When Indians who follow a strict vegan diet started migrating to the western world, a lot of them would begin to suffer from pernicious amaemia, a disease caused by deficiency of dietary vitamin B12. Similar diseases are present in India but appeared to be less common.
Most published research are rather diplomatic about this phenomenon[0]. However, reading between the lines, the reason appears to be the lack of fecal and insect contamination in food supply, which accidentally eliminated a lot of exogenous micronutrients not present in plants.