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.
“Spring and fall sunlight contains less ultraviolet light, though, so it makes sense to get more sun in the spring and fall.”
In the reading I’ve done about Vitamin D I’ve never seen that recommendation, including numerous Vitamin D Council newsletters (John Cannell, M.D.). In fact, the recommendations all point to summer sun as the best for Vitamin D stimulation due to the decreased angle of incidence and resultant increase in ultraviolet energy. Of course, I may have missed it...I’d be interested in any links or studies you might have regarding spring and fall being better for Vitamin D generation.