Quick Answer: Yes, but not mainly through sweat. Regular training is associated with roughly 10-20% higher magnesium requirements, driven more by redistribution into working muscle and increased urinary loss than by the modest amount lost in sweat.

The supplement aisle sells this as a sweat story. It mostly isn’t.

Understanding which mechanism actually matters changes when you’d take it and whether you need more at all.

This is general information, not medical advice.

How much magnesium do you actually lose in sweat?

Less than the marketing suggests. Sweat magnesium concentration is roughly 2-8 mg per liter. Even a hard two-hour session producing 2 liters of sweat costs you somewhere around 4-16 mg.

For context, the adult RDA sits at 310-420 mg per day depending on age and sex. So a genuinely demanding session loses you about 1-4% of a day’s requirement through sweat.

That’s not nothing across a training year, but it isn’t the reason athletes show lower magnesium status than sedentary people.

So why do athletes test lower?

Because magnesium moves during exercise. It shifts from blood plasma into working muscle and red blood cells, which is where it’s needed for ATP production. A blood test taken after training can show a drop that reflects redistribution rather than depletion.

That distinction matters practically. Depletion means you need more coming in. Redistribution means the magnesium is exactly where it should be — doing work in the muscle that just trained — and will return to plasma over the following hours.

This is why post-exercise magnesium readings have been misread for years as evidence that athletes are chronically deficient. Some are. But a low reading taken 30 minutes after a hard session is measuring the wrong moment.

What actually drives the higher requirement?

Three mechanisms, in order of how much they matter:

• Urinary excretion. Intense exercise increases magnesium loss through urine, partly through stress hormone signaling. This is generally the largest single route of extra loss.
• Increased demand in muscle. Magnesium is required for every ATP reaction in your body. More training means more ATP turnover, which means more magnesium doing work rather than sitting in reserve.
• Sweat. Real, measurable, and the smallest of the three for most people.

There’s a fourth factor that isn’t about loss at all: athletes often eat in a way that reduces intake. Cutting calories, restricting carbohydrates, or eating fewer whole grains, legumes, and leafy greens all reduce magnesium coming in — right when demand is up.

How much extra do I actually need?

Estimates cluster around 10-20% above the RDA for people training seriously, which is roughly 40-80 mg more per day. That’s one serving of pumpkin seeds or a large handful of almonds — a food-sized gap, not a supplement-sized one.

Can you cover it with food?

For most people, comfortably. The gap is small enough that a few deliberate choices close it.

Strong sources, per typical serving:

• Pumpkin seeds, 30 g: ~150 mg
• Spinach, cooked, 1 cup: ~157 mg
• Black beans, 1 cup: ~120 mg
• Almonds, 30 g: ~80 mg
• Dark chocolate, 70%+, 30 g: ~65 mg
• Oats, 1 cup cooked: ~60 mg

Two of those a day and you’ve covered the training increment several times over.

Where it genuinely gets hard is on a sustained calorie deficit, or on a very low-carb diet that removes legumes, oats, and most whole grains at once. Those are the situations where supplementing has a clearer case.

Does cooking destroy it?

Magnesium is a mineral, so it doesn’t degrade with heat — but it does leach into cooking water. Boiling spinach and discarding the water loses a meaningful fraction. Steaming, roasting, or using the liquid keeps it.

Does soaking beans and grains reduce the magnesium?

Soaking reduces phytates, which actually improves mineral absorption rather than reducing the magnesium itself. Soaking legumes overnight and discarding the water is a net gain for how much you absorb, even though a small amount leaches out.

What nobody tells you about testing your levels

The standard serum magnesium test is close to useless for this question, and it’s ordered constantly anyway.

Only about 1% of your body’s magnesium is in blood serum. The rest sits in bone and inside cells. Your body defends serum levels tightly, pulling from bone to keep them normal — which means serum can read perfectly normal while tissue stores are genuinely depleted.

RBC magnesium is somewhat more informative because it looks inside cells, but it’s not routinely ordered and interpretation varies.

The practical implication: don’t build a supplementation decision on a normal serum result. Look at intake instead. If you’re eating 400+ mg a day from food and training normally, you’re almost certainly fine regardless of what a serum test says.

What do actual deficiency symptoms look like?

Muscle cramps and twitching, poor sleep quality, and unusual fatigue are the commonly cited ones — but all three have many other causes, and cramps in particular correlate poorly with magnesium status in research. Treat symptoms as a prompt to check intake, not as a diagnosis.

FAQ: Training and Magnesium Questions, Answered

Should I take magnesium before or after training?

Neither timing has strong evidence behind it for performance. If you’re supplementing, take it whenever you’ll remember consistently — and if you use glycinate for sleep, evening makes practical sense. Consistency matters far more than timing.

Does magnesium prevent cramps?

The evidence is weaker than the reputation. Trials on exercise-associated cramps have been largely disappointing, and current thinking points more toward neuromuscular fatigue than electrolyte depletion. If you’re deficient, correcting it may help; if you’re not, it likely won’t.

Can I take too much?

From food, essentially no — your kidneys handle the excess. From supplements, yes: the tolerable upper limit for supplemental magnesium is 350 mg per day for adults, and exceeding it commonly causes diarrhea. People with kidney disease need medical guidance before supplementing at all.

Which form should I use if I do supplement?

Glycinate and citrate are the two most commonly used. Glycinate is gentler on digestion and often chosen for evening use; citrate absorbs well but has a stronger laxative effect at higher doses. Oxide is cheap, poorly absorbed, and the most likely to cause digestive upset for the least benefit.

Does caffeine or alcohol increase magnesium loss?

Both increase urinary excretion modestly. Neither is dramatic on its own, but heavy intake of either on top of hard training adds another small drain to a system that’s already running a higher requirement.

TL;DR:

• Sweat loses you only about 4-16 mg per hard session — roughly 1-4% of a day’s needs.
• Urinary loss and increased muscle demand matter more than sweat.
• Training raises requirements by about 10-20%, or 40-80 mg — a food-sized gap.
• Serum tests can read normal while tissue stores are low; judge by intake instead.

If you train hard and eat a reasonable variety of seeds, legumes, and greens, the training increment takes care of itself. The people with a real case for supplementing are usually the ones in a deficit or cutting out entire food groups.

This is general information, not medical advice. Talk to a clinician before supplementing, especially with kidney conditions.