For people who think that smoking only one or two cigarettes a day carries little cardiovascular risk, a powerful new study maintains the only way to reduce risk is to quit, full stop.
The investigators anticipated that smoking one cigarette a day would be associated with about 5% of the excess relative risk of smoking 20 cigarettes a day, but they found it actually accounts for 46% of excess CHD risk in men and 31% of the risk in women.
For the less commonly reported smoking-related outcome of stroke, the excess risk associated with just one cigarette per day was 41% for men and 34% for women.
The meta-analysis of data from 141 prospective cohort studies was published online today in the British Medical Journal.
"There's been a big shift from people smoking 20 to 25 cigarettes a day to only smoking a few cigarettes a day with the assumption that's good enough for them. Their view is that smoking only a couple a day can't be harmful and that's probably not far off the truth for risk for cancer. For many smokers that's probably the first thing that comes to mind, but cardiovascular risk is the big one," lead author, Dr Allan Hackshaw (University College London, UK), told theheart.org | Medscape Cardiology.
He continued, "So the main public health impact of this is that smokers have done well in reducing and there are various methods to help them quit and cut down, but the aim is to keep on searching for those methods, find one that suits them, and to cut down and then quit completely."
Dr Vincent Bufalino (Advocate Health Care, Chicago, IL), who was not involved in the study, said the findings are not what many physicians would have predicted but that the investigators have built a "very convincing case" in a pretty impressive set of data.
"I think this is actually stunning in that it's an all-or-nothing phenomenon now," he said. "It's going to change our thinking. Of course we wanted people to quit. It's not like we were trying to encourage people to reduce; we would take that only as the third option if everything else failed. But now we have some hard data that says even a cigarette a day is harmful—and not just a little harmful, but increases your risk 50% to 75% of the time. Wow! And it's even more impressive in women."
The excess CVD risk associated with low smoking has been reported since the 1990s and is likely known by expert cardiologists, especially those familiar with tobacco research, but "in terms of a lot of the everyday practicing cardiologists, I have the feeling that they don't," Hackshaw said.
"It could be possibly that when those occasional reports have come out in journals and in the media, they're only based on one study—people say it's only one study, it might be a fluke and that's why it hasn't sunk in properly," he added.
Women, Be Wary
The 141 cohort studies in the meta-analysis were published between 1946 and May 2015 and followed 5.6 million patients for CHD and 7.3 million for stroke. Each study had at least 50 hard events, with 110,000 new cases of CHD and 135,000 cases of stroke reported.
Compared with never smoking, smoking one cigarette a day was associated with a 48% increase in CHD risk for men across all studies (relative risk [RR], 1.48; 95% CI, 1.30–1.69) and a 74% increase in studies controlling for multiple confounders in addition to age and sex (RR, 1.74; 95% CI, 1.50–2.03). For women, smoking one cigarette a day was associated with a 57% (RR, 1.57; 95% CI, 1.29–1.91) to 119% (RR, 2.19; 95% CI, 1.84–2.61) increase in CHD risk.
Further, smoking 20 cigarettes per day was estimated to increase CHD risk in men by 104% (all studies: RR, 2.04; 95% CI, 1.86–2.24) to 127% (adjusted studies: RR, 2.27; 95% CI, 1.90–2.72) and in women by a staggering 184% (all studies: RR, 2.84; 95% CI, 2.21–3.64) to 295% (adjusted studies: RR, 3.95; 95% CI, 3.34–4.67).
"There's a bit of uncertainty why the effect is higher in women," said Hackshaw. There may be biological causes, differences in inhalation, or that women have lagged behind in terms of quit rates. "But we are seeing the effects on a population of what the risks of heart disease and stroke are."
"The unfortunate thing for cardiovascular disease is that the adverse effects seem to come through quite quickly after only 2 or 3 years of smoking and the impact is quite big," he added. "The good thing is that if you quit smoking, a lot of your risk goes away quite quickly as opposed to cancer, where it takes several years to mainly go away."
The risk for stroke was slightly lower than that for CHD, but again the data suggest a slightly larger effect among women than men, particularly among heavy smokers. Across all studies, the relative risks for stroke were 1.25 (95% CI, 1.13–1.38) in men who smoked one cigarette per day and 1.31 (95% CI, 1.13–1.52) in women; the corresponding risk estimates for men and women smoking a pack a day were 1.64 (95% CI, 1.48–1.82) and 2.16 (95% CI, 1.69–2.75), respectively.
While the stroke risk may be lower, Dr Nancy Rigotti (director of Massachusetts General Hospital's Tobacco Research and Treatment Center, Boston) said in an interview, "It's still quite high. A 30% increased risk is huge. So I think it just reinforces the message that even one cigarette puts you at risk of heart attack and strokes."
e-Cigarettes for Harm Reduction?
In an accompanying editorial, Dr Kenneth C Johnson (University of Ottawa, Ontario) writes, "Any assumption that smoking less protects against heart disease or stroke has been dispelled this week in the BMJ."
He notes that light smoking, occasional smoking, and smoking fewer cigarettes all carry substantial risk for CVD and that "only complete cessation is protective and should be emphasised by all prevention measures and policies."
Johnson argues that although e-cigarettes deliver reduced levels of carcinogens, regulatory approval should be withheld for the devices and that "e-cigarettes and heat-not-burn products should not be promoted for 'harm reduction' on the grounds that they lead people to smoke fewer cigarettes, because modest reductions in cigarette consumption are unlikely to have meaningful health benefits and dual use of cigarettes and e-cigarettes may expose smokers to increased total risk."
"I actually think I would disagree with that," Rigotti countered. "e-Cigarettes are nicotine-delivery devices. They don't burn tobacco and so they don't expose users to the products of combustion, and most of what we understand about what causes cardiovascular disease insmokers is really the products of combustion, not the nicotine."
She noted that a National Academy of Science (NAS) e-cigarette reportreleased earlier this week that she coauthored is consistent with that view. "We would expect that someone would have a very different risk if they were vaping the equivalent of a cigarette a day compared to smoking a cigarette per day, so that switching from one cigarette per day to an equivalent amount of vaping would almost certainly reduce risk."
Hackshaw agreed: "It's highly, highly unlikely or unfeasible that e-cigarettes are going to have the same impact on cardiovascular risk factors or the cardiovascular system as smoking one or two cigarettes a day, but there are some studies in place that are going to be looking at the long-term effects of that."
He continued, "There are some people who have been concerned about what the effects of e-cigarettes are on the cardiovascular system. And that's all good and well, but in the current state of play if you've got some current smokers who really cannot quit, do they carry on smoking 25 or 30 a day or do you get them to cut down? And one of the best ways to help them cut down is e-cigarettes."
"Perhaps the most important point to be made there is that if a smoker just substitutes a few cigarettes with e-cigarettes but continues to smoke regular cigarettes as well, they're probably not reducing their risk and that is what this article says," Rigotti said. "Because if they're smoking even one cigarette a day, it doesn't matter how much they're vaping because they're still giving themselves a risk."
However, Johnson also points out that e-cigarettes may attract a whole new generation of young smokers. There is evidence of this in the NAS report, but modeling of overall population risks suggests the number of lives saved by having e-cigarettes available to help people quit is greater than the small number of teens who may be induced to smoke who otherwise would not have, Rigotti said.¨
"Almost certainly there is harm reduction, and furthermore, someone who starts vaping does not necessarily stay vaping forever. Many of them could and do eventually wean off the e-cigarettes," she said.
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