Magnesium Citrate vs. Oxide: Which Absorbs Best?

Key Findings

This 2019 observational study demonstrated that magnesium citrate (MgC) has significantly higher bioavailability than magnesium oxide (MgO). After a single 400 mg dose, MgC supplementation led to a statistically significant increase in 24-hour urinary magnesium excretion (P < 0.05) and elevated plasma magnesium concentrations at 4 h and 8 h post-administration. In contrast, MgO did not show significant changes in urinary excretion or plasma levels compared to baseline. The study concluded that MgC is more effectively absorbed and recommended urinary Mg excretion as a primary endpoint for future bioavailability research.

Study Design

The study was a controlled observational trial involving 14 healthy male participants. Subjects were first saturated with 400 mg/day of Mg for five days to standardize magnesium levels. On separate days, they received a single 400 mg dose of either MgC or MgO. Bioavailability was assessed via 24-hour urinary Mg excretion and plasma Mg concentrations measured at 0, 2, 4, 8, and 24 hours post-supplementation. The crossover design allowed within-subject comparisons, though the small sample size and male-only demographic limit broader applicability.

Dosage & Administration

Participants were pre-saturated with 400 mg of Mg daily (from unspecified sources) for five days prior to testing. A single 400 mg dose of MgC or MgO was administered orally on two distinct test days, likely with water. Timing of blood and urine collection was standardized to capture absorption kinetics.

Results & Efficacy

• Urinary Excretion: MgC caused a significant increase in 24-hour urinary Mg excretion (P < 0.05), indicating higher systemic absorption. MgO did not reach statistical significance.
• Plasma Levels: MgC significantly elevated plasma Mg at 4 h (P < 0.05) and 8 h (P < 0.05) post-dose, with sustained increases at all time points compared to baseline. MgO showed no significant changes in plasma Mg.
• Comparative Bioavailability: MgC’s superior performance aligns with prior hypotheses about organic vs. inorganic Mg salts, though exact effect sizes (e.g., percentage differences) were not quantified in the summary.

Limitations

• Small Sample Size: Only 14 males were studied, limiting statistical power and generalizability to females or clinical populations.
• Short Duration: Focus on acute (24-hour) effects overlooks long-term absorption differences or potential tolerability issues.
• Observational Design: Cannot establish causality; confounding variables (e.g., dietary intake, renal function) were not fully controlled.
• Lack of Kinetic Data: No area-under-the-curve (AUC) analysis or pharmacokinetic modeling was reported.
• Saturation Protocol: Pre-supplementation with Mg may mask differences in absorption efficiency under baseline Mg-deficient conditions.

Clinical Relevance

For supplement users, this study supports choosing magnesium citrate over magnesium oxide to achieve measurable increases in magnesium status. The emphasis on urinary excretion as a reliable biomarker suggests that clinicians could use 24-hour urine tests to assess individual absorption. However, results apply specifically to Mg-saturated individuals; further research is needed for Mg-deficient populations. Practically, MgC may be preferable for correcting mild deficiencies or maintaining optimal levels, while MgO’s poor bioavailability aligns with its common use as a laxative rather than a systemic supplement.

Note: The study’s URL is provided but not directly analyzed here. Findings are based solely on the summary details.