Option A: handle it as Xanthippe suggests. This has definite pros in that it doesn’t make ripples, but it does mean that you’ll still be hearing the suggestions.
Option B:
Your mom: “Dark Light, you need to follow insert latest food-related trend here.”
You: “Thank you, mom. I appreciate your concern, but I’m quite satisfied with the way I’m eating now.” (If appropriate, you can add “as is my doctor.”)
If she continues to push it, say in gentle but firm tones, “I appreciate your concern, but as I said, I’m quite satisfied with the way I eat now. Let’s change the subject.”
If she still continues to push it, then, still in a polite but firm tone, “Mom, I have told you twice that I am quite satisfied with my diet, but you have continued to bring it up anyway. I’m not interested in discussing it further, and I won’t.” Then change the subject, or get off the phone, or leave the room.
This last will probably lead to a certain amount of initial resentment on her part if she isn’t used to people setting boundaries with her, but continue to be polite-but-firm and she’ll probably get it eventually. I recently had to set a similar boundary with someone over my diet, as pregnancy seems to be interpreted by the world at large as a request for their particular diet fads, with the added impetus of “FOLLOW MY DIET FAD OR YOUR CHILD WILL BE DAMAGED FOR LIFE.” After I told this person gently but firmly several times that both my doctor and I are quite satisfied with my current diet, and she continued to insist that I was probably causing my child brain damage by eating a certain way, I reiterated once more that my doctor and I are both happy with my diet and then said that we needed to change the subject. Her reaction was rather strongly resentful, but that was partly my fault; I had not set some proper boundaries as an adult with her, so she was very surprised when I did so now, even though the boundary was a reasonable one. Lesson learned: it’s best to set certain types of boundaries sooner rather than later in relationships!