The food diary is one of those objects that looks obvious and is not. Somebody had to decide that a written record of ordinary meals was worth keeping at all, and that decision belongs to the history of research rather than to the history of cookery.
Early dietary surveys needed to know what populations actually ate, and the only way to find out was to ask people to write it down. The instrument was a form: columns for the meal, the item and the quantity, filled in over a set number of days.
The form comes first
Because the diary began as a survey instrument, its shape was fixed by what the surveyors wanted to count. Columns exist for the things a researcher intended to add up. That is why old forms ask for quantities in household measures rather than in anything more precise: the aim was a record a participant could plausibly complete, not a laboratory reading.
The same design assumption survives in almost every version since. A diary records what its designer thought countable, and quietly ignores everything else. Anyone who has kept one has run into the gap between the columns and the meal.
Known limits, written down early
Researchers were candid about the instrument's weaknesses from very early on. Keeping a record changes the behaviour being recorded. Memory at the end of a day is unreliable. Portions written in household measures cover a wide range. None of this was a secret; it appears in the methods sections of the surveys themselves.
What is interesting is that the instrument survived all of that criticism, because nothing better was available for ordinary use. A flawed record kept by the person eating remained more informative than no record at all.
From instrument to habit
The move from research tool to everyday habit happened gradually, carried by clinics, household writing and later by magazines. Along the way the diary picked up an evaluative column. The research form asked what was eaten; the popular form began to ask how the writer felt about it.
That extra column is the real historical change, and it is the reason the food diary reads so differently depending on the decade it comes from. The columns tell you what the people who printed the form believed was worth knowing.