No safe level of smoking exists when it comes to CVD
Low cigarette consumption and risk of coronary heart disease and stroke: meta-analysis of 141 cohort studies in 55 study reportsHackshaw A, Morris JK, Boniface S, et al. - BMJ 2018;360:j5855
Data show that the percentage of smokers of 1-5 cigarettes per day is increasing, probably because many smokers believe that less smoking results in less health damage [1,2]. This may be true for lung cancer, but limited data suggest that there is a non-linear relation for smoking and cardiovascular (CV) risk [3,4]. In this systematic review and meta-analysis, the relation between smoking 1-5 cigarettes per day and the risk of coronary heart disease (CHD) and stroke was assessed and quantified.
Studies were eligible for the meta-analysis, if they included prospective cohorts of generally healthy populations, and reported at least 50 CV events with hazard ratios (HR) or relative risks (RR), in at least 3 smoking categories vs. never smokers. The main measure was the excess RR for smoking 1 cigarette/day, expressed as a proportion of that for smoking 20 cigarettes/day, expected to be approximately 5%, assuming a linear relation between risk and consumption.
- The meta-analysis included 55 publications containing 141 cohort studies.
- The pooled RRs for CHD in men were 1.48 (95%CI:1.30-1.69, adj 1.74) for smoking 1 cigarette/day and 2.04 (95%CI:1.86-2.24, adj 2.27) for smoking 20 cigarettes/day. Male smokers of 1 cigarette/day had 46% (IQR:24-56%, adj 53%) of the excess RR for smoking 20 cigarettes/day.
- The pooled RRs for CHD in women were 1.57 (95%CI:1.29-1.91, adj 2.19) for smoking 1 cigarette/day and 2.84 (95%CI:2.21-3.64, adj. 3.95) for smoking 20 cigarettes/ day. Female smokers of 1 cigarette/day had 31% (IQR:2-46%, adj 38%) of the excess RR for smoking 20 cigarettes/day.
- The excess risk for smoking 1 cigarette/ day expressed as a percentage of the RR for 20 cigarettes/day for CHD were 35%, 33%, and 20% for men aged 45, 55, and 65 years, respectively. The corresponding figures for women were 11%, 15%, and 36%.
- The pooled RRs for stroke in men were 1.25 (95%CI:1.13-1.38, adj 1.30) for smoking 1 cigarette/day and 1.64 (95%CI:1.48-1.82, adj. 1.56) for smoking 20 cigarettes/day. Male smokers of 1 cigarette/day had 41% (IQR:-7 to 62%, adj 64%) of the excess RR for smoking 20 cigarettes/day.
- The pooled RRs for stroke in women were 1.31 (95%CI:1.13-1.52, adj 1.46) for smoking 1 cigarette/day and 2.16 (95%CI:1.69-2.75, adj 2.42) for smoking 20 cigarettes/day. Female smokers of 1 cigarette/day had 34% (IQR:3-51%, adj 36%) of the excess RR for smoking 20 cigarettes/day.
- The excess risk for smoking 1 cigarette/ day expressed as a percentage of the RR for 20 cigarettes/day for stroke were 22%, 25%, and 15% for men aged 45, 55, and 65 years (based on 2 studies), respectively. The corresponding figures for women were 24%, 20%, and 14% (based on 1 study).
- RRs were generally higher for women than men. For example. RRs for smoking one cigarette per day were: 1.57 vs. 1.48 for CHD, respectively, and 1.31 vs. 1.25 for stroke, respectively. This was consistent with a higher risk of CHD in women reported by others.
Smoking 1 cigarette/day is associated with 30-50% of the excess risk of CHD and stroke when smoking 20 cigarettes/day. These data suggest that there is no safe level of smoking when it comes to CVD; smokers need to quit to avoid the risk of CHD and stroke.
In his editorial article, Johnson  emphasizes that ‘Any assumption that smoking less protects against heart disease or stroke has been dispelled this week in the BMJ.’ He discusses the biological background of the findings of Hackshaw et al., including systemic oxidative stress, inflammatory vascular dysfunction, increased platelet activation and blood viscosity, atherosclerosis, ischemic heart disease, and altered cardiac autonomic function. Moreover, he analyzes the major public health implications of the study, including the need for prevention measures and policies by all involved parties, not only for active smoking, but also for passive smoking, as well as for the use of marijuana and sheesha (hookah) smoke, and e-cigarettes. New products, such as e-cigarettes, still expose users to high levels of ultrafine particles and other toxins that may increase CVD risk. They also should not be promoted to reduce harm on the grounds that people using these products smoke less cigarettes. Finally, e-cigarettes reduce smoking cessation rates and seem to recruit and addict new generation of smokers. He concludes: ‘The take home message for smokers is that any exposure to cigarette smoke is too much. The message for regulators dealing with newly marketed “reduced risk” products is that any suggestion of seriously reduced CHD and stroke from using these products is premature.’