The benefits of magnesium for sub-3 runners

Magnesium is one of those quiet essentials that most runners don’t give much thought to until they start pushing the upper limits of training. When you’re clocking 100km weeks or chasing down a sub-3 marathon, though, the small details matter. Magnesium plays a central role in muscle and nerve function, energy metabolism, recovery and even sleep. Get enough of it and your body hums along smoothly. Skimp on it and you’ll feel the consequences, often in the form of cramps, twitching muscles or sluggish recovery.

For me, magnesium has become a non-negotiable part of my routine. I use it in three ways: as a nightly supplement, as a race-week aid and as a recovery ritual with epsom salts. None of these is a silver bullet, but together they help keep me consistent, calm and cramp-free.

1. Bedtime supplement

Every night I take a 500mg magnesium oxide tablet before bed. It’s as much a habit as brushing my teeth. I was first tipped off by my yoga instructor, who swore by magnesium for strengthening ligaments. The science is stronger on its role in recovery and sleep, but whatever the mechanism, I notice the difference when I skip it. It seems to settle the nervous system, helps my muscles relax and ensures I wake up feeling genuinely rested.

There are fancier formulations such as citrate or glycinate with claims of better absorption. I’ve stuck with oxide for years because it works for me. In sub-3 training, the best supplement is often the one you actually take every day without fuss.

2. Food first

Of course, the best source of magnesium is still real food. Leafy greens, nuts, seeds, beans, wholegrains and even dark chocolate are all rich in it. In theory, you could get everything you need from diet alone. But marathon training makes demands that go beyond “in theory”. Even with a balanced diet, many runners find they dip below optimal levels, especially if they sweat heavily or do not prioritise magnesium-rich foods at every meal. For me, supplements are a safety net. The base should always be diet.

3. Race-week supplement

In the final week before a marathon, I add a second layer: Xendurance’s lactic acid buffer. I take three tablets in the morning and three in the evening. The two biggest ingredients are magnesium oxide and magnesium hydroxide. The company claims these help make your system slightly more alkaline, buffering lactic acid and reducing fatigue. While I’m not entirely sure about the science myself, here’s the thing: since I’ve been taking them, I haven’t cramped in any of my five sub-3 marathons. Placebo or not, I’ll take that.

The ritual itself also matters. By the time race week comes around, my body and mind are conditioned to a routine that signals, “This is what we do before big performances”. That reassurance can be as powerful as the supplement itself.

4. Recovery ritual

Finally, there’s the classic epsom salt bath. Before I started serious marathon training, I honestly can’t remember the last time I had a bath. Showers were quicker, cleaner and more adult. But when you’re stringing together 130km weeks, the normal recovery toolkit of naps, protein and stretching only gets you so far. That’s when I started “soaking my legs”.

Let’s be clear: it’s a bath. A big, steamy bath with a few scoops of magnesium sulfate dissolved in it. Do I believe magnesium is really being absorbed through my skin? Probably not. The evidence is wafer-thin, and the only study suggesting it might be possible has never been replicated. But the heat, the ritual and the calm all work. After 30–45 minutes submerged, my legs feel fresher, looser and ready for another hammering. The placebo, if it is one, is a very effective one.

Even now, I buy epsom salt in bulk 20kg sacks. I scoop in a few cups every time, even though I know the water itself is doing 90 percent of the work. And yes, I still call it “soaking my legs”. Because somehow that sounds more athletic than “having a bath”.

The takeaway

Magnesium will not run the miles for you. It will not replace the hard graft of doubles, tempo runs and long Sunday slogs. But it can be one of those subtle, supportive pillars that keep you injury-free, sleeping soundly and recovering fast enough to go again tomorrow. Whether it’s a simple tablet at bedtime, a race-week insurance policy, food choices that keep you topped up or a hot soak after a brutal block, magnesium has earned its place in my sub-3 toolkit.