Magnesium: What It Actually Does, How Much You Need, and Whether It's Worth It

What magnesium actually does in the body, how much you need daily, which forms are worth buying, and when a supplement makes sense — with real doses and food numbers.

Magnesium is one of the few supplements that genuinely makes sense for some people who train — just usually not for the reasons you hear in the gym. It isn't a cramp cure, it won't raise your testosterone, and it won't add kilos to your squat. Here's what's actually known: how much you need, where to get it, and when a tub of powder earns its place.

What magnesium does in the body

An adult carries roughly 25 grams of magnesium. About 60% sits in bone, around 39% inside soft-tissue cells, and less than 1% circulates in blood. That's why a standard serum magnesium test is a weak signal — the number can look fine while tissue stores are drained.

Magnesium is a cofactor in more than 300 enzyme reactions. Specifically, ATP — the molecule your muscle pulls energy from — is only biologically active as a magnesium complex. It also plays a role in protein synthesis, nerve signalling, muscle relaxation after contraction, blood glucose regulation, and bone structure.

Are you actually short?

Dietary surveys across Europe and the US consistently show a sizeable share of adults eating below the recommended intake. That doesn't mean everyone is deficient, but it does mean intakes often sit near the line. Your risk is higher if you:

diet below 1,800 kcal for more than 8–12 weeks;
eat few legumes, nuts, seeds, or whole grains;
train 8–12 hours a week and sweat heavily;
drink regularly, have type 2 diabetes, or have chronic gut issues;
have been on proton pump inhibitors or diuretics for over a year.

Before you buy anything, count your food. Real per-serving numbers:

pumpkin seeds, 30 g — about 150 mg
almonds, 30 g (a handful) — about 80 mg
cooked spinach, 100 g — about 80 mg
black beans or chickpeas, 150 g cooked — about 90 mg
oats, 60 g dry — about 80 mg
70% dark chocolate, 30 g — about 65 mg

Three of those a day already puts you around 250–300 mg before bread, potatoes, yoghurt, and tap water get counted. For a lot of people, that's most of the job done without a single capsule.

How much per day

The recommended daily intake is 400–420 mg for men and 310–320 mg for women (the higher figure applies past age 31). Pregnancy adds roughly 40 mg.

If you supplement, the sensible range is 200–400 mg of elemental magnesium per day. The word elemental is the whole game. A label saying "magnesium citrate 500 mg" is not 500 mg of magnesium — citrate is about 16% elemental, so that's roughly 80 mg. Find the line that says "of which magnesium."

The upper limit from supplements is around 350 mg a day, and that limit exists because of diarrhoea, not toxicity. Magnesium from food has no such cap — healthy kidneys clear the excess.

Forms, briefly

Citrate — well absorbed, cheap, mildly laxative. A solid default.
Bisglycinate — gentlest on the gut, the pick if citrate sends you to the toilet.
Malate, lactate, chloride — perfectly usable.
Oxide — cheapest and worst absorbed (under 5% in some measurements). Mostly functions as a laxative.
L-threonate — expensive, sold on brain claims; human data is thin.

What magnesium doesn't do

This is the part that saves you money. Controlled trials on nocturnal leg cramps in adults have largely failed to beat placebo. A calf cramp on your 15th rep of leg press is usually neuromuscular fatigue, not an electrolyte shortfall.

On sleep: there's an effect in people with low intakes, but it's modest — small trials show time-to-sleep shortening by roughly 15–20 minutes. That's not a sleeping pill.

On testosterone, strength, and hypertrophy: correcting a deficiency helps; piling magnesium on top of an already adequate intake does nothing. Don't buy it expecting that.

How to actually use it

Track your food intake across 3 typical days first.
If you're under 250 mg a day, add 200–300 mg of elemental magnesium.
Take it with a meal. If your gut complains, split it into two doses of 100–150 mg.
If sleep is the target, take it 30–60 minutes before bed.
Give it 3–4 weeks before judging whether anything changed.

Space it 2 hours away from large zinc or iron doses, and 4 hours away from tetracycline and quinolone antibiotics and levothyroxine — magnesium blunts their absorption.

When to see a doctor

Talk to a doctor, not a supplement seller, if:

you have kidney disease or reduced kidney function — magnesium can accumulate, and that's serious;
cramps come with tingling around the mouth or fingers, muscle twitching, palpitations, or fainting;
you've had diarrhoea for more than two weeks, or repeated vomiting;
you take diuretics, digoxin, or proton pump inhibitors long term;
a cramp is one-sided with calf swelling and pain — a clot needs ruling out.

The short version

Aim for 310–420 mg a day total, food plus supplements.
Three servings of seeds, legumes, oats, or leafy greens covers most of it.
If you supplement: 200–400 mg elemental, citrate or bisglycinate, with a meal.
Don't expect cramps to vanish, testosterone to climb, or lifts to jump — none of that holds up.
Kidney disease, tingling, palpitations, or long-running diarrhoea mean a doctor's check before any supplement.