My suggestion would be to be direct... "Honey (mom, kid) I love that you are supportive of my changes for better health. It would be more helpful to me if you would not comment on what I eat (do, exercise, try...) unless I ask for your advice. These changes are quite a challenge to me and that would really help me. Thank you for understanding!"
I think that it is important to put this in a non-critical way, and to talk about what is or is not helpful to you.
I had to have that conversation with my teen son. He thought that he was being helpful by commenting when I would eat things he thought were not diet food. Rather than finding it helpful, like you, it made me defiant. Plus annoyed!