I'm not convinced by Dr Cui's cancer world map shows that its vitamin D alone that is responsible for lower cancer rates.
Russia for instance has an average lower age of death than most countries which means a lot of cancers formed in the 70/80 year olds will not be formed in Russia's population.
The other Elephant in the room is why Mexico adjacent to the US, has only 1/3rd the rate of cancer of the US, especially when their average vitamin D levels are
low(under 75 nmol/l), not much different than US.
A similar reason could be put forward for the lower rates in lots of Middle East and African countries, who despite having plenty of sunshine still have low vitamin D levels and lower cancer rates.
The real question that should be asked is why the 3 major areas with Western diets, US, Western Europe and Australia & New Zealand appear to have the highest cancer rates irrespective of the amount of sunshine.