Early-life exposures and child health outcomes: A narrative review of LSN21 research in Japan.

BACKGROUND: The Longitudinal Survey of Newborns in the 21st Century (LSN21) tracks two Japanese national birth cohorts-2001 (baseline n = 47,010) and 2010 (n = 38,554)-from infancy through young adulthood, capturing parenting practices and family environments. Most studies analyze single exposures or outcomes. We conducted a narrative review summarizing the findings published by the Okayama University group on diverse health and developmental outcomes. METHODS: We reviewed 59 LSN21 papers (2013-2025), extracting data on exposures, outcomes, and methods. Evidence was categorized into four exposure types (infant feeding, sleep, environmental, and perinatal) and three outcome domains (obesity, allergies/respiratory tract infections, and neurobehavioral development), including cohort comparisons. RESULTS: Exclusive breastfeeding was associated with a lower obesity risk at ages 7 (adjusted odds ratio 0.55, 95% confidence interval 0.39-0.78) and 15, later puberty, and fewer hospitalizations. Short or irregular sleep before age 3 was linked to behavioral problems and injuries. Maternal smoking and prenatal air pollution were associated with respiratory conditions and developmental challenges. Preterm birth and small-for-gestational-age predicted delays, especially without catch-up growth by age 2. Pneumococcal vaccination likely contributed to declining otitis media after 2010. Additional findings included associations between outdoor play and reduced obesity risk, and complex relationships between breastfeeding and food allergies that varied by infantile eczema status. CONCLUSIONS: LSN21 findings highlight modifiable early-life factors (breastfeeding, sleep patterns, and smoke-free environments) and identify preterm and growth-restricted children for priority monitoring. While LSN21's strength lies in longitudinal social assessments, complementary perspectives from other Japanese cohorts could enhance understanding of biological mechanisms and intergenerational effects.