Association between life-course cigarette smoking and metabolic syndrome: a discovery-replication strategy

Background The relation between cigarette smoking and metabolic syndrome (MetS) remains unclear, and previous studies focusing on MetS are limited in sample size. We investigated the association between life-course smoking and MetS with independent discovery and replication samples. Methods Preliminary analysis utilized data from an annual cross-sectional survey of 15,222 participants aged >= 60 years in Tianjin, China. Suggestive associations were followed-up in 8565 adults from the China Health and Nutrition Survey. MetS was identified according to the criteria of the Chinese Diabetes Society in 2013. Life-course smoking was assessed by a comprehensive smoking index (CSI), based on information on smoking intensity, duration, and time since cessation across life-course, collected through standard questionnaires. Participants were divided into four groups: non-smokers; and the tertiles of CSI in ever smokers. Multivariable logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for the association between life-course smoking and MetS. Results In the discovery sample, ORs of MetS were 2.01 (95%CI: 1.64-2.47) and 1.76 (95%CI: 1.44-2.16) for smokers in the highest and second tertile of CSI compared with never smokers. Potential interaction was shown for age, with increased ORs for MetS associated with smoking limited to individuals who aged < 70 years (P-interaction = 0.015). We were able to replicate the association between cigarette smoking and MetS in an independent adult sample (second tertile vs. never: OR = 1.30, 95%CI: 1.04-1.63). The interaction of smoking with age was also replicated. Conclusions Life-course cigarette smoking is associated with an increased odds of MetS, especially among individuals who aged < 70 years.