Influence of vitamin D3 on inhibitory effect of vitamin K2 on bone loss in ovariectomized rats.

To evaluate whether the vitamin D3 level in the plasma influences the inhibitory effect of vitamin K2 on bone loss, vitamin K2 (25 mg/kg/day) was administered to ovariectomized (OVX) rats fed a diet containing vitamin D3 (V.D.(+)) or a diet deficient in vitamin D3 (V.D.(-)). After 3 months of treatment, the plasma 25-OH-vitamin D3 (25-OH-D3) level in the V.D(-)-sham group was about 1/3 of that in the V.D(+)-sham group. The plasma calcium level and alkaline phosphatase activity were also significantly lower in the V.D(-)-sham group than in the V.D(+)-sham group. In the V.D(+) group, the plasma 25-OH-D3 level in the vitamin K2 group was about 1.5 times higher than that in the OVX-control group. Ovariectomy resulted in a significant decrease in bone density, bone mineral content (BMC) and bone mineral density (BMD) of femurs in both the V.D(+) and V.D(-) groups. In the V.D(-) group, vitamin K2 had no marked effect on the bone loss. In the V.D(+) group, the bone density and BMD in the mid portion of the femur were significantly increased by vitamin K2 treatment. These findings suggest that the effect of vitamin K2 on bone loss is affected by the vitamin D3 level in the plasma.