Supplements
Pick up almost any magnesium supplement and the front of the tub says something like 500 mg. That number is usually not magnesium. It is the weight of the whole compound, and magnesium is one part of it. The rest is whatever it is bound to.
Depending on the form, the actual magnesium content ranges from roughly 11 percent to around 60 percent. Then absorption varies on top of that. Between those two variables, two products showing the same number on the label can deliver wildly different amounts into your body.
Magnesium is never sold on its own. It is always bound to something else, and that something makes up most of the weight.
Magnesium oxide is around 60 percent magnesium by weight, which sounds excellent until you look at absorption. Magnesium citrate is around 16 percent. Magnesium glycinate is around 14 percent.
So a 500 mg glycinate capsule contains roughly 70 mg of elemental magnesium. A 500 mg oxide tablet contains around 300 mg. On paper the oxide looks like it wins by a distance. It does not, and the reason is the second variable.
Magnesium oxide is poorly absorbed. Published absorption studies have put it in the single digits as a percentage of the dose, far below the organic forms like citrate and glycinate.
Run the arithmetic on a 500 mg tablet of each.
| Form | Elemental Mg | Roughly absorbed |
|---|---|---|
| Oxide 500 mg | ~300 mg | small fraction |
| Citrate 500 mg | ~80 mg | much higher fraction |
| Glycinate 500 mg | ~70 mg | much higher fraction |
The oxide tablet starts with four times the magnesium and delivers less of it. Most of what is not absorbed stays in the gut, draws water in, and produces the laxative effect magnesium oxide is genuinely useful for.
Magnesium oxide is the cheapest form to manufacture, which is why it is in most retail products. If a tub is unusually cheap and the label says magnesium oxide, that is the whole explanation.
The forms are not interchangeable, and the binding partner does real work.
Glycinate. Bound to glycine, well absorbed, gentle on the gut. The default choice for general repletion and the one most often used where sleep is the reason for taking it, partly because glycine has its own effects.
Citrate. Well absorbed, cheap relative to glycinate, mildly laxative at higher doses. A reasonable general-purpose option.
Malate. Bound to malic acid, which participates in energy metabolism. Popular with people who train, though the evidence for it being superior to other absorbable forms is thin.
Threonate. Marketed for cognitive benefit on the basis that it crosses the blood-brain barrier more readily. Expensive, and the human evidence is early rather than established.
Oxide. Poorly absorbed. Genuinely useful as a laxative, and that is essentially its role.
Sulfate. Epsom salts. Topical and bath use. Absorption through skin is not well established despite how confidently it gets claimed.
This catches people constantly. Roughly one percent of the magnesium in your body is in your bloodstream. The rest sits in bone and inside cells. Your body defends serum magnesium tightly, pulling from stores to keep it steady.
Serum magnesium stays normal while your stores deplete. A normal result does not mean adequate status, which is why serum magnesium is a poor screening test and why deficiency is easy to miss. Red cell magnesium is somewhat better and still imperfect.
In practice, magnesium status is usually judged from intake and symptoms rather than from a blood test, which is unusual among nutrients and worth knowing. If you are working out which markers are actually worth testing, the blood test selector sorts them by diagnostic value and flags the ones that will not tell you much.
Adult requirements sit around 400 to 420 mg elemental for men and 310 to 320 mg for women, from food and supplements combined. Food sources are genuinely good: leafy greens, nuts, seeds, legumes, whole grains and dark chocolate. Someone eating a varied diet built on whole foods is often fine.
Requirements and losses rise with:
Supplemental doses commonly sit between 200 and 400 mg elemental, taken in the evening. Whether the timing matters is less established than the internet suggests, but it is harmless and the gut tolerates it better away from a large meal.
That last point is the one that separates a serious product from a cheap one, because a magnesium product that hides its composition is usually hiding the oxide. If you want a named-form glycinate rather than a mystery "complex", God of Supps stocks one.
It is a cofactor in hundreds of enzymatic reactions, including every reaction that uses ATP, which is to say every reaction that uses energy. The areas with the most consistent evidence:
Sleep quality, particularly where intake was inadequate to begin with. Not a sedative, and it will not fix sleep that is broken for other reasons.
Muscle function, including involvement in cramping, though cramps have several causes and magnesium is only sometimes the answer.
Insulin sensitivity, where low magnesium status associates with worse glucose handling in population data.
Blood pressure, with modest effects in supplementation trials.
What it does not do is act as a stimulant, a fat loss aid or a testosterone booster, regardless of how it gets marketed.
Read the elemental number, not the front of the tub. Choose glycinate or citrate over oxide unless you specifically want the laxative effect. Do not rely on a serum magnesium test to tell you whether you need it. Most people taking magnesium are taking a poorly absorbed form at a dose that looks larger than it is.
Glycinate for general repletion and tolerability, citrate as a cheaper well-absorbed alternative. Oxide is poorly absorbed and is better understood as a laxative than a magnesium supplement.
Adult requirements are around 400 to 420 mg elemental for men and 310 to 320 mg for women, from food and supplements combined. Supplemental doses commonly sit between 200 and 400 mg elemental.
The actual weight of magnesium in a compound, as distinct from the weight of the whole compound. A 500 mg magnesium glycinate capsule contains roughly 70 mg elemental magnesium, because glycine makes up most of the weight.
Poorly. Around one percent of body magnesium sits in serum, and the body defends that level by drawing from stores. Serum magnesium can read normal while tissue stores are depleted.
It is commonly taken in the evening and is well tolerated then, though the evidence that timing matters is weaker than commonly claimed. Taking it away from a large meal generally suits the gut better.
Usually because the form is poorly absorbed, most often oxide or citrate at a higher dose. Unabsorbed magnesium stays in the gut and draws water in. Switching to glycinate typically resolves it.
This article is educational and is not medical advice. See the medical disclaimer.
Want a programme built around your own numbers?
Work With Dinesh
Newsletter
One email when a new guide, calculator or breakdown goes live. Nothing else.
