Development and validation of a photograph-based instrument to assess nutrition literacy: the NUTLY

BACKGROUND: Most instruments measuring nutrition literacy are difficult to apply to less educated people, or assess theoretical knowledge, not necessarily evaluating the ability to make healthy food choices. We aimed to develop and validate a photograph-based instrument to assess nutrition literacy (NUTLY) among adults in Portugal. METHODS: NUTLY assesses the ability to identify the food with the highest energy/sodium content from sets of three photos (two foods with similar content and one with higher content). A version of 79 sets was obtained after a stepwise process involving expert and lay panels. It was applied to a sample representing different age, gender and education level groups (n = 329). We assessed internal consistency (Cronbach's α) and associations between each test item (set of three photos) and the overall score, defined as the sum of correct answers (latent trait models); sets performing worse were excluded. Reproducibility was assessed after one month (n = 158). To assess construct validity, NUTLY was compared to the Medical Term Recognition Test (METER) and the Newest Vital Sign (NVS), and across groups with varying education and training in nutrition/health. Latent class and Receiver Operating Characteristic (ROC) analyses were used to define the cut-off point. RESULTS: The final NUTLY includes 48 sets (energy: 33; sodium: 15; α = 0.74). Mean scores were highest in nutritionists (39.9), health professionals (38.5) and decreased with decreasing education (p < 0.001). Those with adequate nutrition literacy (based on NVS) showed higher NUTLY scores (37.9 vs. 33.9, p < 0.001). For METER, the differences were not significant. Reproducibility was high (ICC=0.77). The Area Under the ROC Curve was 0.97. Adequate nutrition literacy was defined as a NUTLY score≥35 (sensitivity: 89.3%; specificity: 93.7%). CONCLUSIONS: NUTLY is a valid and reliable tool for assessment of nutrition literacy, with the potential to be adapted to other cultures and specific population groups. KEY MESSAGES: • A new photograph-based nutrition literacy measuring tool was developed and validated in Portugal, overcoming some limitations of previous instruments. • In the future, this instrument could be used as part of a public health surveillance system, and as a tool to assess the success of nutrition education interventions.