Sorry I missed your question. I looked and there was no mention - and I'm approaching ancient at a rapid pace! So that's good info. I also tan easily, so with all of my recent cycling I'm darker, so probably not getting as much vitamin D as before(?). I probably should supplement. And I don't ride in a bathing suit (see below).

He did have this which is very interesting (and new to me) - Chapter 11, Environmental Toxins, Sun, Vitamin D, and Nature (starts on page 132):

Recommendations for determining how much sun exposure you should get

An adult wearing a bathing suit and exposed to one minimal erythemal dose (MED) of sunlight produces the equivalent of ingesting between 10,000 and 25,000 IUs of vitamin D (we will say 17,500 average). An MED is the amount of exposure needed to produce a slight pinkness of the skin the day after exposure. If just your arms and legs are exposed to an MED, you produce 6,000 IUs.