tanning is via UVA rays

which penetrate the atmosphere and reach the surface better for more hours

VitD generation when UVindex>3 is via UVB

a shorter wavelength (295-297nm)

which best reaches the surface

when the sun is 45° (there IS some question if this could be as low as 30°) above the horizon

hence you can tan well without making VitD

AND by doing so make it harder to😬

D-fortified milk has 100IU per 8oz👍