Vitamin D is a steroid hormone and is synthesized in one's body after skin is exposed to sunlight.
Once the body has made enough, it will produce no more. Too much Vitamin D can be toxic and lead to bone loss. Testing of 10 samples of infant formula revealed seven with more that twice the Vitamin D content reported on the label, one of which had more than four times the label amount. Vitamin D is toxic in overdose. This demonstrates adequate mineral absorption occurs from a predominantly vitamin D-free transport mechanism.