This is pretty well documented in Weston Price's book, where the same thing took place 80 years ago across the world.
Indigenous people typically are totally ignorant that their diet keeps them healthy. Unfortunately they are also the least adapted of all of us to modern foods. When modern foods arrive they are embraced as status symbols, tasting good, and the old ways typically are discarded.
For some unknown reason, these people usually don't grasp the correlation between their change in diet and their new medical problems. The scientific method must not be instictive.
In one case of an Eskimo family, they had abandoned their self-heated igloos and built a "modern" home that required a constant wood fire to stay warm. They also changed their diet to bread, jams, etc. Their daughter developed tuberculosis as a result of malnutrition. An explorer realized that their lifestyle had kept them healthy and literally threw them out of their house. They were forced to go back to their previous way of living, and their daughter recovered.
I think that basically what we are seeing is Survival of the Fittest. There are not enough animals to feed all the people that we have on this planet. So, people will have to adapt to be able to eat mostly grains.