Yep. Thank you for scientific explanation.UVB rays from the sun cause 7-DHC in the body to synthesize D3....my guess is that tanned skin has more resistance to UVB rays.
Also there is easier to absrob Vit D3 and and less easy. Use oil gel caps or cyclosome delivery (uses plant fats) D3, thet are more effective.
D2 is not the same aa D3.