Have you ever thought about whether it might not just be about the diet, but rather that she's expressing her concern in the wrong way?
I think the thing is: moms make comments like that often out of some kind of need for control, because they think they're still "responsible" for you. But that doesn't make it any less exhausting for you. What the others already said is right - you need to set your boundary - but I'd package it differently. Instead of confronting her directly, you could just stay calm in the moment next time and say: "Mom, I know my health is important to you, but I'm making my own decisions about this. That works for me right now." Not accusatory, just matter-of-fact. And then change the subject. If she starts again, just repeat it without letting yourself get pulled in. She just needs to know from you that this isn't up for discussion anymore.
The other important part: she'll probably still feel weird about it because she's worried and that's... okay. That's her problem, not yours. Some people have to learn first that grown children go their own way. That takes time. But if you keep responding, justifying, debating - then you're sending her the signal that the topic is still open. So: clear boundary, stay calm, don't argue. It might take a few visits before she gets that this is really over, but that usually works better than having some big serious conversation.