WOW! I find that behavior relatively bizarre. My guess is that they have a fair amount of issues with an awful lot of life. To feel threatened by someone else’s health is rather mind numbing to me. Truly, the most constructive remedy I can think of is for them to act more mature.
That being said, you really can’t do anything about their mindset. I’m kind of an upfront and in-your-face person when it comes to this sort of thing. If confronted with this sort of attitude, my response would be to look at them sympathetically and ask why they are so frightened of me taking care of my health. I would not let them off the hook until they gave me an acceptable answer or they came unraveled But that is me.
But I think that it would pay for you to try and have an honest dialogue with them the next time this comes up. I I would ask them why this bothers them so much when it is something that you have no choice in doing. What makes them uncomfortable and how can you help them get over that. Offer them support.
The other thing that I would do, and in fact is what I do, is to make sure that I eat before I go there for a meal. Also, just because someone fixes food for a meal does not mean you must eat it if it poses a danger to you. They just have to understand that and deal with it. You are in charge of your health, not them. You have nothing to apologize for in being proactive and taking care of yourself.