Magnesium bioavailability from magnesium citrate and magnesium oxide.
Level 3 - non-randomized controlled study
Controlled clinical trial with in vitro testing; abstract does not specify random allocation.
PubMed 2407766 · doi:10.1080/07315724.1990.10720349
What was done
In vitro solubility of 25 mmol magnesium citrate and magnesium oxide was measured in solutions with varying hydrochloric acid concentrations (0 to 24.2 mEq/300 ml) simulating achlorhydria to peak acid secretion, followed by titration to pH 6 and 7 to simulate pancreatic secretion. In vivo absorption was evaluated in normal volunteers by measuring the increase in urinary magnesium relative to creatinine following an oral load of 25 mmol of each compound.
What was found
In vitro, magnesium oxide was virtually insoluble in water and 43% soluble under peak acid conditions, whereas magnesium citrate was 55% soluble in water and substantially more soluble across all acid levels. About 65% of magnesium citrate remained complexed in soluble form, with no reprecipitation observed at pH 6 to 7 for either salt. In vivo, urinary magnesium increment was significantly higher with magnesium citrate than magnesium oxide during 0–4 hours post-load (0.22 vs 0.006 mg/mg creatinine, p < 0.05) and during the second 2 hours post-load (0.035 vs 0.008 mg/mg creatinine, p < 0.05).
Why it matters
This study provides direct physical and human pharmacokinetic evidence that magnesium citrate provides substantially greater gastrointestinal bioavailability than magnesium oxide, which may explain differences in clinical supplement efficacy.
Limits
The abstract does not report the sample size (n), volunteer demographics, or whether human testing followed a randomized crossover design. Bioavailability was measured indirectly via short-term urinary excretion rather than total systemic retention or clinical outcomes.
Cited by
- supports Magnesium oxide has very poor bioavailability compared to organic magnesium salts such as magnesium citrate, malate, taurate, and glycinate.