Low-energy reporting and duration of recording period.

Three-day, rather than 7-day, food records are frequently used because mis-reporting of food intake is believed to increase with recording period. Data from the National Diet and Nutrition Survey of adults were used to explore trends in reported energy intake (REI) with day of recording and to compare average REIs from Thursday to Saturday and from Sunday to Tuesday to the complete 7-day record. Although REIs decreased from days 2 to 7, this was by a quantitatively insignificant 49 kJ per day (P=0.026) and well within the measurement error of recorded food intakes. Furthermore, REIs were lowest on the first recording day. The 3- and 7-day averages were similar (mean difference 0.039 (s.d.±1.0) MJ (NS), range −3.3 to +4.2 MJ). However, the difference was greater for those reporting higher than average energy intakes because of higher REIs on weekend days. Food intake reporting periods of longer than 3 days and ideally 7 days are preferable.