Testing an online measure of portion size selection: a pilot study concerned with the measurement of ideal portion size

BACKGROUND: Portion size is known to be a key driver of food intake. As consumed portions are often pre-planned, ‘ideal portion size’—an individual’s preferred meal size selected prior to eating—has been identified as a strong predictor of actual consumption. However, assessments of ideal portion size have predominantly relied on laboratory-based computer tasks, limiting use online. Therefore, this cross-sectional study sought to pilot test the validity of a web-based tool to measure ideal portion size. METHODS: In an online study (N = 48), participants responded to images of a range of foods. Each food was photographed in a series of different portions and loaded into an ‘image carousel’ that would allow participants to change the size of the displayed portion by moving a slider left-to-right. Using this image carousel, participants selected their ideal portion size. They also completed measures of expected satiety and expected satiation and self-reported their age and body mass index (BMI). A non-parametric correlation matrix was used to explore associations between ideal portion size and identified predictors of food intake. RESULTS: Supporting convergent validity of this measure, ideal portion size was significantly correlated with expected satiety (r(s) = .480) and expected satiation (r(s) = −.310) after controlling for effects of baseline hunger and fullness, consistent with past research. Similarly, supporting divergent validity of this measure, ideal portion size was not significantly correlated with age (r(s) = −.032) or BMI (r(s) = −.111,). CONCLUSIONS: Pilot results support the validity of this web-based portion size selection tool used to measure ideal portion size, though further research is needed to validate use with comparisons to actual food intake. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s40814-021-00908-x.