Background: Adolescence to early adulthood is a period of multiple life transitions. Changing resources and contexts along with these transitions could contribute to significant changes in diet, which may persist into later adulthood.

Objective: We investigated diet quality trajectories from age 15 to 31 years and changes in diet quality associated with life transitions, by sex.

Methods: Data from the Project EAT study in Minnesota, the US were used to examine diet quality among a longitudinal cohort (n=2,524) across four waves (mean ages of 15, 19, 25 and 31 years). Average within-person changes in DASH (Dietary Approaches to Stop Hypertension) scores were analysed using sex-specific latent growth models, incorporating underlying growth trajectories, five life transitions, and baseline socio-demographic and health characteristics.

Results: Both sexes followed a quadratic trajectory of DASH scores, showing decreases in diet quality from wave 1 to 2 followed by increases until wave 4, although males had increasingly worse diet quality than females. Leaving the parental home between waves 1 and 2, compared to no such transition, was associated with transient decreases in diet quality at wave 2 only for males [β -2.34, 95% confidence interval (CI) -3.57, -1.11]. For females, cohabitating with a partner and becoming a parent between waves 3 and 4 were related to decreases (β -1.96, 95% CI -3.45, -0.47) and increases (β 1.85, 95% CI 0.47, 3.23) respectively in diet quality at wave 4. Leaving full-time education and starting full-time employment showed negative and positive associations respectively with long-term diet quality for both sexes.

Conclusions: Diet quality remained suboptimal throughout adolescence but mildly improved across early adulthood. Targeted dietary interventions are welcome for young people who leave their parental home early or who do not enter a structured school or workplace environment, and for addressing sex differences in diet quality associated with family-related life transitions.