Lindberg et al. (1990) conducted one of the foundational bioavailability comparisons, administering equimolar doses of magnesium citrate and magnesium oxide to healthy subjects and measuring urinary magnesium excretion as a proxy for absorption [1]. Magnesium citrate produced significantly higher bioavailability, a finding that has been replicated in subsequent studies and fundamentally shaped clinical supplementation recommendations [1].
Blancquaert, Vervaet, and Derave (2019) published a comprehensive review of magnesium formulations in Nutrients, evaluating organic salts (citrate, glycinate, taurate, malate) against inorganic forms (oxide, chloride, sulfate) [3]. They confirmed that organic chelates consistently outperform inorganic salts in absorption studies, though they noted that the clinical significance of these differences depends on the dose, the individual's baseline magnesium status, and the therapeutic goal [3]. They also highlighted that magnesium glycinate's chelation with glycine protects against the osmotic laxative effect that limits tolerance of other forms at higher doses [3].
The magnesium L-threonate story originates from Bhatt et al. (2010), who showed in a landmark Neuron paper that supplementation with this form increased cerebrospinal fluid magnesium concentrations in rats and enhanced both short-term and long-term memory via increased synaptic density in the hippocampus [2]. The key finding was that other magnesium forms, including chloride and gluconate, failed to elevate brain magnesium levels despite raising serum levels, suggesting that threonate's carrier molecule provides a unique transport advantage across the blood-brain barrier [2].
Gröber, Schmidt, and Kisters (2015) provided clinical guidance on form selection: oxide for constipation and when cost is the primary concern, citrate as a general-purpose supplement, glycinate for patients needing high-dose therapy without GI disturbance, and threonate when cognitive endpoints are the target [4]. They emphasized that the "best" form depends entirely on the clinical context, and that switching forms can sometimes resolve non-response in patients who appear resistant to supplementation [4]. The NIH recommends that the Tolerable Upper Intake Level of 350 mg per day applies to supplemental magnesium from any form, not magnesium from food sources [5].