessions. The quality of that rest decides how much you actually get out of the work.

Magnesium contributes to several parts of the recovery picture:

• Muscle relaxation and tension release after the contraction-heavy stress of running

• Energy-yielding metabolism, which the body needs to rebuild glycogen stores

• Nervous-system regulation, which supports the shift from sympathetic dominance (training, racing, stress) to parasympathetic dominance (digestion, repair, sleep)

For runners building up volume, peaking for a race, or stacking sessions across a heavy week, recovery is where the gains hide. Magnesium is one of the cleaner inputs for supporting it.

The fatigue runners feel isn't only muscular. A heavy training block taxes the central nervous system, hormonal balance and sleep architecture. You can run with tired legs. You can't run well with a wired-but-tired nervous system that won't let you sleep properly.

Magnesium — particularly glycinate — supports the body's shift into rest states. The effect is subtle in week one and clearer by weeks two to four: faster wind-downs in the evening, less middle-of-the-night waking, and a quieter baseline that makes hard sessions feel less expensive the next day.

For runners on heavy training loads, this is often where consistent magnesium pays off most. Not the dramatic cramp prevention. The boring, accumulating improvement in how recovered you feel between runs.

If you've been taking magnesium daily, you don't need a different protocol on race day. Your stores are what matter, and they're built over weeks.

A few practical notes for race week:

• Don't introduce new supplements in the final 7–10 days. Race week is for repeating what's been working, not experimenting.

• Race morning isn't a top-up window. A magnesium capsule with breakfast on race day won't meaningfully change race-day mineral status. The work was done in the months before.

• Acute electrolyte needs during the race — sodium especially, alongside carbohydrate — are different from chronic mineral status. That's what gels, chews and electrolyte drinks are for.

The right way to think about race-day magnesium is the same as race-day sleep: the quality you get is the average of the previous several weeks.

For most runners:

• Around 375mg of elemental magnesium per day, ideally from a multi-form complex. This is roughly 100% of the daily recommended intake (EU Nutrient Reference Value) and sits comfortably within safe supplemental ranges for long-term use.

• A four-form blend rather than a single form. Different forms suit different jobs — malate for energy, glycinate for relaxation and the nervous system, taurate for cellular function, Aquamin for bioavailable magnesium with trace minerals.

• Daily, not just on training days. Magnesium isn't stored long-term, so consistency is the lever that matters.

• Anytime that fits your day. Mornings work if you train early. Evenings work if you sleep poorly. After training works if you stack sessions and remember best in the recovery window.

If you want a deeper look at the forms themselves, we've written a full guide to the types of magnesium worth taking.