High-Dose Vitamin D Supplementation Alters Tissue-Specific Vitamin D Metabolite Distribution in Mice During Pregnancy

Pregnancy is a major modifier of vitamin D metabolism, which is suggested to serve an immunomodulatory role, support placental function, and aid fetal development. However, little is known about local tissue concentrations of vitamin D metabolites following supplementation during gestation.

We aimed to quantify intracellular concentrations of 25-hydroxyvitamin D₃ (25[OH]D) and the bioactive 1,25-dihydroxyvitamin D₃ (1,25[OH]₂D) across tissues following vitamin D supplementation and to determine how this distribution is altered during gestation in mice.

Forty nulliparous female C57BL/6 (wild-type) mice were fed a semipurified diet supplemented with either a normal (1000 IU/kg) or high-dose (6000 IU/kg) vitamin D₃ (cholecalciferol) for 4 wk. Thirty females were mated with 10 diet-matched C57BL/6 males, whereas 10 females served as nonpregnant controls. Maternal serum, lung, liver, kidney, and placental tissue and fetal samples were collected at gestational day 18.5. Concentrations of 25(OH)D and 1,25(OH)₂D were measured using high-pressure liquid chromatography and tandem mass spectrometry and compared between groups using t tests in R (R Foundation for Statistical Computing).

High-dose vitamin D supplementation increased serum 25(OH)D (calcidiol) across all groups, with a clear dose response. Serum 1,25(OH)₂D (calcitriol) concentrations were substantially increased in pregnant compared with nonpregnant females (mean difference: 74 pg/mL for control dose, P < 0.05 and 65 pg/mL for high dose, P = 0.1). Tissue analysis revealed the lungs as important targets of 25(OH)D accumulation, with significantly higher concentrations than liver tissue in nonpregnant (mean difference: 27 ng/g, P < 0.05) mice. Pregnancy induced notable shifts in vitamin D metabolite distribution, including reduced serum 25(OH)D concentrations and enhanced renal conversion to serum 1,25(OH)₂D. Despite high placental accumulation of 25(OH)D, fetal 25(OH)D concentrations were significantly lower in the high-dose group (mean difference: -2.6 ng/g, P < 0.05), suggesting a protective saturation mechanism. High-dose vitamin D supplementation was well tolerated without any adverse gestational events.

Our findings suggest that pregnancy results in a redistribution of vitamin D metabolites in tissues, with the kidneys and placenta playing central roles. This distribution is responsive to prenatal vitamin D supplementation; however, under supraphysiological maternal dosing, fetal 25(OH)D uptake may be decreased.