Development and Construct Validation of a Food-Based Diet Quality Score Using Image-Based Food Records

Background/Objectives: Diet quality indices (DQIs) are commonly used to evaluate the alignment of dietary intake with national dietary guidelines. However, few methods have been developed to apply DQIs to image-based dietary assessment tools. This study aimed to develop a novel food-based Diet Quality Score (DQS) and examine its construct validity in Australian adults aged 18–65 years living with a higher weight (BMI 30–45 kg/m2), using the validated Healthy Eating Index for Australians 2013 (HEIFA-2013) for comparison. Methods: This cross-sectional study assessed dietary intake in 260 adults (mean age 48 ± 12 years; BMI 35.0 ± 4.2) collected over 4 days using the mobile Food Record (mFR™) and scored using the HEIFA-2013 and DQS (both scored out of 100). Descriptive statistics summarised participant characteristics with DQS and HEIFA-2013 scores. Construct validity was examined by assessing the associations between sociodemographic, health behaviour, anthropometric, and clinical blood marker variables and DQS and HEIFA-2013 scores. Multivariable binary logistic regression identified variables associated with being in the lowest diet quality tertile for both DQIs. Results: The mean scores were 47.4 ± 8.7 for the DQS and 52.0 ± 8.6 for HEIFA-2013. Both scores were positively associated with age (DQS: r = 0.216, p < 0.001; HEIFA-2013: r = 0.265, p < 0.001) and attention paid to the health aspects of their diet (DQS: p < 0.023; HEIFA-2013: p < 0.010). Increasing age (OR = 0.958, 95% CI: 0.932–0.985, p = 0.003) and moderate versus low physical activity (OR = 0.40, 95% CI: 0.21–0.80, p = 0.009) decreased odds of being in the lowest diet quality tertile for both DQI scores. Conclusions: The DQS demonstrated acceptable construct validity, providing a valid method for using image-based dietary assessments to evaluate diet quality in Australian populations.