A food diary is a written or digital record of everything a person eats and drinks over a defined period, usually three to seven days. Dietitians ask clients to keep one so that energy intake, nutrient balance, meal patterns, portion sizes, and problem habits can be assessed against requirements. A useful diary notes not just the foods but quantities, preparation methods, times, and sometimes hunger levels or symptoms, which helps link eating patterns to health issues. Accuracy depends on honest, prompt recording — people tend to under-report snacks and indulgences. Smartphone apps have largely replaced paper diaries, adding barcode scanning and automatic nutrient analysis, but the principle is unchanged: you cannot improve a diet you have not measured.