How old are you?
1. If old enough you can simply wait till you are on your own and then do what you want.
2. You can challenge them to prove it and if they challenge you back then well you know where to come to get pointed in the right direction for your research.
3. Do what they ask but try to limit the grains you eat. Young people are resilient and you will survive eating grains although you may end up fat like me.
4. I work in the medical field...... you don't have to tell me how important Dr.'s think they are.