Secular trends in under-reporting in young people

National survey data show that reported energy intake has decreased in recent decades despite a rise in the prevalence of obesity. This disparity may be due to a secular increase in under-reporting or a quantitatively greater decrease in energy expenditure. This study examines the extent of under-reporting of energy intake in the National Diet and Nutrition Survey (NDNS) in young people aged 4–18 years in 1997 using published equations to calculate estimated energy requirements. It explores secular changes by comparison with the Diets of British School Children (DBSC) survey in 10–11- and 14–15-year-olds in 1983. In the NDNS, under-reporting (estimated energy requirements – energy intake) represented 21 % of energy needs in girls and 20 % in boys. The magnitude of under-reporting increased significantly with age (P<0·001) and was higher in overweight than lean individuals over 7 years of age. To compare reported energy intake in DBSC and NDNS, the estimated physical activity level from dietary records (dPAL=reported energy intake/predicted BMR) was calculated. If there were no under-reporting, dPAL would represent the subject's true activity level. However, dPAL from the NDNS was significantly lower than that from the DBSC by 8 % and 9 % in boys and girls for those aged 10–11 years, and by 14 % and 11 % for 14–15-year-olds respectively, reaching physiologically implausible levels in the 14–15-year-old girls (dPAL=1·17). If activity levels have remained constant between the two surveys, under-reporting has increased by 8–14 %. The evidence supports a secular trend towards increased under-reporting between the two surveys, but the precise magnitude cannot be quantified in the absence of historical measures of energy expenditure.