Food diary — how to keep one and what to bring to a dietitian

A food diary is the simplest diagnostic tool in nutrition and also the one most often kept badly. Below: what to log, for how long, and what specifically a dietitian is trying to read out of it.

Why logging alone changes something

Before anyone analyses the diary, it is already doing work. The phenomenon is well described in health psychology as self-monitoring: knowing an action will be recorded changes the action.

In practice that means the second biscuit stops being a reflex and becomes a decision — because it will have to be written down. This is not about guilt. It is that some eating happens outside conscious attention, and a record puts the attention back.

What to log and what to skip

The most common reason diaries get abandoned is trying to record everything at maximum precision. After four days it becomes a second job. A workable split looks like this:

The first column is mandatory, because without it the diary does not describe reality. The second turns a list of food into something you can draw conclusions from — times and hunger levels are what explain why a craving for something sweet turns up at ten in the evening. The third adds nothing that cannot be filled in later.

A photo instead of a description

Typing a meal in takes a minute and requires estimating weights at the exact moment you are hungry and want to eat. A photo takes three seconds. For the completeness of a diary that difference is decisive — estimating from a photo is less accurate than a kitchen scale, but a diary that actually exists beats a more accurate one that does not.

For how many days

It depends on the purpose, and that distinction saves a lot of unnecessary work:

What a dietitian actually wants to see

Counter to intuition, it is not the calories. A dietitian can total those in a few minutes. What they read from the diary are the things an interview cannot recover, because nobody remembers them:

• The rhythm of the day. When the first meal happens, how many hours sit between meals, whether most of the energy lands in the evening.
• How protein is distributed. Spread across the day, or all sitting in dinner.
• Fruit and vegetables. The number of portions a day, not the claim that you “try to eat healthily”.
• Fluids. How much and what kind — that alone settles a lot of questions about fatigue and headaches.
• Patterns, not events. Whether snacking has a time of day, or follows particular situations.
• Weekday versus weekend. Usually the single largest source of discrepancy.

What form to bring it in

Ideally as one PDF or printout, in chronological order, with dates and times. Screenshots from an app are fine too, as long as the dates are visible. What to avoid: loose sheets, notes spread across several places, and an export in a format nobody can open without that same app.

If your app supports exporting the diary, export a full week and check whether the file contains photos or meal descriptions — a table of calorie totals is far less useful than a list of what was actually on the plate.

Five mistakes that ruin a diary

1. Writing it up in the evening from memory. After eight hours, 20–30% of what was eaten has disappeared on average. Log at the moment of eating or immediately after.
2. Leaving out caloric drinks. Juice, a latte, a beer, a sports drink — for many people that is 300–600 kcal a day that never reaches the record.
3. Not counting cooking fats. Two tablespoons of olive oil is about 240 kcal. It is the single most frequently omitted item in any diary.
4. Eating a “nicer” week for the diary's benefit. If you eat differently for those seven days, you are diagnosing somebody else. The diary is there to describe the starting point, not to make a good impression.
5. Quitting after a bad day. One unlogged day does not invalidate the week. A gap in the data is far less damaging than an abandoned diary.

When to stop logging

A diary is a tool, not a lifestyle. It is worth keeping for as long as it is still teaching you something.

After a few months most people estimate their own meals markedly better than they did at the start — a skill the diary builds incidentally and which stays once you put it down. At that point it makes sense to drop to two or three days a week: enough to catch drift, without occupying attention day to day.

This article is informational and is not medical advice. The formulas below are estimates for healthy adults. If you have a medical condition, take medication, are pregnant or are breastfeeding, set your targets with a clinician rather than a calculator.