Systematic review of the portion size norm of discretionary foods

CONTEXT: Portion size norm is described as the perception of how much of a given food people choose to eat. Reducing the portion size norm of foods that are high in saturated fat, added sugar, and added salt toward smaller sizes might be a potential strategy to promote appropriate portion size selections. However, an overview of existing portion size norms for discretionary foods has yet to be established. OBJECTIVE: The aim of this systematic review was to examine the portion size norm of discretionary foods and assess the methodologies used to investigate the norm. DATA SOURCES: The literature search was conducted in 6 databases following the PRISMA guidelines (from inception to January 2022). DATA EXTRACTION: Forty studies were eligible and grouped into 3 categories by portion size norm measures: normal (n = 26), appropriate (n = 8), and preferred portion sizes (n = 3). Study quality was assessed using the Joanna Briggs Institute Critical Appraisal Tools. DATA ANALYSIS: A wide range of portion sizes were considered normal for each food type, with means/medians varying from 2- to 4-fold among studies. Studies differed considerably in design, with variables including the setting, food type, food presentation, the manner in which portion-size-related questions were formulated, and the range and number of displayed serving size options. The quality of reviewed studies was mixed (25 studies had low or moderate risk of bias, 15 had high risk of bias), and the method of assessing portion size was not validated in 15 of 33 quantitative studies. CONCLUSION: The assessment of portion size in future studies should be conducted using tools that are validated for the population of interest so that more definitive conclusions can be drawn regarding portion size norms for discretionary foods. SYSTEMATIC REVIEW REGISTRATION: PROSPERO registration number CRD42021249911.