It is well-established that smoking is hazardous to human health,1 and is a major preventable cause of premature deaths. In many developing Asian countries, the awareness of tobacco-related health risks remains low, and smoking is increasing in popularity. In fact, tobacco-related diseases have become a leading health problem ahead of infectious diseases in many of these Asian countries.2 Previous cohort studies on the association between smoking and subsequent mortality in Asian populations3–6 have had a methodological limitation because the smoking behavior was only measured at baseline. As a result, changes in smoking behavior during follow-up could result in misclassification bias. Consequently, the results according to smoking behavior at baseline only tend to have sick-quitter bias or survivor bias. This common problem in prior cohort studies can be overcome by repeated assessment of smoking behavior. Our cohort provided an opportunity to examine changes in smoking behavior between 1991 and 1997 and to relate these changes to mortality risk during 1998–2008, with a total follow-up of 18 years.
2.1. Study population
The Kinmen Study is a Chinese population survey, which started in 1991. A number of population-based studies have been conducted in Kinmen,7–15 and the details of the methods have been reported previously.16,17 From 1991 to 1992, a baseline survey was conducted among 11,338 registered residents aged > 30 years in Kinmen, Taiwan. Follow-up interviews were conducted from 1997 to 1998, with a total of 5136 participants successfully re-contacted. Participants whose responses were inconsistent (n=124) and former smokers who resumed smoking at the follow-up interview (n=26) were excluded, leaving a total of 4986 participants included in this study. This study was approved by the Institutional Review Board of National Yang-Ming University, Taipei, Taiwan.
2.2. Smoking and smoking cessation categories
According to the smoking behavior from the baseline and follow-up interviews, participants were divided into five groups: (1) continuing smokers who were smoking at both the 1991 and 1997 interviews; (2) new quitters who were current smokers in 1991 baseline but had quit by the 1997 follow-up interview; (3) new smokers who were nonsmokers at the 1991 baseline but started smoking by the 1997 follow-up interview; (4) long-term quitters who were those who had quit smoking at both the 1991 and 1997 follow-up interviews; and (5) never smokers including those who had never smoked both in 1991 and 1997 interviews.
2.3. Ascertainment of mortality
Deaths were identified through linkage of records with the Taiwan Registry of Deaths. For the current analysis, we updated mortality data up to December 31, 2008.
2.4. Statistical analysis
Multivariable Cox proportional hazard regression models were used to examine the association between the changes in smoking behaviors and the risk of all-cause mortality. The covariates in the model were sex, age, education, body mass index (< 18.5 kg/m2, from 18.5 kg/m2 to < 24 kg/m2, from 24 kg/m2 to < 27 kg/m2, and ≥ 27 kg/m2), alcohol consumption, dietary patterns (more meat than vegetables, equal amounts of meat and vegetables, and more vegetables), and a history of diabetes mellitus, hypertension, cardiovascular disease, or chronic liver disease during follow-up interview. The interaction between smoking and sex was tested and, as it was not significant, men and women were analyzed together with an adjustment for sex. All the reported p-values are two-sided, and p<0.05 was considered statistically significant. The statistical software SPSS for Windows, version 19.0 (SPSS Inc., Chicago, IL, USA) was used for the analysis.
Among the 4986 participants in the analysis, 608 were continuing smokers (12.2%), 349 were new quitters (7.0%), 104 were new smokers (2.1%), 187 were long-term quitters (3.8%), and 3738 were never smokers (75.0%). In relation to all-cause mortality, there was statistical significance for sex, age, education, body mass index, smoking status, and alcohol consumption (Table 1). For morbidity with all-cause mortality, there was the expected statistical significance for diabetes mellitus, hypertension, and cardiovascular disease (p<0.001), but not for chronic liver disease (Table 2). When considering morbidity with the change in smoking status, there was statistical significance for hypertension, cardiovascular disease, and chronic liver disease but not for diabetes mellitus (Table 3). Long-term quitters had the highest rates of diabetes mellitus, hypertension, and cardiovascular disease.
Table 4 showed the adjusted hazard ratio for all-cause mortality. We examined the association between the changes in smoking behavior and mortality, with the inclusion of age, lifestyle factors, and comorbidities as potential confounders. Compared with never smokers, continuing smokers had the highest risk of 1.84 [95% confidence interval (CI): 1.38, 2.45] for all-cause mortality, new quitters had a risk of 1.49 (95% CI: 1.04, 2.15), new smokers had a risk of 1.26 (95% CI: 0.59, 2.68), and long-term quitters had a risk of 1.11 (95% CI: 0.64, 1.91). Compared with continuing smokers, there was a significant 19% [(1.84–1.49)×100/1.84] risk reduction in mortality for new quitters, while no significantly increased risk was observed for long-term quitters.
In this study, we compared all-cause mortality among continuing smokers, new quitters, new smokers, long-term quitters, and never smokers. No significantly increased risk was observed for long-term quitters, and the excess risk decreased to the level of never smokers. There was a significant 19% risk reduction in all-cause mortality for new quitters. Smoking cessation was associated with a significant reduction of mortality risk within approximately 6 years. These results show that the smoking-related mortality risk can diminish upon smoking cessation.
Over the past decade, there have been only seven cohort studies with repeated measures of smoking status, which used different classifications for changes in smoking status.18–24 In a large study of 104,519 women aged 30–55 years in the USA, a greater number of years since quitting were associated with a reduction in all-cause mortality.20 A study in the USA with 14,200 participants with a follow-up over 17 years showed that early age at smoking initiation was independently associated with increased cardiovascular risk among all participants, irrespective of race.22 Two studies in Israel21 and Scotland23 that categorized smoking intensity as increased, maintained, reduced, or stopped concluded that reducing cigarette consumption should not be promoted as a means of reducing mortality, although it may have a valuable role as a step toward smoking cessation. A study in Norway with 49,539 participants aged 40–70 years found that continuing smoking strongly increased the risk of death, whereas smoking cessation reduced the risk.19 None of these studies were conducted in Asian populations or used classifications for changes in smoking status that were similar to the present study.
There have been only two studies with repeated measures of smoking status conducted in Asian populations.18,24 A study in Singapore involving 48,255 Chinese participants classified as never-smokers, long-term quitters, new quitters, and current smokers showed that new quitters had a 16% reduction in all-cause mortality, whereas the long-term quitters had a 39% reduction in all-cause mortality.18 This study investigated the middle-aged, and elderly people and new smokers were not included. In a study of 1494 Chinese participants in China, new quitters who had stopped smoking for 2–7 years had an 11% reduction in all-cause mortality, and those who had stopped smoking for at least 8 years had a 36% reduction in all-cause mortality.24 That study differed from ours in including fewer participants and focusing on new quitters who recalled the number of years since they ceased smoking.
In the present study, the long-term quitters had the highest prevalence of diabetes mellitus, hypertension, and cardiovascular disease. This was similar to the findings of the Singapore Chinese Health Study.18 No significantly increased risk was observed for long-term quitters and the excess risk decreased to the level of never smokers. The result was the same as found in other studies, namely that a greater number of years since quitting were associated with a reduction in all-cause mortality.18,20 The risk of new smokers did not reach the highest risk of continuing smokers, which could be due to the shorter smoking time. In the Nurses’ Health Study, the earlier the current smokers started smoking, the more hazards they encountered.20 In addition, new quitters had a significant 19% risk reduction in all-cause mortality. In the Singapore Chinese Health study, new quitters had the same 16% reduction in all-cause mortality.18 In the study of 1494 Chinese participants described earlier, new quitters who had stopped smoking for 2–7 years had an 11% reduction in all-cause mortality.24 Therefore, our findings were consistent with those of these two studies.18,24 Smoking cessation is associated with significant reduction of mortality risk within approximately 6 years.
The strengths of this study were the repeated assessments of smoking status and the long-term follow-up in Asian populations, which were genuinely rare in the literature. There were, however, certain limitations. First, the limited number of participants with repeated assessments formed a relatively small sample size that did not allow for additional cause-specific mortality and subgroup analysis. Second, the percentage of the participants who were successfully re-contacted in the follow-up interview was not high, which could lead to selection bias. However, it was not easy to re-contact participants in the real field study after a 6-year time interval. A third limitation was the lack of data on changes in smoking behavior from 1998 to 2008. Fourth, although analyses were adjusted for socio-demographic variables, lifestyles such as dietary patterns and alcohol drinking, and comorbidities such as diabetes mellitus, hypertension, cardiovascular disease, and chronic liver disease, it was the limitation to fully control of physical activity and other smoking-related diseases.
In conclusion, smoking cessation was associated with a significant reduction in mortality risk within approximately 6 years, while no significantly increased risk was observed for long-term quitters. The results show that the smoking-related mortality risk can diminish upon cessation. Hence, smokers should be encouraged to quit smoking as soon as possible.
1. Kenfield SA, Wei EK, Rosner BA, Glynn RJ, Stampfer MJ, Colditz GA. Burden of smoking on cause-specific mortality: application to the Nurses’ Health Study. Tob Control
2. Dans A, Ng N, Varghese C, Tai ES, Firestone R, Bonita R. The rise of chronic non-communicable diseases in southeast Asia: time for action. Lancet
3. Barzi F, Huxley R, Jamrozik K, Lam TH, Ueshima H, Gu D, et al. Association of smoking and smoking cessation with major causes of mortality in the Asia Pacific Region: the Asia Pacific Cohort Studies Collaboration. Tob Control
4. Lam TH, He Y, Shi QL, Huang JY, Zhang F, Wan ZH, et al. Smoking, quitting, and mortality in a Chinese cohort of retired men. Ann Epidemiol
5. Lam TH, Li ZB, Ho SY, Chan WM, Ho KS, Tham MK, et al. Smoking, quitting and mortality in an elderly cohort of 56,000 Hong Kong Chinese. Tob Control
6. Kondo T, Osugi S, Shimokata K, Honjo H, Morita Y, Maeda K, et al. Smoking and smoking cessation in relation to all-cause mortality and cardiovascular events in 25,464 healthy male Japanese workers. Circ J
7. Chen CH, Chuang JH, Kuo HS, Chang MS, Wang SP, Chou P. A population-based epidemiological study on cardiovascular risk factors in Kin-Chen, Kinmen. Int J Cardiol
8. Chen CH, Lin HC, Kuo HS, Chang MS, Chou P. Epidemiology of hypertension in Kin-Hu, Kinmen. Am J Hypertens
9. Chen CH, Lin KC, Tsai ST, Chou P. Different association of hypertension and insulin-related metabolic syndrome between men and women in 8437 nondiabetic Chinese. Am J Hypertens
10. Chou P, Liao MJ, Kuo HS, Hsiao KJ, Tsai ST. A population survey on the prevalence of diabetes in Kin-Hu, Kinmen. Diabetes Care
11. Chou P, Lin KC, Lin HY, Tsai ST. Gender differences in the relationships of serum uric acid with fasting serum insulin and plasma glucose in patients without diabetes. J Rheumatol
12. Lin KC, Lin HY, Chou P. Community based epidemiological study on hyperuricemia and gout in Kin-Hu, Kinmen. J Rheumatol
13. Lin KC, Lin HY, Chou P. The interaction between uric acid level and other risk factors on the development of gout among asymptomatic hyperuricemic men in a prospective study. J Rheumatol
14. Lin KC, Tsai ST, Lin HY, Chou P. Different progressions of hyperglycemia and diabetes among hyperuricemic men and women in the Kinmen study. J Rheumatol
15. Lin KC, Tsao HM, Chen CH, Chou P. Hypertension was the major risk factor leading to development of cardiovascular diseases among men with hyperuricemia. J Rheumatol
16. Chou P, Kuo HS, Chen CH, Lin HC. Characteristics of non-participants and reasons for non-participation in a population survey in Kin-Hu, Kinmen. Eur J Epidemiol
17. Chou P, Liao MJ, Kuo HS, Wu GS, Hsiao KJ, Jap TS. Program description and preliminary health survey data in Kin-Hu, Kinmen. J Chin Med Assoc
18. Lim SH, Tai BC, Yuan JM, Yu MC, Koh WP. Smoking cessation and mortality among middle-aged and elderly Chinese in Singapore: the Singapore Chinese Health Study. Tob Control
19. Vollset SE, Tverdal A, Gjessing HK. Smoking and deaths between 40 and 70 years of age in women and men. Ann Intern Med
20. Kenfield SA, Stampfer MJ, Rosner BA, Colditz GA. Smoking and smoking cessation in relation to mortality in women. JAMA
21. Gerber Y, Myers V, Goldbourt U. Smoking reduction at midlife and lifetime mortality risk in men: a prospective cohort study. Am J Epidemiol
22. Huxley RR, Yatsuya H, Lutsey PL, Woodward M, Alonso A, Folsom AR. Impact of age at smoking initiation, dosage, and time since quitting on cardiovascular disease in African Americans and whites: the Atherosclerosis Risk in Communities study. Am J Epidemiol
23. Hart C, Gruer L, Bauld L. Does smoking reduction in midlife reduce mortality risk? Results of 2 long-term prospective cohort studies of men and women in Scotland. Am J Epidemiol
24. He Y, Jiang B, Li LS, Li LS, Sun DL, Wu L, et al. Changes in smoking behavior
and subsequent mortality risk during a 35-year follow-up of a cohort in Xi’an, China. Am J Epidemiol