Which form of magnesium is best to take?
Magnesium citrate has the strongest evidence for good absorption in humans; if you want to take a magnesium supplement, an organic form such as citrate is a reasonable choice, but pay attention to product quality as well, because it varies considerably.
In a randomised double-blind study of 46 people, magnesium citrate showed the strongest improvement in blood magnesium levels compared with magnesium oxide and an amino-acid-bound form. In that study, magnesium oxide did not differ from a placebo. That is a concrete starting point if you are looking for a supplement.
Broader research confirms this picture: a systematic review of 14 studies concludes that organic forms of magnesium, such as citrate, gluconate and lactate, are generally better absorbed than inorganic forms, such as oxide, carbonate and sulphate. In animal research, magnesium gluconate achieved the highest absorption, but the difference from other organic forms was limited. Animal data cannot be automatically translated to humans, but they point in the same direction.
A caveat is in order here. Even when you choose a form with higher bioavailability, only a portion of it reaches your bloodstream. A study using extended-release magnesium lactate measured a serum bioavailability of 12 to 38 percent compared with an intravenous infusion, depending on whether you take it on an empty stomach or with a meal. Taking it on an empty stomach appears to be more favourable for blood levels.
There is also a contradictory finding. A short-term study in 61 young women found, after 10 days, better results for magnesium oxide on the directly available fraction in the blood, while citrate lowered total serum magnesium. Most researchers believe that the short duration and the specific measurement time point explain this difference, but it shows that the science is not entirely unanimous here.
Finally, even within the same salt form, quality can vary considerably. Research involving 30 people and 15 commercial products found enormous variation in absorption, including among products of the same type. The rise in serum levels ranged from +4.6 to +6.2 percent for the highest versus the lowest bioavailability. Brand and formulation therefore matter, not just the name on the packaging.
Evidence is based on a mix of a randomised double-blind study (n=46), a systematic review of 14 studies, a small crossover study (n=12), a study in 30 people, a short-term study in 61 women, animal research and in-vitro studies.