The OP writes that a food journal is being kept. Obviously you cannot write in the journal ON Shabbos but you can write before Shabbos. Here’s the idea…PLAN in advance what you will eat. Write it down and refer back to it over Shabbos.
Stick to what you write. If you end up with something different than what you wrote you can correct it after Shabbos.
Planning meals is very helpful for dieting. Good Luck!