Validation of the Thumbs food classification system as a tool to accurately identify the healthiness of foods

The Thumbs food classification system was developed to assist remote Australian communities to identify food healthiness. This study aimed to assess (1) the Thumbs system's alignment to two other food classification systems, the Health Star Rating (HSR) and the Northern Territory School Canteens Guidelines (NTSCG); (2) its accuracy in classifying 'unhealthy' (contributing to discretionary energy and added sugars) and 'healthy' products against HSR and NTSCG, and (3) areas for optimisation. Food and beverage products sold between 05/2018-05/2019 in 51 remote stores were classified in each system. System alignment was assessed by cross-tabulating percentages of products, discretionary energy sold and added sugars assigned to the same healthiness levels across the systems. The system/s capturing the highest percentage of discretionary energy sold and added sugars sold in 'unhealthy' products, and the lowest levels in 'healthy' products, were considered the best performing. Cohen's kappa was used to assess agreement between the Thumbs system and the NTSCG for classifying products as healthy. The Thumbs system classified product healthiness in line with the HSR and NTSCG, with Cohen's kappa showing moderate agreement between the Thumbs system and the NTSCG (k=0.60). The Thumbs system captured the most discretionary energy sold (92.2%) and added sugar sold (90.6%) in unhealthy products, and the least discretionary energy sold (0%) in healthy products. Modifications to optimise the Thumbs system include aligning several food categories to the NTSCG criteria and addressing core/discretionary classification discrepancies of fruit juice/drinks. The Thumbs system offers a classification algorithm that could strengthen the HSR system.