Changes in secondhand smoke exposure levels and risk of type 2 diabetes in middle age: the Korean Genome and Epidemiology Study (KoGES).

Objectives: Secondhand smoke (SHS) was known as one of the risk factors for type 2 diabetes. So far, some studies revealed the association of SHS exposure and type 2 diabetes, however, no studies to show the relationship of cumulative SHS exposure with type 2 diabetes exist. Therefore, the objectives of this study were to identify subgroups of participants who share similar trajectories in SHS exposure levels in middle age by using latent class growth modeling, and determine the independent association of these SHS exposure level trajectories with risk of incident type 2 diabetes. Methods: In Korean Genome and Epidemiology Study (2001-2014), 2079 participants aged 40 years and above who received biennially health check-up to follow-up and with available information of SHS exposure were selected. Four distinct trajectory groups (low-stable, moderate to low, moderate, and high to low) were identified for SHS exposure levels using trajectory modeling methods. Multivariable Cox proportional hazards model was used to examine the association of trajectories with risk of type 2 diabetes. Results: During 24 083.3 person-years of follow-up (mean follow-up duration, 11.6 years), 200 incident cases of type 2 diabetes and 640 incident cases of impaired fasting glucose (IFG) were identified. In multivariable Cox model, 'High to low' trajectory was significantly associated with risk of type 2 diabetes (OR 1.9; 95% CI 1.3 to 2.8) compared with 'Low-stable'. For IFG, all trajectories had significantly 30%-30% higher risk of type 2 diabetes compared with the 'Low-stable' trajectory. Conclusions: Changes in SHS exposure levels have been shown to associate with subsequent type 2 diabetes risk. Reversing high exposure level of SHS in middle-aged adulthood may still lead to worse progressions of type 2 diabetes than remaining stable exposure level.