How to keep a food diary
I've logged what I eat for a few years, and the thing that made it last is a boring one: being able to do it quickly. Take the phone out, 4 taps, done. The first app I built for it was almost as easy, I just didn't have actual items, meaning no tap tap to add, it was always written by hand. And it kinda lapsed eventually.
Most of this guide follows from that. A food diary is only worth reading back if nearly every meal is in it, and nearly every meal only gets in if writing it down takes seconds. The rest is a few small habits that keep each entry short and the same from one week to the next. If you want the columns ready to copy, there's a template with a worked week.
Write it down as you eat
At the table or right after, not in the evening, when the afternoon snack has probably gone missing already and lunch has turned into "around noon, I think". Use whatever you have on you while you eat. For most of us that's the phone, so a notes app or a diary app (a small notebook works just as well, if it's always in your bag).
What to write for each meal
The time, the foods and roughly how much. One name per food, and the same name every time: if it's "sourdough" on Monday and "toast" on Thursday, you'll only find half of them when you go back. Split a dish into the parts you'd recognize again, "rice, chicken, yogurt" and not "lunch".
Drinks go in too, marked as drinks, so a coffee on its own doesn't read as a meal.
How rough can the amounts be? Pretty rough: 2 slices, a big bowl. Leaving out the weighing and the calories is really most of what keeps an entry to a few seconds, and a separate page covers keeping rough amounts consistent.
Make it yours
Frictionless is one half of a diary that lasts, the other is that it's bespoke: it holds what you care about, not what a form asks for. So add one column of your own, a word or a color with a meaning you decide. Ate out, ate late, cooked from scratch, bought ready, whatever you'd want to see again. Write the key down, keep it to a handful, and don't make any mark mean good or bad; it says something about the meal, it doesn't grade it.
Stools, if you want them
The stool log is optional on every page here. If you keep one, write it as its own list under the meals: the time, a number from 1 to 7 for consistency, whether you had to rush, and a note when there's something to note. The template has the scale with a plain word for each number, so the same thing gets the same number every time.
When you miss a meal
It'll happen, and it's not a big deal. Add it when you remember, with the time as close as you can get it, and if you can't remember what it was, write "lunch, don't remember". An empty slot reads as a skipped meal a month later, and a marked one doesn't.
How long to keep a food diary
There's no right length, and it depends on why you keep it, which is up to you. Some people keep one for a couple of weeks, I've kept mine for years. What decides how long it lasts is mostly whether it stays quick: at 10 seconds a meal it barely registers in a day, and at 2 minutes a meal it probably won't make it past the first busy week.
Reading it back
Read it by the week. A day on its own is too little and a month is too much for one sitting, while a week fits on a page, with the marks counted per day in the margin. Paper's one weak spot is going back for a single food, since a notebook has no search.
More guides in Keeping a food and stool diary.
Full disclosure: I make Digest, an iOS app built around the first half of this page. You type a meal the way you'd say it, "two eggs and toast around 8", and it reads the foods and the time out of it for you to check. Foods you've had lately come back as one-tap chips, a template adds the breakfast you repeat in one go, and a widget or a Lock Screen control opens the log in one tap, which keeps a meal to about 10 seconds. History shows the weeks, and search lists the days you had a food or a color. There's no account. It's a journal, not a medical device, and nothing in it is medical advice.