Diet quality is lower among adults with a BMI ≥40 kg m⁻² or a history of weight loss surgery

Background: Poor diet is a major public health issue requiring strategies to support improvements. Nutrition knowledge influences eating behaviours, yet few studies have examined relationships with diet quality. The current study aimed to explore relationships between demographic characteristics, nutrition knowledge, and diet quality using the Australian Recommended Food Score (measuring diet variety). Methods: Adults 18–60 years completed a 210-item survey including questions on demographics, health, nutrition knowledge, and diet. Statistical analysis used chi-square tests, linear and multiple regression, adjusted for covariates. Results: 480 respondents with a mean (SD) age 39.1 ± 11.6 years (18% male) completed all questions. Overall diet quality scores were high (ARFS 39.5 ± 9 points). Nutrition knowledge (p < 0.001) and BMI (p < 0.001) were positively associated with ARFS. ARFS scores were higher for those with higher nutrition knowledge scores (ARFS 42 ± 8 points) and of lower BMI (ARFS 40 ± 8 points) compared to those with lower knowledge (ARFS 37 ± 11) and higher BMI (ARFS 35 ± 10 points). Those with BMI ≥ 40 kg·m⁻² and weight loss surgery reported the lowest diet quality (ARFS 31 ± 10 points). Conclusion: Diet quality was highest among those with high nutrition knowledge and lower BMI. Those with a BMI ≥ 40 kg·m⁻², particularly those with past weight loss surgery reported the lowest diet quality, despite comparable levels of nutrition knowledge. It remains unclear which factors explain the variation in diet quality in the weight loss surgery group and this deserves further attention given the growing popularity of weight loss surgery.