Turning food down

yes, I have the same problem. I ate some really nice food I was offered last week, because I hadn't flared for a while, and was starting to get slightly more adventurous at trying small amounts of things I used to be able to eat before. But I hadn't eaten onions for over 2 years and the recipe had onion in it.
It upset my belly for a couple of days. Not horrifically but I would have been better not eating it.
It's hard to get into the conversation of what foods are okay and what you can't eat. Sometimes there will be a number of things in a recipe, and no way of even knowing what might cause bad effects, unless you get the cook to let you know every ingredient! I wouldn't want to get into asking that, before I ate, unless I was seriously allergic of course.
Then they want to know what bad effects the foods have, and that's not a reply anyone wants to hear! Too much information.
It's really difficult. As I'm slightly built to start with people suspect I might have anorexia because I often have to refuse food when out or visiting people.
I tell them all sorts...that I just ate a big lunch before I came...didn't know there was food offered....have to go early to make an important call from home....
I make sure those are not complete lies as I hate lying to people. So I often will eat something safe before going out somewhere food might be offered.

Or sometimes I will accept a doggy-bag offering gracefully, when I'm ready to leave. The food always smells so good and I long to eat it but know I can't.
A doggy bag will always be appreciated by another member of the family, microwaved for someone's lunch the next day!
But I have to go home and eat my own food.
I like it best when something is offered that I actually CAN eat, like a piece of cake, or some plain cookies!

I also have NCGS, allergies to shellfish and avocado's, and can't eat whole nuts, seeds, or any greens or lettuces.
I bring my own food everywhere I'm invited to. And I don't explain. I just eat.

I can relate. There are certain foods I just cannot eat and I am fine with that. I would rather not eat certain foods than suffer for it later. My problem is why do people think they have a say in my diet. Whenever I turn down salad at a restaurant or someones house they become shocked and say "You don't like salad." Or when I passed on the ice cream and I was asked why and explained I am lactose intolerant the would say "It sucks to be you." It is hard enough living with IBS without being made to feel as if I am some sort of freak. Does anyone else feel like this.

Yupe I can feel you, even though I don't dare to refuse people, because for me it's very rude to say people no.