RUOkie can correct me, but I think we start with 1500-1800 calories in reserve. Keeping it somewhat in balance is the goal, without going into a negative. Eventually you will have to add protein to the mix, which is why I always had Perpetuem on hand. Whey protein is a bit easier on the system, but many folks can't seem to handle Hammer products. Gels and blocks are also on the menu, and Hostess Snowball are a certainty after 200 miles. I have yet to figure out how people can eat a whole meal while on a ride, but I have seen many a Rando do it. Not my thing, though the only full meal I eat each day is the salad I have for dinner.
Why protein?
You only eat one meal a day normally? Or when you are cycling? If you only eat one meal a day, my mind is totally blown.