The practise of remembering
Right now, can you list everything you ate the day before?
Don't worry about whether it's good/bad. It's important to be in the practise of awareness. If you need to journal as a habit that's a good way to begin.
But ultimately you should be in the habit of remembering what you ate the day before.
You need to know how to do this before you check your vitals as a daily habit.
(side note - any way you can practise remembering is a good thing. The world is becoming forgetful, have you noticed?)