You need to have a more general discussion about how you all work together to make decisions.
And in that process you absolutely respect their opinions and ideas, but there are times when you may not agree. Also make it clear that your diet choices are not a protest or some crazy rebellion thing, but a lifestyle choice.
Ask if this is really bothering them and they can not stop bringing it up an and bugging you about it then, you suggest we all go to a family counselor and dietitian expert. Then if they decline, respond with the same thing. Please stop bringing this up, I asked to go to an expert to help us discuss this issue and you declined. If you think it is so important, than I am sure you can find the time to go to the counselor. Also you need to pick out someone independent, not one of their friends.
It is a bit odd, I mean you could be eating fast food, chips and candy, soda etc, like most younger folks. I guess that would be OK? Show your parents my note.
PS, be fair, do not nag them if they have a steak.