Background Adolescence and early adulthood is a period characterised by poor diet quality. Development of socioeconomic position (SEP) during this period may have long term impacts on diet quality and contribute to observed dietary inequalities in adulthood. This study aimed to identify the dietary trajectories present across adolescence and early adulthood; describe the food intakes driving diet-quality differences by trajectory and explore the contribution of parental and individual socioeconomic position to dietary trajectory membership.

Methods The Raine Study is a large multigenerational cohort, with participants (Generation 2) recruited prenatally during 1989–1991, Western Australia. Dietary data for participants (n = 1984) were self-reported using food-frequency questionnaires at up to five time points from ages 14 to 27 years. Growth mixture modelling was conducted in R to identify distinct diet quality trajectories based on DaSH diet scores over time. Food groups driving differences in diet quality were described. Multinomial logistic regression models were used to determine the associations between both parental SEP (maternal education level; and household income at age 14 years) and individual SEP (school leaving age) and diet trajectory membership. Associations between individual SEP and diet quality trajectory were adjusted for parental SEP, and all models were adjusted for sex and ethnicity.

Results Three diet quality trajectories were identified, characterised by diets of consistently low-quality (41% of participants), low-quality but improving over time (42%), and consistently high-quality (16%). Diet quality between the trajectories differed according to all 10 food groups considered, apart from total dairy and total grains. Having a mother who completed education beyond Year 12 was positively associated with membership in the high-quality diet trajectory (OR = 1.44 [95% CI: 1.07, 1.94] and negatively associated with membership in the low-quality diet trajectory (OR = 0.72 [95% CI: 0.57, 0.91]. Leaving school before Year 11 rather than completing school (Year 12) was positively associated with membership in the low-quality diet trajectory (OR = 1.59 [95% CI: 1.13, 2.23] but not significantly associated with membership in the high-quality diet trajectory, after adjustment for parental SEP.

Conclusion Our study demonstrates the development of inequalities in diet quality across adolescence and early adulthood, and highlights the importance of individual, in addition to parental socioeconomic position in shaping dietary trajectory membership during this period. Interventions to improve diet quality in early adulthood are particularly needed among those who leave education at a young age.