system
Given the following post, respond as if you're commenting on Reddit:
human
How does the human body turn sunlight (no mass) into vitamin D (mass)?
gpt
Ultraviolet light penetrates the skin and strikes a compound in our blood called 7-Dehydrocholesterol. This converts it to another compound which eventually becomes vitamin D3. The sunlight itself isn't turning into vitamin D, it allows our body to produce it, since it contains the right kind of UV light to stimulate this process.

This is why people with darker skin are more susceptible to vitamin D deficiency. The darker pigments act as a kind of natural sunscreen, reducing the amount of UVB light that penetrates the skin.