I just posted about two large trials where high-dose vitamin D increased fractures and falls.

This one found that taking vitamin D decreased bone mineral density:

Effect Of High-Dose Vitamin D Supplementation On Volumetric Bone Density And Bone Strength, A Randomized Clinical Trial, JAMA, August 2019

In this randomized clinical trial that included 311 healthy adults, treatment with vitamin D for 3 years at a dose of 4000 IU per day or 10 000 IU per day, compared with 400 IU per day, resulted in statistically significant lower radial bone mineral density (BMD).

Even participants who took 400 IU had a reduction in bone mineral density.

If high-dose vitamin D does stimulate an increased rate of bone loss, this could have greater clinical significance [fracture risk] in older individuals with osteoporosis.

Fracture risk … Didn’t we just see that?

Another problem with taking vitamin D, especially with calcium, is that it can increase the amount of calcium in blood (hypercalcemia) and urine (hypercalciuria), which it did in this study. Too much calcium in urine is a common cause of kidney stones. Too much calcium in blood effects many systems, has many side effects.

There was a dose-response effect:

Episodes of hypercalcemia and hypercalciuria were more common with increasing vitamin D dose, consistent with previous reports.

Surprisingly, it also affected those in the 400 IU group:

However, in the 400-IU group that was following the National Academy of Medicine–recommended dietary allowance of calcium and vitamin D3, 17% had hypercalciuria on at least 1 occasion over the study duration.

Conclusions:

These findings do not support a benefit of high-dose vitamin D supplementation for bone health.