Two bottles can carry the same words — “magnesium 300 mg” — and still deliver different amounts. A mineral in a tablet is always part of a salt: oxide, citrate, gluconate, carbonate. The salt decides how well it dissolves and how much reaches your blood. Here are the three minerals people most often buy on their own.
Magnesium: citrate, not oxide
Magnesium oxide is the cheapest form, but it dissolves poorly. In a study by Lindberg and colleagues (1990), oxide barely dissolved in water and only 43% dissolved even in acid like that of a digesting stomach, while citrate was 55% soluble in plain water. After the same dose, the rise in urinary magnesium — a sign of how much was absorbed — was many times greater after citrate.
In a 60-day study, 46 people took 300 mg of magnesium a day in different forms. Citrate and amino-acid chelate were absorbed better than oxide, and oxide did no better than placebo (Walker et al., 2003).
Zinc: gluconate or citrate
In a study by Wegmüller and colleagues (2014), 15 adults took 10 mg of zinc in different forms on an empty stomach. They absorbed 61.3% from citrate, 60.9% from gluconate and only 49.9% from oxide, and three participants absorbed almost nothing from oxide.
Calcium: carbonate with food, and better in parts
Here the data disagree. A meta-analysis of 15 studies found calcium citrate absorbed 22–27% better than carbonate, both on an empty stomach and with meals (Sakhaee et al., 1999). But in a study by Heaney and colleagues (1999), where both forms were taken with a light breakfast, carbonate was absorbed just as well as citrate. Food helps in general: a light meal raised calcium absorption from carbonate by 10–30% (Heaney et al., 1989).
One more finding from the 1999 study: 36% of a 300 mg dose of calcium was absorbed, but only 28.4% of a 1,000 mg dose. So a large dose is better split across the day.
How to read the label
What counts is the amount of the mineral itself, not of the salt. Magnesium citrate is only about one-sixth magnesium, calcium carbonate two-fifths calcium, zinc gluconate one-seventh zinc. So “magnesium citrate 1,000 mg” is about 160 mg of magnesium, and “calcium carbonate 1,250 mg” is 500 mg of calcium. Good labels give both numbers; look at the one next to the name of the mineral itself.
In short
- Magnesium — citrate or chelate, not oxide.
- Zinc — gluconate or citrate, not oxide.
- Calcium: carbonate with food, citrate whenever suits you; split a large dose.
- Iron is a story of its own — see How to take iron.
More isn’t better: the safe upper limit is 350 mg a day for magnesium from supplements, 40 mg for zinc and 2,000–2,500 mg for calcium including food. If you take medicines, check supplement interactions with your doctor.
The CookCount diary has a Supplements section: enter the contents of one tablet — the amount of the mineral itself from the label. The calculator adds supplements to your food in the charts and history and warns you if a day goes over the safe upper limit.