First up, the quantity and quality of calories matter both to health and to weight. You can’t gain without a surplus. You can’t lose without a deficit. And the quality of the calories you’re consuming will affect health and satiety which in turn will affect the quantity of them that you consume. Moreover, the bioavailable calories you consume will differ by food, and also likely differs by individual (which is why some gain and lose with more ease than others).
Next up, we’re crappy food historians. We may forget portions, choices, or both, not all the time, but certainly some of the time. We can’t possibly know what’s in meals we haven’t cooked ourselves. And even if we are cooking ourselves, most aren’t going to be weighing and measuring everything and eyes are terrible at both.
And a recent study confirms some of the above whereby researchers looking at users of myfitnesspal found the average user was missing nearly a meal’s worth of calories a day (445). Yet studies on food diary use pretty much invariably report they markedly benefit weight loss efforts.
Personally, though I think having some rough inaccurate sense of caloric intake is valuable (if you were in a foreign country and didn’t know the exchange rate, price tags would still be somewhat helpful), more valuable is the use of the food diary to remind yourself that you’re trying to eat thoughtfully and likely differently.
Human nature being what it is, without a system designed to consciously remind you to change your usual default behaviours, you’re likely to drift back to those behaviours, healthy or not, and a food diary, even if inaccurate, if kept in real-time, will remind you many times a day that you’re trying to change.
So long as you’re not using your food diary as a tool of judgment, as it’s not meant to be there to make you feel badly about your choices, chances are it’ll be of benefit, and likely it’ll be of benefit regardless of what it is you’re tracking (calories, macros, carbs, whatever) and even if inaccurate, because it’s primary job is to serve you as your constant change reminder service, not as your judge and jury.