Even 'light' smoking may raise women's arthritis risk

"Smoking just a few cigarettes a day more than doubles a woman's risk of developing rheumatoid arthritis," the Mail Online website reveals. The website reports on a Swedish study that found women who only smoke a small number of cigarettes a day significantly increase their risk of developing the condition.

This large study followed more than 30,000 women over a seven-year period to look at whether smoking increased their risk of developing rheumatoid arthritis. Smoking is already recognised as a possible risk factor for developing the condition. 

But this study showed that the risk increased even at relatively low levels of smoking. It found that even smoking as many as between one and seven cigarettes per day more than doubled a woman's chance (2.31 times) of developing rheumatoid arthritis compared with a woman who had never smoked.

Although this study provides further evidence about the dangers of smoking, it does have a number of limitations. For example, it is not clear how many women dropped out of the study, which could have biased the results. It is also unclear whether similar risk patterns would be seen in men (the condition is more common in women) or a more ethnically diverse group.

Nonetheless, this study provides evidence of yet another disease that smokers may be at significantly increased risk of developing, even if they are considered to be 'light' smokers.

Where did the story come from?

The study was carried out by researchers from the Karolinska Institutet in Sweden and was funded by research grants from the Swedish Research Council's Committee for Medicine, Committee for Research Infrastructure for maintenance of the Swedish Mammography Cohort, and the Swedish COMBINE inflammation research consortium.

It was published in the peer-reviewed science journal Arthritis Research and Therapy.

The Mail Online's coverage of the study was generally accurate, and included information about the study sample size and an idea of the prevalence of the condition.

What kind of research was this?

This was a cohort study that aimed to discover how much a woman needs to smoke in order to increase her risk of developing rheumatoid arthritis.

The authors pointed out that previous studies have shown that cigarette smoking was directly associated with a higher risk of developing rheumatoid arthritis. What was unclear was whether this risk was associated with so-called 'light' smoking and whether quitting smoking reduced the risk.

The focus of this study was therefore on examining how much the risk of developing rheumatoid arthritis increased depending on how much a woman smoked and how long they had smoked for, and whether it was possible to reduce this risk if a woman quit smoking.

Rheumatoid arthritis is what is known as an autoimmune condition, where the body's own immune system starts to attack the cells that line the joints, causing pain and swelling. Hands, feet and wrists are commonly affected, but it can also damage other parts of the body.

The condition is estimated to affect more than 580,000 people in England and Wales, and occurs more frequently in women than men. It is most common between the ages of 40 and 70, but can affect people of any age.

Exactly how smoking can increase the risk of a person developing rheumatoid arthritis is still uncertain. One theory is that that it can disrupt the normal workings of the immune system, leading to the type of abnormal immune response associated with the condition.

What did the research involve?

The researchers used an existing cohort of women called the Swedish Mammography Cohort, which included 34,101 women aged 54 to 89 years. For the current study, the group were followed from January 1 2003 to December 31 2010, in which time 219 cases of rheumatoid arthritis occurred.

Women were asked about various aspects of their diet and lifestyle via questionnaire, as well as additional questions about their smoking habits and history, physical activity, and their use of some medications and dietary supplements.

The current study population of 34,101 excluded women from the mammography cohort who had missing data on their smoking status (797), as well as women with non-rheumatoid arthritis joint conditions (2,052). Women already diagnosed with rheumatoid arthritis were also excluded.

Cases of rheumatoid arthritis were identified by linking the records of women in the cohort to medical databases. The researchers also had access to a national rheumatology register so they would be notified if a diagnosis of rheumatoid arthritis was made.

The analysis estimated the relative risk (RR) between various aspects of smoking behaviour – such as intensity, duration and time since quitting – and the risk of developing rheumatoid arthritis. The analysis took into account a variety of potentially modifying factors (confounders), including alcohol consumption, menopausal status, educational level and body mass index.

What were the basic results?

Over the seven-year study period, 219 cases of rheumatoid arthritis occurred from within the group of 34,101 (0.6% of the cohort). There was a statistically significant association between smoking intensity and the risk of developing rheumatoid arthritis.

Women who smoked between one and seven cigarettes per day were 2.31 times more likely to develop the condition compared with never smokers (RR 2.31 95% confidence interval (CI) 1.59 to 3.36) over the course of the seven-year study.

There was also a statistically significant association between how long a woman had smoked for and the risk of developing rheumatoid arthritis. Women who had been smoking for between one and 25 years were 1.60 times more likely to develop the condition compared with never smokers (RR 1.60, 95% CI 1.07 to 2.38).

Compared with never smokers, the risk for these smokers was still significantly elevated (to about twice the risk of never smokers) 15 years after the women had quit smoking (RR 1.99, 95% CI 1.23 to 3.20).

Among former smokers, there was a suggested trend that the risk of rheumatoid arthritis decreased over time since stopping smoking. For example, women who stopped smoking 15 years before the start of the study had a non-significant 30% lower risk of rheumatoid arthritis compared with those who had stopped only a year before the start of the study (RR 0.7, 95% CI 0.24 to 2.02).

How did the researchers interpret the results?

The researchers concluded that, "even light smoking is associated with increased risk of rheumatoid arthritis in women and that smoking cessation [stopping smoking] may reduce, though not remove, this risk".

Conclusion

This research indicates that a relatively low level of smoking (one to seven cigarettes per day) is associated with an increased risk of developing rheumatoid arthritis compared with women who had never smoked. This adds further knowledge to previous research that has suggested that cigarette smoking is directly linked to a higher risk of developing rheumatoid arthritis.

While this study is relatively robust and its results believable, it does have limitations that should be considered. It was not clear how many women dropped out of the study. If this was a large proportion of the women who started, it could significantly bias the results of the study.

The study also only recruited women. The same risk pattern may not have been seen if the study had recruited men, who are at lower risk of developing the condition than women. Similarly, the size of the risk differences between different smoking behaviours may be slightly different in men than women. Further research is needed to establish whether this is the case.

Another drawback is that women were recruited from just two Swedish counties. While no ethnicity data was reported, it is likely they were fairly similar ethnically, and it could be that different ethnicities may have different risk profiles for developing rheumatoid arthritis. This means that the results may differ if the study was repeated in a more ethnically diverse population.

This relatively robust study suggests that smoking can significantly increase a woman's risk of developing rheumatoid arthritis compared with women who never smoked, even if a woman only smokes relatively low levels of between one and seven cigarettes per day.

It adds to a growing body of evidence that there is no such thing as a safe level of smoking. Aside from the risk of rheumatoid arthritis, 'light' smoking can significantly increase your risk of developing lung cancer, heart disease and stroke.

Read more about how the NHS can help you quit smoking.