Are you sure? Melanin the skin pigment converts 99.9% of UV radiation into heat otherwise it damages deeper layers of the body. I'd say Melanin is there to protect us, so at polar latitudes, the need to protect from UV radiation drops off. Plus things like copper which is one of the chemicals used to make melanin is also used in other processes, for example some of the aromatic neurotransmitters. If you reduce chemicals to some neurotransmitters because its needed elsewhere in the body, what does it do to the brain? Look into hair pigmentation to get a better understanding. Remember diet restricts our intakes so some processes can appear to be rate limiting, when they are not, we are just a complex real-time chemical reaction. Feed any chemical reaction the right amount of chemicals for the environment and you can keep a chemical reaction going forever.