Evaluating the change in the dietary assessment tool for the UK National Diet and Nutrition Survey

The NDNS was set up in 2008 as a cross-sectional survey with a continuous programme of fieldwork. It provides the only source of nationally representative data on the diet and nutritional status of the UK population (1) . For its first 11 years, dietary data was collected using a paper food diary. In 2019, to modernise methods, increase cost efficiency and provide opportunities for scaling the survey in the future, the paper diary was replaced with an online 24-hour recall tool, Intake24. The new dietary assessment method in NDNS is a significant change in the way dietary data is collected, processed and analysed. The move also necessitated a major review and rationalisation of the NDNS nutrient databank (NDB) (2) . Given the importance of the survey to monitor UK dietary trends over time, this study aimed to identify aspects of data discontinuity and the feasibility of continuing to track trends over time. A trend analysis from 2008 to 2023 (survey years 1 to 15) was carried out on key foods and nutrients using a weighted regression modelling approach. This included a break for the diet method change such that regression lines were fitted separately for 2008 to 2019 (years 1 to 11 using the diary) and 2019 to 2023 (years 12 to 15 using Intake24). This approach enabled the quantification of any step changes at the point of the method change. A number of step changes were seen in reported consumption of foods including a downward step change in 5 A Day fruit and vegetables portions for adults aged 19 to 64 (1.1 portions per day, 95% CI 0.8, 1.3) and consequently a downward step change in fibre (3.6g per day, 95% CI 2.7, 4.5). However, the majority of the Intake24 recall data collection period coincided with the COVID-19 pandemic and significant increases in the cost of living in the UK, both of which impacted diets due to changes in lifestyle, food availability and affordability. A doubly labelled water (DLW) sub-study showed that use of Intake24 in NDNS was associated with a similar degree of underreporting of energy intake to that found by previous NDNS DLW studies for the paper diary method. The study identified a number of step changes in reported intakes between Intake24 and the paper diary. However, as it was not possible to definitively distinguish between differences in the data due to the method change and those that signified real changes in diets, it was not appropriate for NDNS to continue the time trend over this period. Instead a new time trend will be built up from 2019 and presented in the future when sufficient data are available.