Do you remember that I was told by Dr. Ann that she was the gurus' doctor? Well, I worked with a number of them after she died. And, I want to give you permission to tell your friends that your doctor (me) will only allow you to eat certain things, and give them some options: for you to bring your stuff along, or for you to give them a list of things you can eat to stay on the program to sustain helping me to help our students, before you go on that trip this weekend.
They will respect your 'doctor's' orders and supply some local fruits or so and you won't get knocked off for three days or more. One man who is in charge of over 3 million people is in serious troubles now because he is doing just what you mentioned, eating foods that his faithful followers are making for him (tangible evidence of their love, even though it might take all of their foodstuffs they own for a couple of weeks or more to make the dainties they lay at his feet).
"How can I turn them down?" he said to me. That's the excuse a lot of people say when their wives or other family members like moms fry them chicken, bake ham, etc. and then they die because they can't use the old "My doctor doesn't allow me the normal celebratory foods; I would be happy to either bring what I can eat, or here's a list of the whole foods I can eat to help you in your thoughtful preparation of what I will eat."
You could use this as a quiet way to teach, not by talking about what you bring or eat, but just by your example as you eat it. let them help you stay on the program. This is an opportunity to let people know why you are able to do to help them, what you can do without even speaking.
Thank you for allowing me to share.