How to keep a food and symptom diary

Write down three things every time you eat: what it was, when you ate it, and how you felt afterwards. Do it at the time rather than from memory at bedtime. Keep it up for two to three weeks without changing your diet. That is the whole method — the difficulty is not understanding it, it is doing it on the days nothing interesting happens.

What goes in each entry?

Four fields, and none of them needs to be precise.

Dish names are where most diaries fail. "Sandwich" is not an entry. Bread, cheese, tomato, mayonnaise is an entry — because the answer might be any one of them.

Why does logging at the time matter so much?

Because memory smooths things out. At the end of the day a large portion becomes a normal one, a snack disappears entirely, and "around four o'clock" becomes "the afternoon". None of those distortions are dishonest; they are just how recall works. And every one of them removes exactly the detail — portion, timing — that the diary exists to capture.

An entry written ninety seconds after eating is worth more than a page written from memory at midnight.

Why record the good days?

This is the step almost everyone skips, and it is the one that makes the diary work.

If you only write down the bad days, every food you ever ate on a bad day looks guilty. The days you ate bread and felt fine are what clear bread. A pattern is a difference between two groups, and you cannot see a difference if you only recorded one of them.

How long before you look for a pattern?

Two to three weeks. Less than that and a single unusual week dominates everything.

When you review it, go looking for the boring answer first:

1. Sort by symptom, not by food. Take your worst days and list what you ate in the six hours before.
2. Look for what is missing. Does the suspect food appear on good days too? If it does, it is probably not the answer on its own.
3. Check the amount. A food that causes nothing in small portions and trouble in large ones behaves exactly like an intolerance.
4. Write down your best two or three candidates. Then stop, because the next step is not yours to take alone.

What happens after the diary?

You take it to a dietitian or a doctor. A properly supervised elimination diet follows, and Monash University — whose researchers developed the low FODMAP approach — describes it as three phases: a restricted phase of 2 to 6 weeks, reintroduction over roughly 6 to 8 weeks, then personalisation. Monash is explicit that the first two phases should be done with dietitian guidance.

The diary does not replace any of that. It makes it faster, because the elimination starts from a short list instead of the whole supermarket.

A note on what a diary cannot do

It cannot diagnose anything. It cannot distinguish an intolerance from an allergy — that difference is about the immune system and needs a clinician. And it cannot tell you that a food is safe, only that on the days you recorded, it did not coincide with symptoms.

What it does well is one narrow thing: it turns a vague suspicion into a written record with dates and portions in it. We built FoodLog because doing this in a notes app collapses after about four days, and the entries that matter most are the ones logged on the days you did not feel like logging.

More on this in food and symptoms.