"Sun" is definitely not sufficient. Vitamin D synthesis requires UVB exposure [1] and that plummets to effectively zero in the higher latitudes in the winter: h

"Sun" is definitely not sufficient. Vitamin D synthesis requires UVB exposure [1] and that plummets to effectively zero in the higher latitudes in the winter: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5037520/#sec3-nutrients-08-00533title https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5037520/#sec3-n... By "higher latitudes" I mean considered on a global scale; continental Europe is effectively all at these "higher latitudes". [1]: https://en.wikipedia.org/wiki/Vitamin_D https://en.wikipedia.org/wiki/Vitamin_D