High dose dietary vitamin D₃ increases bone mass and strength in mice

Vitamin D plays a critical role in skeletal homeostasis. Vitamin D supplementation is used worldwide to maintain optimal bone health, but the most appropriate level of supplementation remains controversial. This study aimed to determine the effects of varying doses of dietary vitamin D₃ on the mechanical properties and morphology of growing bone. Eight-week-old female mice were supplied with one of 3 diets, each containing a different dose of vitamin D₃: 1000 IU/kg (control), 8000 IU/kg or 20,000 IU/kg. Mice had ad libitum access to the specialty diet for 4 weeks before they were culled and their tibiae collected for further analysis. The collected tibia underwent three-point bending and reference-point indentation from which their mechanical properties were determined, and cortical and trabecular morphology determined by micro computed tomography. Dietary supplementation with 20,000 IU/kg vitamin D₃ resulted in greater ductility (~ 200%) and toughness (~ 150%) compared to the 1000 IU/kg control. The 20,000 IU/kg diet was also associated with significantly greater trabecular bone volume fraction and trabecular number. The 8000 IU/kg diet had no significant effect on trabecular bone mass. We conclude that vitamin D₃ supplementation of 20,000 IU/kg during early adulthood leads to tougher bone that is more ductile and less brittle than that of mice supplied with standard levels of dietary vitamin D₃ (1000 IU/kg) or 8000 IU/kg. This suggests that dietary vitamin D₃ supplementation may increase bone health by improving bone material strength and supports the use of vitamin D₃ supplementation, during adolescence, for achieving a higher peak bone mass in adulthood and thereby preventing osteoporosis.