Magnesium glycinate vs. citrate vs. oxide: only one is worth taking

The magnesium supplement industry generates roughly $1.5 billion in annual revenue in the US. Most of that revenue is in magnesium oxide, which is the cheapest form to manufacture and one of the worst-absorbed.

The forms, ranked by bioavailability

1. Magnesium glycinate — chelated to glycine. ~80% absorbed. Well-tolerated and gentle on the stomach. The form we recommend for daily, long-term supplementation and for use in the evening.
2. Magnesium citrate — ~25–30% absorbed. Has a mild laxative effect at doses above 400 mg, which is its primary clinical use (constipation).
3. Magnesium malate — ~40% absorbed. Some preliminary evidence for daytime muscle fatigue.
4. Magnesium L-threonate — proprietary form (Magtein®) marketed for cognitive benefits. The evidence base is small but interesting; the price-per-mg is the highest in the category.
5. Magnesium oxide — < 4% absorbed. This is the form in nearly every drugstore multivitamin and nearly every Costco-tier supplement. It is functionally a placebo at typical doses.

How much to take

The US RDA is 320 mg/day for women and 420 mg/day for men. Most US adults consume around 250 mg/day from food. The supplement gap is therefore real but small — 100 to 200 mg from a high-bioavailability source covers it.

When to take it

Magnesium glycinate is calming and best taken in the evening. Magnesium citrate, because of the laxative effect, is best taken in the morning — or not at all, depending on your priorities.

What to look for on a label

The supplement facts panel must disclose the form. “Magnesium 200 mg” alone is not enough — it must say “magnesium (as glycinate) 200 mg.” If a label just says “magnesium,” it is almost certainly the oxide form, and you are paying for filler.