If one has to go totally native. Ie foraging only. There is a side effect some may incounture. It's called the runs found out the hard way.
As mentioned earlier above start now. Learn how to find and start eating what you find in your area.
Something's may agree with you and others will not.
While doing this experiment make sure you don't forget the toilet paper. Also mint tea if it is avalible. Will help settle your stomach.