Smoking just one cigarette a day carries a much higher risk of developing coronary heart disease and stroke than expected – about half the risk of smoking 20 per day – concludes a University College London (UCL)-led review of the evidence. The researchers say their findings have important consequences for many smokers and health professionals who believe that smoking only a few cigarettes per day bears little or no harm.
“What this tells us is that people who smoke shouldn’t just cut down – they should aim to stop smoking altogether. There is no safe level of smoking,” said the study’s lead author, Professor Allan Hackshaw (UCL Cancer Institute).
Individual studies have reported that smoking only one to five cigarettes per day is associated with a higher than expected risk of heart disease. To investigate this further, the researchers analysed the results of 141 studies and estimated the relative risks for smoking one, five, or 20 cigarettes per day.
They found that men who smoked one cigarette per day had 46% of the excess risk of heart disease and 41% of the excess risk of stroke associated with smoking 20 cigarettes per day (much higher than the expected one 20th, or 5%).
For women, those who smoked one cigarette per day had 31% of the excess risk of heart disease and 34% of the excess risk of stroke associated with smoking 20 cigarettes per day. Their heart disease risk was more than doubled with one cigarette per day, when only studies that controlled for several factors were included in the analysis.
Cardiovascular disease, not cancer, is the greatest mortality risk for smoking, causing about 48% of smoking-related premature deaths.
According to a recent Health Survey for England, one in four smokers are trying to cut down but not trying to stop altogether. “People who smoke less can benefit from big risk reductions for disorders like lung cancer. But we would encourage those who are cutting back to go one step further and stop smoking entirely to reduce their risk of heart disease and stroke – two common diseases,” Hackshaw said.
In other terms, when compared with people who don’t smoke, women who smoke about one cigarette per day have a 57% higher risk of heart disease and a 31% higher risk of for stroke (or 119% and 46% when allowing for multiple confounders). For men, the increased risk of heart disease posed by smoking once a day is 48%, and 25% for stroke compared to non-smokers (or 74% and 30%, respectively, when allowing for confounding factors).
“As we have found that a large proportion of the cardiovascular risk caused by smoking comes from just one cigarette per day, we hope that our findings could be used to strengthen public health campaigns and provide increased incentive for people to stop smoking. Smokers could make use of various smoking cessation aids, with positive support from friends and their family doctor. The great news is that much of the risk of heart disease and stroke goes away only a few years after stopping,” Hackshaw said.
The study involved researchers from UCL, Queen Mary University of London, King's College London, Chinese University of Hong Kong, and was funded by Cancer Research UK.
Objective: To use the relation between cigarette consumption and cardiovascular disease to quantify the risk of coronary heart disease and stroke for light smoking (one to five cigarettes/day).
Design: Systematic review and meta-analysis.
Data sources: Medline 1946 to May 2015, with manual searches of references.
Eligibility criteria for selecting studies: Prospective cohort studies with at least 50 events, reporting hazard ratios or relative risks (both hereafter referred to as relative risk) compared with never smokers or age specific incidence in relation to risk of coronary heart disease or stroke.
Data extraction/synthesis: MOOSE guidelines were followed. For each study, the relative risk was estimated for smoking one, five, or 20 cigarettes per day by using regression modelling between risk and cigarette consumption. Relative risks were adjusted for at least age and often additional confounders. The main measure was the excess relative risk for smoking one cigarette per day (RR1_per_day−1) expressed as a proportion of that for smoking 20 cigarettes per day (RR20_per_day−1), expected to be about 5% assuming a linear relation between risk and consumption (as seen with lung cancer). The relative risks for one, five, and 20 cigarettes per day were also pooled across all studies in a random effects meta-analysis. Separate analyses were done for each combination of sex and disorder.
Results: The meta-analysis included 55 publications containing 141 cohort studies. Among men, the pooled relative risk for coronary heart disease was 1.48 for smoking one cigarette per day and 2.04 for 20 cigarettes per day, using all studies, but 1.74 and 2.27 among studies in which the relative risk had been adjusted for multiple confounders. Among women, the pooled relative risks were 1.57 and 2.84 for one and 20 cigarettes per day (or 2.19 and 3.95 using relative risks adjusted for multiple factors). Men who smoked one cigarette per day had 46% of the excess relative risk for smoking 20 cigarettes per day (53% using relative risks adjusted for multiple factors), and women had 31% of the excess risk (38% using relative risks adjusted for multiple factors). For stroke, the pooled relative risks for men were 1.25 and 1.64 for smoking one or 20 cigarettes per day (1.30 and 1.56 using relative risks adjusted for multiple factors). In women, the pooled relative risks were 1.31 and 2.16 for smoking one or 20 cigarettes per day (1.46 and 2.42 using relative risks adjusted for multiple factors). The excess risk for stroke associated with one cigarette per day (in relation to 20 cigarettes per day) was 41% for men and 34% for women (or 64% and 36% using relative risks adjusted for multiple factors). Relative risks were generally higher among women than men.
Conclusions” Smoking only about one cigarette per day carries a risk of developing coronary heart disease and stroke much greater than expected: around half that for people who smoke 20 per day. No safe level of smoking exists for cardiovascular disease. Smokers should aim to quit instead of cutting down to significantly reduce their risk of these two common major disorders.
Allan Hackshaw, Joan K Morris, Sadie Boniface, Jin-Ling Tang, Dušan Milenković