Self-reported food intake decreases over recording period in the National Diet and Nutrition Survey

From 2008 the UK’s National Diet and Nutrition Survey (NDNS) changed the method of dietary data collection from a 7-d weighed diary to a 4-d unweighed diary, partly to reduce participant burden. This study aimed to test whether self-reported energy intake changed significantly over the 4-d recording period of the NDNS rolling programme. Analyses used data from the NDNS years 1 (2008/2009) to 8 (2015/2016) inclusive, from participants aged 13 y. and older. Dietary records from participants who reported unusual amounts of food and drink consumed on one or more days were excluded, leaving 6932 participants. Mean daily energy intake was 7107kJ (1698kcal), and there was a significant decrease of 164kJ (39kcal) between days one and four (P < 0.001). There was no significant interaction of sex or low-energy reporter status (estimated from the ratio of reported energy intake to BMR) with the change in reported energy intake. The decrease in reported energy intake on day four compared to day one was greater (P < 0.019) for adults with higher BMIs (>30kg/m2) than it was for leaner adults. Reported energy intake decreased over the 4-d recording period of the NDNS rolling programme suggesting that participants change their diet more, or report less completely, with successive days of recording their diet. The size of the effect was relatively minor, however.