Associations of childhood smoking initiation age with cardiometabolic multi-morbidity and mortality: a multi-national cohort study

BACKGROUND: The smoking initiation age may influence risks for chronic diseases and their progression toward multi-morbidity and mortality. However, evidence on the long-term associations of initiating smoking in childhood (< 10 years) with transitions among cardiometabolic conditions remains limited, particularly across countries with varying socio-economic contexts. We aimed to examine the associations of early smoking initiation with the risk and longitudinal progression of heart problems, diabetes, and mortality in older adults. We further explored the associations of daily smoking intensity, and current smoking status with these health outcomes. METHODS: In this multi-national, prospective cohort study, we used data from three nationally representative longitudinal aging cohorts across three countries from 2010 to 2020: the Health and Retirement Study (HRS, 2010-2020), the China Health and Retirement Longitudinal Study (CHARLS, 2011-2020), and the Mexican Health and Aging Study (MHAS, 2012-2018). We included adults aged ≥ 50 years without heart problems, diabetes, or all-cause mortality at baseline. Participants were categorized based on smoking initiation age. Cox proportional hazards models were used to assess associations between smoking initiation age, incident heart problems, diabetes, and all-cause mortality, by adjusting different covariates. Multi-state models explored the associations of smoking initiation age with different transitions from baseline to individual diseases and their co-morbidity. RESULTS: Among 21,018 eligible older adults, 2,447 participants developed heart problems, 3,078 developed diabetes, and 1,165 reported all-cause mortality. Compared with smoking initiation age ≥ 10, initiating smoking before age 10 was consistently associated with increased risks of heart problems across HRS (adjusted hazard ratio aHR 1.52, 95% CI 1.04-2.21), CHARLS (aHR = 1.43, 1.04-1.96), and MHAS (aHR = 1.45, 1.12-1.88). Childhood smoking initiation significantly associated with diabetes (aHR = 1.54, 1.13-2.10) and mortality risk (aHR = 2.27, 1.12-4.63) in the HRS cohort. Multi-state models showed that childhood smoking initiation (age < 10) significantly associated with transition risks from baseline to heart problems in all cohorts. Significant association of smoking initiation age < 10 with risk of direct transitions from baseline to cardiometabolic multi-morbidity (co-occurrence of heart problems and diabetes) was found in CHARLS (aHR = 1.71, 1.12-2.61). Smoking cessation was associated with a reduced risk compared to current smokers; however, the impact of smoking intensity on health outcomes varies across cohorts from different countries. CONCLUSIONS: Initiating smoking in early childhood (before age 10) significantly associate with long-term risks of heart disease, diabetes, and premature mortality for older adults, and influences the progression toward cardiometabolic multi-morbidity. Our findings highlight the importance of targeting younger children, beyond current adolescent-focused tobacco intervention strategies. We also emphasize the necessity for early-life smoking prevention, which could substantially reduce the future burden of chronic diseases and associated healthcare expenditures globally. In addition, promoting smoking cessation at any stage of life, particularly among current smokers, remains critical, as quitting smoking can meaningfully lower health risks and improve long-term outcomes.