You have options that range from polite to potentially mean. What style you choose is up to you.
If you have the opportunity to speak to her alone, tell her politely that her comments make you feel uncomfortable and you need her to stop saying these things. Repeat this to anything she says, except “I won’t do that any more.” This will work better as an in person conversation. No matter what other things she says in the course of the conversation, don’t go off topic. She may try to deflect with comments about your weight, your health, etc. NONE of those things are her business. Simply repeat that her comments make you uncomfortable and you need her to stop saying them. No matter what tangent she tries to go on, and she will, stick to the subject. Once she says that she will stop, thank her. Use her name during the conversation. You would be surprised the impact that has. Then hold her to it at dinner the next time you go out. Remind her that she agreed to stop commenting of your eating habits.
Another option to try takes place during your night out. When she makes her first comment, look her in the eye and politely, firmly, say, “XXXX, your comments about how much I eat are unnecessary and upsetting. I must ask you to stop.” When/if she objects, stick to what you said.
Option three: “I didn’t realize that you are the food police. I know I’ve not committed any food violations, so you’re trying to give me a citation without cause.” Any upset on her part requires you to react calmly and go immediately back to the point of her remarks making you feel uncomfortable.
Option four: After she makes her first comment, respond with something equally rude. For example, if she says something like “are you even eating your food?” respond with something like, “Did you forget to comb your hair?” When she acts startled/surprised/angry/whatever, your response can be, “Oh, I thought we were asking rude and unnecessary questions at dinner tonight.” (Obviously, you can come up with your own rude question.) The hope is that she will understand your discomfort and finally shut up about how you eat.
Options three and four have backfire potential if you don’t remain calm. Be prepared to sit in an uncomfortable silence. It might not be easy, so remind yourself that she has been rude to you and you need to have her stop.
I’m interested to know how your friends react when she makes these comments to you.