Just one cigarette a day increases heart disease and...

"Just one cigarette a day is 'almost as dangerous as 20 – hiking your heart attack and stroke risk by 40%'," The Sun reports. A new review of 141 studies covering more than 12 million people showed that people who think "light" smoking is relatively harmless are wrong.

The researchers looked at the risks of getting heart disease or stroke, comparing the risks of smoking:

no cigarettes a day
1 to 5 (so-called "light" smoking)
20 cigarettes a day

They found the risk of heart disease in people smoking 1 cigarette a day rose by 48% for men and 57% for women – around a third to a half of the extra risk of smoking 20 a day. There were similar trends for stroke.

In conclusion, this study shows that, if you really want to reduce your risk of common killers like heart disease and stroke, you need to stop smoking altogether. Find out more about ways to help you stop smoking.

Where did the story come from?

The study was carried out by researchers from University College London, the University of London, King's College London, and the Chinese University of Hong Kong. It was funded by Cancer Research UK. It was published in the peer-reviewed British Medical Journal (BMJ) and is free to read online.

The UK media carried mostly accurate and balanced reports of the study. Several reports included a warning from one doctor that heavy smokers should not be discouraged from cutting down, as they might then go on to quit once they had reduced their tobacco intake.

What kind of research was this?

The researchers carried out a systematic review and meta-analysis of prospective cohort studies. This type of research is useful for getting an overall picture of the state of research about a topic, pooling the evidence from many (in this case millions) of trial participants.

What did the research involve?

Researchers used a database of medical research to look for large-scale prospective cohort studies that investigated the risk of heart disease or stroke for people smoking different quantities of cigarettes each day, from 1946 up to 2015. They pooled the results to find relative risks (compared to non-smokers) of smoking 1, 5 or 20 cigarettes a day, for men and women. They then looked to see how much of the excess risk of smoking 20 a day also applied to smoking 1 to 5 cigarettes a day.

They used several statistical checks to ensure their results were reliable. All the studies had to include information about age and sex, as these are known to affect the risk of heart disease and stroke. The researchers looked separately at studies which also took account of other factors such as blood pressure and cholesterol level. They investigated whether a number of factors could have affected their results – for example, whether heavy smokers might have cut down over the years.

What were the basic results?

Looking at the risk of heart disease, the researchers found that, compared to non-smokers:

women who smoked 20 a day had almost 3 times the risk (relative risk (RR) 2.84, 95% confidence interval (CI) 2.21 to 3.64)
women who smoked 1 cigarette a day had 57% higher risk (RR 1.57, 95% CI 1.29 to 1.91)
men who smoked 20 a day had more than double the risk (RR 2.04, 95% CI 1.86 to 2.24)
men who smoked 1 cigarette a day had a 48% higher risk (RR 1.48, 96% CI 1.30 to 1.69)

The researchers calculated that women who smoked 1 cigarette a day had 31% of the excess risk of those smoking 20 a day and men who smoked 1 cigarette a day had 46% of the excess risk of those smoking 20 a day.

The risk of stroke, compared to non-smokers, was:

more than double for women who smoked 20 a day (RR 2.16, 95% CI 1.69 to 2.75)
31% higher for women who smoked 1 a day (RR 1.31, 95% CI 1.13 to 1.52)
64% higher for men who smoked 20 a day (RR 1.64, 95% CI 1.48 to 1.82)
25% higher for men who smoked 1 a day (RR 1.25, 95% CI 1.13 to 1.38)

Women who smoked 1 cigarette a day had 34% of the risk of those smoking 20 a day and men who smoked 1 a day had 41% of the risk of those smoking 20 a day.

Using only data from studies which took account of factors like cholesterol and blood pressure showed an even greater risk associated with smoking 1 cigarette a day – more than double the risk of heart disease for women smoking 1 cigarette a day (RR 2.19, 95% CI 1.84 to 2.61) and 74% increased risk for men smoking 1 a day (RR 1.74, 95% CI 1.5 to 2.03).

The researchers said statistical tests did not uncover any likely sources of bias to the findings.

How did the researchers interpret the results?

The researchers said their results show that smoking 1 to 5 cigarettes a day carries a risk of cardiovascular disease "substantially higher than many health professionals or smokers recognise."

They said the results "show clearly that no safe level of smoking exists for cardiovascular disease at which light smokers can assume that continuing to smoke does not lead to harm."

They went on to advise: "Smokers need to quit completely rather than cut down if they wish to avoid most of the risk associated with heart disease and stroke."

Conclusion

The study adds to the evidence that smoking increases the risk of serious conditions like heart disease and stroke, even if you smoke only a few cigarettes a day.

Around 7.6 million adults in the UK smoke, around 16% of the population. The average number of cigarettes smoked per day is 12 for men and 11 for women.

The study was generally very strong, and researchers made efforts to check whether potential problems – such as people wrongly reporting how much they smoke – could have affected the results. Limitations include that the researchers were unable to look at individual patient data and could not take account of how long people had smoked. In addition, there was wide variability between the study types which reduces the validity of the pooled results. However, the size of the study and the consistency of the results suggest the overall findings are likely to be reliable.

If you smoke, the best thing you can possibly do for your health is to stop smoking altogether. As well as reducing your risk of heart disease, stroke and cancer, stopping smoking has many other benefits to how you feel and how you look. Find out more about the benefits of stopping smoking.

That's not to say that there's no point in cutting down – the research did not look at lung cancer, but previous studies have shown that your chances of lung cancer do reduce when you cut down on cigarettes. However, heart disease and stroke are much more common than lung cancer, so it does make sense to aim to cut cigarettes out completely, even if you do so by cutting down first.