Magnesium status and supplementation influence vitamin D status and metabolism: results from a randomized trial.

This was a double-blind randomized ancillary trial within a colorectal cancer prevention study at Vanderbilt University Medical Center in Nashville, Tennessee, USA. Adults aged 40 to 85 were enrolled; 87 participants were assigned to magnesium and 93 to placebo, and baseline dietary patterns showed a relatively high calcium-to-magnesium intake ratio.

Personalized oral magnesium capsules were given daily for 12 weeks in a double-blind design. The mean daily magnesium dose was 205.52 mg, with a range of 77.25 to 389.55 mg. Placebo participants received microcrystalline cellulose capsules.

Magnesium supplementation altered vitamin D metabolism in a baseline-dependent way rather than producing one uniform effect. At baseline 25(OH)D of 30 ng/mL, magnesium increased 25(OH)D3 versus placebo by 2.79 ng/mL, but at 50 ng/mL it lowered 25(OH)D3 by 6.87 ng/mL and lowered 24,25(OH)2D3 by 1.85 ng/mL. Magnesium also increased 25(OH)D2 as baseline 25(OH)D rose, with the largest difference at 50 ng/mL: 8.39 ng/mL. The authors concluded that optimal magnesium status may be important for optimizing 25(OH)D status and that further dosing studies are needed to clarify clinical relevance.

The trial was relatively small, short in duration, and conducted at a single U.S. center in a largely White population from a colorectal cancer prevention cohort, which limits generalizability. Findings were based on biomarker outcomes and subgroup analyses by baseline 25(OH)D, so clinical significance remains uncertain. There were 11 withdrawals overall, including 6 attributed to self-reported adverse events.