(Is It Really a Vitamin?)* D
I try to increase my intake of vitamin D to reduce cancer risk. The best would be long outdoor exposure to the sun. Unfortunately, this would increase the risk of skin cancer. So why can the human body not synthesize this vitamin (without sunlight)? What kind of intelligent design is this?
So what's special about sunlight, in particular, sunlight in the UV? Energetic photons, that's what. My guess is that it's either hard or very inconvenient to store a big enough chunk of energy to complete some crucial part of the synthesis - and sunlight is a widely available resource.
The photosynthesis of vitamin D evolved over 750 million years ago; the phytoplankton coccolithophor Emeliani huxleii is an early example. Vitamin D played a critical role in the maintenance of a calcified skeleton in vertebrates as they left their calcium-rich ocean environment for land over 350 million years ago. "Because vitamin D can only be synthesized via a photochemical process, early vertebrates that ventured onto land either had to ingest foods that contained vitamin D or had to be exposed to sunlight to photosynthesize vitamin D in their skin to satisfy their body’s vitamin D requirement."
It seems that plants and fungi synthesize a different form of Vitamin D which may be used as a sort of sunscreen! In any case, nature seems to have had plenty of time to think about devising alternative strategies for its manufacture, but doesn't seem to have developed them. On the other hand:
The naked mole rat appears to be naturally cholecalciferol deficient as serum 25-OH vitamin D levels are undetectable. Interestingly, the naked mole rat is resistant to aging, maintains healthy vascular function and is the longest lived of all rodents.
*Because we can synthesize it.