估计来源是这个：
It's important to understand that vitamin D3 is an oil soluble steroid hormone. It's formed when your skin is exposed to ultraviolet B (UVB) radiation from the sun (or a safe tanning bed). When UVB strikes the surface of your skin, your skin converts a cholesterol derivative in your skin into vitamin D3.
However, the vitamin D3 that is formed is on the surface of your skin does not immediately penetrate into your bloodstream. It actually needs to be absorbed from the surface of your skin into your bloodstream. The critical question then is: how long does it take the vitamin D3 to penetrate your skin and reach your bloodstream? If you're thinking about an hour or two, like I did until recently, you're wrong. Because new evidence shows it takes up to 48 hours before you absorb the majority of the vitamin D that was generated by exposing your skin to the sun!
怪不得过去中国北方的农民不缺维生素D，因为几个月才洗一次澡。
