@whiteybeefcake
Did you read anything from this abstract? This isn't a study on D3 supplementation, it's a dermatological study of cultured skin cells incubated with a topical D3 and retinol application.
This is relevant for understanding the mechanisms of topical Vitamin A and D applications on the skin, e.g. for skin diseases, but it does not investigate or study general bodily metabolism of Vitamin D and Vitamin A supplements whatsoever, nor can any conclusions be drawn about that from this study.
To investigate the potential effect of vitamin D on retinol metabolism in human skin keratinocytes, HaCaT cells were preincubated with various vitamin D3-analogs at 10(-7)M for 24 h followed by the addition of [3H]retinol for another 24 h period.
Did you read anything from this abstract? This isn't a study on D3 supplementation, it's a dermatological study of cultured skin cells incubated with a topical D3 and retinol application.
This is relevant for understanding the mechanisms of topical Vitamin A and D applications on the skin, e.g. for skin diseases, but it does not investigate or study general bodily metabolism of Vitamin D and Vitamin A supplements whatsoever, nor can any conclusions be drawn about that from this study.