Is there an association between food portion size and BMI among British adolescents?

The prevalence of obesity has increased simultaneously with the increase in the consumption of large food portion sizes (FPS). Studies investigating this association among adolescents are limited; fewer have addressed energy-dense foods as a potential risk factor. In the present study, the association between the portion size of the most energy-dense foods and BMI was investigated. A representative sample of 636 British adolescents (11-18 years) was used from the 2008-2011 UK National Diet and Nutrition Survey. FPS were estimated for the most energy-dense foods (those containing above 10·5 kJ/g (2·5 kcal/g)). Regression models with BMI as the outcome variable were adjusted for age, sex and misreporting energy intake (EI). A positive association was observed between total EI and BMI. For each 418 kJ (100 kcal) increase in EI, BMI increased by 0·19 kg/m2 (95 % CI 0·10, 0·28; P< 0·001) for the whole sample. This association remained significant after stratifying the sample by misreporting. The portion sizes of a limited number of high-energy-dense foods (high-fibre breakfast cereals, cream and high-energy soft drinks (carbonated)) were found to be positively associated with a higher BMI among all adolescents after adjusting for misreporting. When eliminating the effect of under-reporting, larger portion sizes of a number of high-energy-dense foods (biscuits, cheese, cream and cakes) were found to be positively associated with BMI among normal reporters. The portion sizes of only high-fibre breakfast cereals and high-energy soft drinks (carbonated) were found to be positively associated with BMI among under-reporters. These findings emphasise the importance of considering under-reporting when analysing adolescents' dietary intake data. Also, there is a need to address adolescents' awareness of portion sizes of energy-dense foods to improve their food choice and future health outcomes.