Wednesday May 1 2013
The more a woman smokes, the higher her cancer risk
BBC News has revealed that, "Women who smoke have a higher risk of cancer than men," reporting the results of a new study examining the relationship between gender and bowel cancer caused by smoking.
The large-scale study found that smoking increased the risk of bowel cancer in women by 19% compared with women who had never smoked. This was much larger than the (non-significant) 8% risk increase seen in male smokers.
Smoking is a recognised risk factor for bowel (colon) cancer and several other life-threatening diseases in both men and women. It is important to bear in mind that this research only looked at colon cancer. Whether there are gender differences in other smoking-related cancers, such as lung cancer, is uncertain based on the findings of this study alone.
The authors point out that their study did not take into account important risk factors known to be linked to bowel cancer, such as family history, diet, and alcohol consumption. If these had been accounted for the results may well have been different.
The study also didn't produce any firm evidence to explain why there may be a difference in risk between women and men. Future research will need to address these limitations to see if the gender differences in risk still apply and, if so, why.
Where did the story come from?
The study was carried out by researchers from the University of Tromsø, Norway in collaboration with researchers from institutions in Hawaii and Finland, and was funded by the Norwegian Cancer Society.
It was published in the peer-reviewed journal Cancer Epidemiology, Biomarkers and Prevention.
The BBC's coverage was generally accurate, although it wasn't initially obvious that the study only related to bowel cancer rather than all cancers, which readers may have assumed from the headline.
The BBC also discussed a second recent study (also covered by the Mail Online website) that reportedly showed how teenage girls exposed to passive smoking had lower levels of the "good" form of cholesterol that reduces heart disease risk. This, the BBC reported, gave a possible explanation as to why women who start smoking increase their risk of a heart attack.
What kind of research was this?
The researchers reported how smoking is a recently established risk factor for what medical professionals refer to as colon cancer, or cancer of the large bowel. They explained that the levels of colon cancer in Norwegian women are unusually high when compared with similar countries.
In men, smoking levels peaked during the late 1950s, while in women levels did not peak until the 1970s. The fact that historically women smoked less but still had high levels of colon cancer could mean they were more vulnerable to the harmful effects of smoking in terms of colon cancer risk.
To test this, the researchers carried out a cohort study to see if women may be more susceptible to smoking-related colon cancer than men.
What did the research involve?
The researchers recruited 602,242 Norwegians who were aged 19 to 67 at enrolment between 1972 and 2003.
They combined the information gathered from four separate cohort studies into one larger study. The researchers linked unique IDs assigned to each of the study participants to National Cancer Registry databases so they could establish whether any of the study group developed cancer.
At enrolment, and at various other points throughout the study period, participants filled out multiple questionnaires about a wide range of health and lifestyle behaviours, such as smoking, diet and physical activity levels, as well as demographic information.
Smoking levels were categorised into two main groups for analysis:
- those who had never smoked (never-smokers)
- a pooled group of current smokers and ex-smokers (ever-smokers)
The main analysis looked at how the two levels of smoking influenced the risk of developing colon cancer overall, as well as specific subgroups of colon cancer. That is, whether the cancer was located in the first part of the colon (proximal colon cancer) or lower parts of the colon (distal colon cancer).
The main analysis took account of age at enrolment, level of physical activity, body mass index (BMI) and years of education. These represented factors known to influence the risk of developing colon cancer (confounders).
What were the basic results?
The study followed people for an average of 14 years, during which time 3,998 people (46% women) developed colon cancer.
Female ever-smokers had a 19% increased risk of colon cancer compared with female never-smokers (hazard ratio [HR] 1.19, 95% confidence interval [CI] 1.09 to 1.32). This was much larger than the non-significant 8% increased risk found between male ever-smokers compared with male never-smokers (HR 1.08, CI 0.97 to 1.19).
Women categorised in the groups who started smoking the earliest, smoked for longest, or smoked the most cigarettes per day were at more than 20% higher risk of colon cancer (range 28-48%) than women never-smokers.
The increase in risk was much larger for proximal colon cancer, with female ever-smokers more than 40% more at risk of developing the disease compared with female never-smokers.
The researchers also tested for differences in the findings between men and women. They found this was only the case for the association between female ever-smokers and the risk of proximal colon cancer.
How did the researchers interpret the results?
The researchers concluded that their findings meant that, "Female smokers may be more susceptible to colon cancer, and especially to proximal colon cancer, than male smokers."
In this study, the researchers suggest that smoking played a role in increasing the risk of colon cancer in both sexes, but it seemed to play more of a role in women smokers. This particularly increased the risk of cancer of the first part of the large bowel (proximal colon cancer).
The study had many strengths, including its large size and long follow-up time. However, the research suffers from some limitations, meaning that we can't be sure that women smokers really do have a higher risk of colon cancer based on this study alone.
These limitations include:
- The study did not take account of many factors known to increase the risk of colon cancer, such as higher alcohol and red meat consumption. Had it done so, the results may have been different. The researchers point out that, generally, alcohol and red meat consumption is higher in men, putting them at an increased risk of colon cancer. Not taking these factors into account means it was less likely to find the results they did.
- The study only looked at colon cancer. This tells us nothing about whether women smokers are more susceptible than men to other types of cancers. This would need direct investigation.
- Smoking was categorised into only two groups rather than a more detailed breakdown, and did not account for passive smoking levels. There will therefore have been some error in using this simple categorisation method.
Overall, the study suggests the effect of smoking on the risk of developing colon cancer may differ by gender, but it cannot confirm this is definitely the case, or explain why this may be. Further research is required to confirm both of these questions.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.