Importance of smoking cessation for cardiovascular risk reduction

This editorial refers to ‘Smoking cessation, but not reduction, reduces cardiovascular disease incidence’, by S.-M. Jeong et al., https://doi.org/10.1093/eurheartj/ehab578.

Graphical Abstract

The mapped data are from the Global Burden of Disease Study 2019, accessed July 19, 2021 at http://ghdx.healthdata.org/gbd-2019.

Nearly 60 years ago, the Framingham Heart Study first reported on the association between smoking and coronary heart disease on a population-based level.^{1  ,  2} Since then, innumerable data have confirmed a strong, graded relationship of smoking, smoking duration, and number of cigarettes smoked per day with cardiovascular and non-cardiovascular morbidity and mortality risks. Although smoking prevalence has globally decreased by ∼25% from 1980 to 2012,³ at least 25% of the world’s male and 5% of the world’s female population are still actively smoking.⁴ Moreover, the smoking prevalence trends have in many places been shown to differ among young and old and rich and poor, which raises concerns for an expansion of the already unfavourable social gradients that exist in cardiovascular health. In particular, it is notable that although tobacco use among young people appears to be declining in many places in the Western world,³ disturbingly increases have been observed in some countries with a lower human development index, secondary to cigarettes becoming more affordable.⁴

In 2015, smoking was estimated to cause >11% of the world’s deaths, and >27% and 12% of all ischaemic heart disease-related mortalities in 2017 among men and women, respectively.^{5  ,  6} Although the population-attributable fractions of ischaemic heart disease mortality associated with smoking on a global level declined by a modest 33% in men vs. 17% in women during the last century, substantial differences were noted between countries.⁶ For instance, in high-income North America and Western European regions, fractions have declined by ∼50% and 40% in both women and men, whereas in, for example, Eastern Europe and East Asia, declines have been almost absent.⁶ In the Western Pacific and South-East Asian countries, smoking still accounts for up to 30% of all ischaemic heart disease and stroke events, illustrating a true ongoing epidemic in this region.⁷ Smoking prevalence is also high and accounts for a considerable population-attributable fraction of cardiovascular disease risk in the Western world, South America, and parts of Africa (Graphical Abstract).⁵ Thus, more than half a century after the negative effects of tobacco use on cardiovascular disease risk and mortality were established, we are still facing tobacco use as one of the most important preventable factors of premature mortality and cardiovascular morbidity worldwide.

Although still disturbingly high, South Korea has experienced substantial declines in the prevalence of smoking over the past few decades following several directed smoking cessation initiatives (the Health Promotion Act), including advertising bans, media campaigns, the implementation of smoke-free policies in public places (such as restaurants), visual warning labels on tobacco products, and a more than doubling of tobacco prices since 1995.⁸ Between 1995 and 2016, the rates of smoking were reported to have declined from 71.5% to 39.7% (i.e. by 44%) in men and from 6.5% to 3.3% (i.e. by 49%) in women.⁹ Data from South Korea may therefore inform us about several important public health implications of smoking cessation initiatives and guide future smoking cessation efforts. In the present issue of European Heart Journal, Dr Shin and colleagues utilized a large South Korean population-based database (KNHANES)¹⁰ collected during this dynamic period and performed a set of excellent analyses to address the question of whether smoking reduction and smoking cessation were associated with lowered prospective risks of cardiovascular disease.¹¹ Using a population sample of nearly 900 000 individuals aged ≥40 years in 2009 (95% men), all active smokers, the association of reducing daily cigarette consumption by 20% to <50% vs. reducing >50% vs. quitting completely with incident stroke and myocardial infarction was evaluated over a mean follow-up of 6 years. Between 2009 and 2011, 21% stopped smoking, while another 20% reduced their daily smoking intake. In contrast, 15% increased their smoking by at least 50% during this period. At first glance, a 21% relative reduction of smokers over a few years seems a lot but, given the country’s many initiatives to reduce smoking rates, it may not be very disconcordant with the overall reported trends in South Korea. The authors observed that smoking cessation was associated with an adjusted hazard ratio of 0.77 and 0.74 for stroke and myocardial infarction, respectively, compared with sustained smoking, whereas reducing cigarette intake was not associated with a differential risk compared with sustained smoking.

While smoking cessation has been shown to lower cardiovascular risk before (with longer cessation duration being associated with greater cardiovascular disease risk reduction), it has remained unclear what impact smoking reduction has on cardiovascular health.^12–14 The study by Shin et al. sheds important light on this topic. Their data convincingly suggest that efforts towards complete smoking cessation and not just reduction must be achieved to show cardiovascular benefits.¹¹ Similar conclusions were drawn from the Copenhagen Centre for Prospective Population Studies over a 16-year follow-up, where reductions in all-cause and cardiovascular-specific mortality were only observed in quitters, in contrast to reducers (hazard ratio 1.02 and 1.01 for all-cause and cardiovascular mortality compared with continuing smokers, respectively).¹⁵

It is not very intuitive why smoking reduction should not lead to lowered cardiovascular risks, when greater intensity of smoking so convincingly increases the cardiovascular disease risk in a dose-dependent manner (the latter also being confirmed in the present study, where those who increased the daily number of cigarettes smoked had greater risks than those who sustained their usual number of cigarettes, and where those who relapsed had greater risks than sustained quitters). One potential explanation is of course residual confounding and socioeconomic differences between quitters, sustainers, and reducers. It is noteworthy that, at baseline, people who quit smoking were on average smoking fewer cigarettes daily than people who continued to smoke, whereas individuals who decreased their daily amount of cigarettes were smoking more cigarettes than the sustainers to start with. Moreover, people who quit smoking have previously been reported to often be of higher socioeconomic class than individuals who continue to smoke.⁹ However, the associations remained robust in multiple sensitivity analyses, including inverse probability weighted models, and multiple important variables were available for adjustment in their models, including total pack-years, duration of smoking, income level, blood pressure, anthropometric measures, usual exercise levels, and blood biochemistry. Further, follow-up was started in 2012, 1 year after defining the smoking trajectory, to avoid reverse causality. This is appropriate, since people who are diagnosed with ischaemic heart disease may be more likely to stop smoking, probably already before diagnosis (due to concern for health, shortness of breath, etc.).¹⁶ The risk of indication bias (i.e. what caused a person to stop or decrease smoking) should also be somewhat diminished in this South Korean sample, given the country’s many governmental smoking cessation interventions that occurred during the study period. Another plausible explanation is that smokers who reduced their consumption have already accrued harmful effects of high-intensity tobacco exposure and that reducing smoking intensity does not allow, to the same extent, the reversibility of damage that is seen for smoking cessation. Possibly, low-grade inflammation and alterations in DNA methylation that are caused by heavier smoking may be sustained even by just continuing to smoke a few cigarettes daily.¹⁷

Another interesting observation in the current study was that young individuals had greater relative risk reductions associated with smoking cessation, compared with older individuals, which may relate to their relatively fewer accrued pack-years prior to stopping and the known greater relative importance of smoking for development of cardiovascular disease in young individuals.¹⁸ However, in terms of absolute risk reduction, given the much more common occurrence of cardiovascular disease in older adults, substantial benefits can still be gained from smoking cessation in older adulthood, as also elegantly illustrated in the present study. Since almost all current smokers are introduced to smoking during their youth, efforts to target smoking cessation already in younger individuals are, however, warranted from so many different perspectives.¹⁹

The low prevalence of women (5%) must be acknowledged as a limitation of the present study. Although this is likely to reflect the epidemiology of smoking in East Asia, under-reporting of smoking in women may be a concern.⁸ In addition, as tobacco use is becoming more equally distributed among men and women in many places, more data on the sex-specific associations of smoking cessation vs. smoking reduction are needed.⁵ Finally, non-cigarette tobacco products have become very common in some populations.²⁰ In South East Asia, smokeless tobacco has been reported to be one of the most common types of tobacco among the youth, and e-cigarettes have also gained a lot of popularity among the youth especially in high-income countries.¹⁹ Whether switching to such products alters cardiovascular disease risks is not well understood yet and remains another research priority.²¹

Conflict of interest: none declared.

The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.