Age-and sex-specific variation in diet quality across the lifespan and its lifestyle and socio-demographic determinants in Japan

Variations in diet quality across the lifespan remain insufficiently characterised, particularly in non-Western populations. We aimed to examine age-and sex-specific variation in diet quality and identify associated dietary components and socio-demographic and lifestyle factors in a Japanese population. In this cross-sectional study of 4455 Japanese individuals aged 1-79 years, dietary intake was assessed using 8-day weighed dietary records collected across four seasons. Diet quality was evaluated using the Healthy Eating Index-2020 (HEI-2020) and Nutrient-Rich Food Index 9·3 (NRF9·3). Associations with dietary components and lifestyle and socio-economic factors were examined separately in children and adults, with tests for interaction by age. Both indices showed marked age-and sex-specific variation. HEI-2020 scores reached estimated nadirs at 16·1 years in females and 23·4 years in males, whereas NRF9·3 scores were lowest in early adulthood (approximately 24 years) in both sexes; older adults consistently exhibited the highest scores. Across age-sex strata, the components contributing most to total score variation were dairy, fatty acids, Na and saturated fats for HEI-2020 and Na and added sugars for NRF9·3. Socio-economic disparities emerged early, with higher household income and parental education associated with better diet quality in children. In adults, lower diet quality was associated with lower income and education, smoking and breakfast skipping in both sexes and full-time employment in males. In conclusion, diet quality was lowest in adolescence and early adulthood in this Japanese population, with distinct socio-economic and lifestyle determinants across life stages, thereby highlighting critical periods for targeted, age-appropriate nutritional interventions.