Smoking and atherosclerotic cardiovascular disease in men with low levels of serum cholesterol: the Korea Medical Insurance Corporation Study

CONTEXT: Few studies have examined the interactive effects of smoking and serum cholesterol level on morbidity and mortality from cardiovascular dieseases. In East Asia, where the prevalence of smoking is among the highest in the world, morbidity and mortality from ischemic heart disease (IHD) is rapidly escalating. OBJECTIVES: To determine whether cigarette smoking is an independent risk factor for atherosclerotic cardiovascular disease (ASCVD) in the Republic of Korea (South Korea), a population that has relatively low levels of serum cholesterol, and to determine whether serum cholesterol levels modify the risk relationship between smoking and ASCVD. DESIGN: Prospective cohort study with a follow-up period of 6 years (1993-1998). SETTING AND SUBJECTS: A total of 106745 Korean men aged 35 to 59 years who received health insurance from the Korea Medical Insurance Corporation and who had biennial medical evaluations in 1990 and 1992. MAIN OUTCOME MEASURES: Hospital admissions and deaths from IHD, cerebrovascular disease (CVD), and total ASCVD. RESULTS: At baseline, 61389 (58%) were current cigarette smokers and 64482 (60%) had a total cholesterol level of less than 5.17 mmol/L (200 mg/dL). Between 1993 and 1998, 1006 IHD events (176 per 100000 person-years), 1364 CVD events (238 per 100000 person-years), and 716 other ASCVD events (125 per 100000 person-years) occurred. In multivariate Cox proportional hazard models controlling for age, hypertension, hypercholesterolemia, and diabetes, current smoking increased the risk of IHD (risk ratio RR], 2.2; 95% confidence interval CI], 1.8-2.8), CVD (RR, 1.6; 95% CI, 1.4-1.8), and total ASCVD (RR, 1.6; 95% CI, 1.5-1.8). For each outcome, there were significant dose-response relationships with amount and duration of smoking. Throughout the range of serum cholesterol levels, current smoking significantly increased the risk of IHD and CVD. In the lowest quartile of serum cholesterol levels (<4.42 mmol/L 171 mg/dL]), the RR from current smoking was 3.3 (95% CI, 1.7-6.2) for IHD and 1.6 (95% CI, 1.2-2.3) for CVD. There was no evidence of an interaction between smoking and serum cholesterol (P for interaction = .75, .87, and .92 for IHD, CVD, and total ASCVD, respectively). CONCLUSIONS: This study demonstrates that in Korea smoking is a major independent risk factor for IHD, CVD, and ASCVD and that a low cholesterol level confers no protective benefit against smoking-related ASCVD.