Vitamin D is not directly necessary for bone growth and mineralization

To determine whether vitamin D has a direct action on bone growth and mineralization, severely vitamin D-deficient rats were infused continuously for 10 days with calcium and phosphorus solutions intrajugularly to maintain plasma calcium and phosphorus levels equal to those in rats on the same diet given vitamin D. Rats from the same litters identically treated were given 75 IU of vitamin D3 every 3 days and infused with saline for the 10-day period for comparison. Infusion of calcium and phosphorus to the vitamin D-deficient rats induces bone growth and mineralization equal to that of the rats given vitamin D as revealed by femur length, femur weight, total femur ash, percent femur ash, and epiphyseal plate width. The mineral deposited had a calcium-to-phosphorus ratio of 1.6 identical to the mineral deposited in the rats given vitamin D and unlike controls maintained in the vitamin D-deficient state without infusions. The vitamin D-deficient rats infused with calcium and phosphorus had greater amounts of mineral and matrix in their femurs than did those treated with vitamin D. This result demonstrates that some form of vitamin D does have a direct effect on bone. The most likely possibility is that bone resorption in relation to modeling or calcium mobilization for calcium homeostasis or both are compromised in extreme vitamin D deficiency. The above results strongly support the concept that vitamin D is not necessary for and does not stimulate directly bone growth and mineralization. Vitamin D therefore supports bone growth and mineralization indirectly by elevating plasma calcium and phosphorus levels.