Vitamin D refers to a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, iron, magnesium, phosphate, and zinc. In humans, the most important compounds in this group are vitamin D3 (also known as cholecalciferol) and vitamin D2 (ergocalciferol).[1] Cholecalciferol and ergocalciferol can be ingested from the diet and from supplements.[1][2][3] Very few foods contain vitamin D; synthesis of vitamin D (specifically cholecalciferol) in the skin is the major natural source of the vitamin. Vitamin D is made in the skin from cholesterol dependent on sun exposure (specifically UVB radiation).
https://en.wikipedia.org/wiki/Vitamin_D

I wonder how the study would have fared with a group subscribed regular sun exposure instead of supplements.
I like how the imagery is sun centered, the yellow and then the sunset/sunrise, subtle :D