How accurately can a diary or app describe someone's usual diet?

Understanding the question

A diary or app can describe reported food intake, but its accuracy depends on completeness, portion estimation, the food database, and whether recording changes eating behaviour. It is not a direct measurement of absorbed nutrients or long-term nutritional status. Different dietary assessment tools answer different questions: a recall reconstructs a recent period, a diary records intake as it happens, and a food frequency questionnaire estimates habitual patterns across a longer interval. Each has characteristic errors. Several days of data can help describe variation, but a short record may still miss weekends, seasonal foods, or unusual training days. The National Cancer Institute's Dietary Assessment Primer emphasizes matching the method to the research question. Useful interpretation treats an intake estimate as evidence with uncertainty, rather than as an exact account of everything consumed.

What a useful investigation needs to consider

A database entry may not match the brand, recipe, preparation method, or portion that was actually eaten. Extra decimal places in an app output do not remove these underlying uncertainties.

Group-level associations and individual adequacy assessments require different reasoning. A questionnaire suitable for ranking a large study population may not accurately quantify a particular person's daily nutrient intake.

Read the detailed explanation

The companion article explores a food record is an estimate, not a direct measurement of nutrition in more depth, with topic-specific explanations and source material.