through the action of UV-B light (wavelength range 230-313 nm). Therefore, technically in a perfectly controlled experiment, I GUESS that 防晒霜 may slow down 维生素D的 formation when compared to not wearing 防晒霜, since 防晒霜 usually blocks UV-B. But in practice, I doubt that it has any real effect. Sunscreen won't block all that range of the UV light. And I think that you also get V-D from your food.