Despite evidence showing the contrary, many smokers and some health professionals commonly assume that light smoking may be harmless or is associated with little risk for heart disease, according to survey results. British and American national studies show a trend for heavy smokers to cut down their use rather than quit, making low cigarette consumption an important topic to research.
Building an Evidence Base
For a study published in BMJ, Allan Hackshaw and colleagues sought to combine the results of previous studies in order to produce a single reliable evidence base that quantified the risk of coronary heart disease or stroke among light smokers (one to five cigarettes per day).
The team performed a systematic review and meta-analysis using 55 publications containing 141 cohort studies. Hackshaw and colleagues estimated the relative risks for these disorders based on smoking one, five, or 20 cigarettes per day by using regression modelling between relative risk and cigarette consumption.
Assumptions Versus Reality
The most important finding of the study refutes the assumption that smoking only a couple cigarettes per day, as opposed to 20, has the same proportionate reduction in the risk of cardiovascular disease, according to Hackshaw. “One would intuitively think that smoking one cigarette per day would have one-twentieth (or 5%) of the excess risk associated with smoking 20 cigarettes per day (ie, 5% of the extra risk due to smoking, above the background risk), but the studies we examined show that it is more like 40% to 50%,” he explains. Among men who smoked one or 20 cigarettes per day, the relative risks for coronary heart disease were 1.74 and 2.27, respectively, in studies that adjusted relative risk for multiple confounders. Among women, the respective relative risks were 2.19 and 3.95. Using relative risks adjusted for multiple factors, men and women who smoked one cigarette per day had 53% and 38% of the excess relative risk, respectively. For stroke, the relative risks adjusted for multiple factors were 1.30 and 1.56 for men who smoked one or 20 cigarettes per day, respectively (so that one cigarette per day represents 64% of the excess risk of 20 per day). In women, the respective estimates were 1.46 and 2.42 (with one cigarette per day representing 36% of the excess risk).
“The body’s cardiovascular system is very sensitive to even small levels of toxins in cigarette smoke,” explains Hackshaw. “This also explains why never smokers have a 30% increased risk of heart disease—compared with unexposed non-smokers—if they are regularly exposed to secondhand smoke. These and our own findings confirm that much damage is done at low levels of smoking, rather than building gradually when smoking up to 20 cigarettes per day.”
While estimates of risk varied slightly by age and gender, there was consistency in the findings (Figure). Relative risks were generally higher among women than men. “This gender disparity has been reported in previous research,” says Hackshaw, “but the reasons for it are unclear. It could be biological differences, differences in characteristics or lifestyle habits, or a mixture of them.”
Taking the Next Step
Hackshaw notes that many physicians do an excellent job in positively encouraging smokers to quit. “That said, smoking is highly addictive, and thus many smokers (especially heavy smokers) understandably find it hard to stop completely,” he says. “Cutting down on one’s smoking levels is certainly better than continuing at the same amounts, which should be a goal for smokers who can’t quit at that moment. Our findings could help encourage these people to take the next step and quit smoking. For other smokers, knowing that even light smoking is very harmful might be enough for them to attempt to quit.”
Hackshaw A, Morris J, Boniface S, Tang J, Milenkovic D, Low cigarette consumption and risk of coronary heart disease and stroke: meta-analysis of 141 cohort studies in 55 study reports. BMJ. 2018, Jan 24 (Epub ahead of print). Available at www.bmj.com/content/360/bmj.j5855.