Elemental magnesium

Magnesium supplements come in a variety of forms, including magnesium oxide, citrate and chloride [5], and the partner molecule adds weight. US labeling rules require the amount of each dietary ingredient to be the weight of the ingredient rather than of its source, with the example of the weight of calcium rather than calcium carbonate [4]. The source may be named in parentheses after the ingredient, as in "Calcium (as calcium carbonate)" [4]. NIH's Office of Dietary Supplements notes that, for magnesium, the Supplement Facts panel declares the elemental magnesium, not the weight of the whole compound [5].

The partner also changes how much magnesium the body takes up. A review of magnesium salts says the anion attached to magnesium, such as oxide, chloride, gluconate or lactate, affects how much elemental magnesium reaches its target [1]. In one laboratory and volunteer study, magnesium oxide was virtually insoluble in water, and magnesium citrate was more soluble and more bioavailable than oxide [2].

When studies report doses as elemental magnesium, the amounts can be compared across forms. Not every trial does; some give the weight of the whole compound, so check which one a study reports. For example, a randomized, placebo-controlled trial in 155 adults reporting poor sleep gave 250 mg of elemental magnesium a day as magnesium bisglycinate [3].

Two products with the same elemental number can still differ, because forms that dissolve well in liquid tend to be absorbed more completely than less soluble forms [5][2]. The magnesium page lists the doses used in studies by form.

• Label claim: the amount a label declares, which for magnesium is the elemental weight [4].