It was drilled into me by my grandfather that when you're a guest in someone's home, it's nothing but rude to ask for anything special. So I am trying to overcome it.

Thanks so much for the invitation, I would love to come! I hate to inconvenience you, but I just wanted you to know that [insert dietary restriction or hatred here]. I'm really looking forward to it!

I am imagining Brenda's friends as People of the Carbs. (Plus Occasional Chicken.)

How do you LIVE without vegetables? Vegetables are good, people! I like almost all of them!

Communication, it's what for dinner.

Thanks so much for the invitation, I would love to come! I hate to inconvenience you, but I just wanted you to know that [insert dietary restriction or hatred here]. I'm really looking forward to it!

This is perfect.

Something like not eating meat, or some category of meat, I think I'd just say "I'd love to come for dinner. I don't eat red meat though - I hope that's not a problem?". For an actual bbq, telling them "I'm happy to bring something I can grill" would also be nice, but I'm betting nine times out of ten the response would be "oh don't worry about it, we can take care of you." Or it ought to be.

Other likes/dislikes I think are context dependent. If you know they're having pizza, I don't see anything wrong with saying "can I make a request for please no mushrooms?" If it's more a dinner party, I'd let it go and assume I could eat around them in most cases.

Depends on how ubiquitous or central the particular food is likely to be I guess - if you don't eat meat, it's going to be really obvious if you don't touch the main course. If it's a veggie or something more likely to come up as a side element, you can probably get away with taking a tiny portion or pushing it around on the plate or something without making it a big deal.

Now that I am willing to learn to pipe up, what is the absolute most polite way to say it?

"I'm a vegetarian" or "I don't eat meat" both sound fine to me.

It was drilled into me by my grandfather that when you're a guest in someone's home, it's nothing but rude to ask for anything special.

I think it's rude if you wait until the food is on the table (or later), but as long as you let the host know before s/he goes grocery shopping you're doing them a favor. "I'd love to come to dinner. By the way, I'm a vegetarian. Is there anything I can bring?"

Thanks so much for the invitation, I would love to come! I hate to inconvenience you, but I just wanted you to know that [insert dietary restriction or hatred here]. I'm really looking forward to it!

This. This is pretty much what Pete and I say. (He can't deal with nitrites, which means that unless he knows for sure, he can't have bacon, sausage, or ham. Brunch can sometimes be a very sad meal for him.)

Note: I make sure to have nitrite-free foods for Pete.

Forgot to add him to the list of "people whose food issues I have on speed dial in my brain."

I am imagining Brenda's friends as People of the Carbs. (Plus Occasional Chicken.)

How do you LIVE without vegetables? Vegetables are good, people! I like almost all of them!

Yeah, I don't even know. When the onion in a cheddar onion brat is enough to put it off limits? (To be clear, A eats veggies like a normal person. It's just her husband who is wacky about it.)

To which a good host says "thank you for letting me know". Not knowing about a guest's food restrictions ruins the evening for me as well, because it makes me feel like I haven't been a good host because my guest isn't enjoying themselves. Plus, as a general rule, I usually have several thoughts for dishes, and it would be easy to make something vegetarian, or without cheese or pork or whatever.