en ● Health Systems مقاله اصيل پژوهشي Orginal Article <div style="text-align: justify;"><strong>Background</strong>: The healthy eating index (HEI) is a valuable tool in health and nutrition research, providing a measure of diet quality aligned with key recommendations from the Dietary Guidelines for Americans. This study aimed to examine the trends and sociodemographic correlates of dietary quality, as measured by the HEI-2015, among Iranian households over 30 years from 1991 to 2020.<br> <strong>Methods</strong>: Data from the Household Income-expenditure Survey of 665,254 Iranian households (out of 926,339 surveyed from 1991 to 2020) were analyzed in this cross-sectional study. A hierarchical age-period-cohort (HAPC) model, implemented in SAS software, version 9.4, was used to disentangle age, period, and cohort effects.<br> <strong>Results</strong>: HEI scores initially increased from 1999 to 2017, indicating improved dietary quality, then declined since 2018. Significant sociodemographic disparities were observed, with higher household income (P<0.0001), education level (P<0.0001), and urban residence (P<0.0001) associated with higher HEI scores. The HAPC model enabled disentangling age, period, and cohort effects, providing insights into the distinct influences shaping dietary patterns across population subgroups over time. This study revealed a statistically significant positive trend in scores, with a mean increase of 8 points over 18 years (1999-2000 to 2015-2016). Scores rose from 43.86 (1999-2000) to 51.86 (2015-2016).<br> <strong>Conclusion</strong>: This study highlights the need for targeted interventions to promote healthy eating habits, particularly among socioeconomically disadvantaged and rural populations. Addressing the observed decline in dietary quality and sociodemographic disparities is crucial for improving the overall population health and well-being.</div> Healthy eating index (HEI)-2015, Diet quality, Hierarchical age-period-cohort (HAPC), Household income and expenditure survey  379 390 http://jrh.gmu.ac.ir/browse.php?a_code=A-10-2542-4&slc_lang=en&sid=1 Omid Emami oe.info@yahoo.com 100319475328460047149 100319475328460047149 No Department of Food and Nutrition Policy, School of Nutritional Sciences and Dietetics, Tehran University of Medical Sciences, Tehran, Iran. Ahmadreza Dorosty Motlagh dorostim@tums.ac.ir 100319475328460047150 100319475328460047150 No Health Equity Research Center (HERC), Tehran University of Medical Sciences (TUMS),Tehran, Iran. Amirhossein Takian dorostim@tums.ac.ir 100319475328460047151 100319475328460047151 Yes Department of Food and Nutrition Policy, School of Nutritional Sciences and Dietetics, Tehran University of Medical Sciences, Tehran, Iran. Ehsan Mousa Farkhani farkhanie@mums.ac.ir 100319475328460047152 100319475328460047152 No Department of Epidemiology and Biostatistics, School of Health, Mashhad University of Medical Science, Mashhad, Iran.