Sunscreen blocks UVB radiation, and reduced sun exposure contributes to declining vitamin D levels.

“Sunscreen blocks UVB radiation and most people are wearing sunscreen. Most people are indoors now. You know, we're on our computers, we're not in agrarian society like we used to be. You know, people aren't out in the sun all the time. People are inside. I mean, if you look at, you know, enhanced data over the past couple of decades, you see vitamin D levels are steadily just going down, down, down, down.”