I don’t understand that statement.
Cholesterol is what the skin uses to make vitamin D. Its kinda cool, really. The two molecules are the same except for the position of a single bond, but they have such different effects.
Cholesterol:
Vitamin D:
“...just 20 minutes in the summer sun is enough to deplete all the cholesterol in the skin necessary for the production of D.”
I don’t understand that statement.
The body is supplied naturally with vitamin D by the conversion of cholesterol in the skin to vitamin D by ultraviolet light of the sun.
Cholesterol moves slowly into the skin, though, so after just 20 minutes exposed to the ultraviolet light of the full summer sun, all the available cholesterol has been converted to D.
Spring and fall sunlight contains less ultraviolet light, though, so it makes sense to get more sun in the spring and fall.