Wednesday August 18 2010
Family history is the only certain risk factor for psoriasis
“Women who drank five or more beers a week doubled their chances of developing psoriasis,” said a report in the Daily Mail.
This study examined the drinking habits of more than 80,000 US nurses over an average of 14 years, during which 1.4% developed psoriasis. Women who drank an average of 2.3 or more alcoholic drinks a week were 72% more likely to develop the condition. For specific types of drinks, women who drank five or more non-light beers a week were found to be 76% more at risk.
This research has several limitations. Only a few women who developed psoriasis were also in these highest categories of alcohol consumption, which increases the probability that these results are due to chance. The study is also based on individuals’ responses to a questionnaire, raising the possibility that the women’s responses were inaccurate and leaving the question of the size and strength of alcoholic intake open to interpretation.
A link between psoriasis and high alcohol intake is feasible, and other studies have indicated an association between the two. However, the evidence to date is not strong and the only proven risk factor is down to genetics. Further study is needed to clarify whether alcohol also has an effect.
Where did the story come from?
The study was carried out by researchers from Brigham and Women’s Hospital, Harvard Medical School and Harvard School of Public Health, and Boston University. It was funded by the National Institutes of Health and the National Cancer Institute. The study was published in the peer-reviewed medical journal, Archives of Dermatology.
Both BBC News and Daily Mail have correctly reported the findings of the study, but they have not mentioned certain limitations.
What kind of research was this?
This cohort study assessed how alcohol in general, and particular types of alcoholic beverage, affect the risk of developing psoriasis. Psoriasis is a skin condition in which the skin cells replace themselves faster than usual, leading to a build up of plaques (red scaly, flaky patches) on the skin. The condition commonly affects the back of the elbows, knees and scalp, but can affect other areas depending on the type of psoriasis.
Aside from family history, the causes and triggers for psoriasis are not known, although certain drugs and stress have been implicated. Several studies have noted an association between alcohol consumption and an increased risk of psoriasis. This research used a cohort study design, which is the most suitable method for assessing this possible cause-and-effect relationship. However, it is likely that the study has some limitations due to the difficulty in accurately quantifying a person’s alcohol intake, which is also likely to vary over time.
What did the research involve?
This study used members of the Nurses’ Health Study II (NHS II), an ongoing study of registered female nurses from across the United States. The nurses were enrolled in 1989 when they were between the ages of 25 and 42, and since then they have been asked to complete questionnaires every two years. Of the 116,430 women enrolled, 82,869 responded to a question in the 2005 questionnaire asking if they had ever been diagnosed with psoriasis. A subset of the women who responded yes had their diagnosis confirmed by a special screening tool for psoriasis.
The 1991 version of the survey was the first to ask about alcohol intake. Of the women who said they had psoriasis in 2005, the researchers excluded 1,280 women who had also reported it in 1991. This left them with 1,069 women who had developed psoriasis since first answering a question on alcohol intake in 1991.
Questions on alcohol included the women’s average intake of beer (light and non-light), red wine, white wine and liquor. The women were asked to pick one of nine answers to rate how many alcoholic drinks they consumed, ranging from no drinks or less than one a month, to six or more drinks a day. Alcohol content was estimated to be 12.8g for a glass, bottle, or can of beer (360 ml), 11g for a glass of wine (120 ml), and 14g for a shot of liquor (45 ml). One drink was considered to be 12.8g of alcohol.
The researchers examined how total alcohol intake affected the risk of developing psoriasis. Intake was put into the following caegories:
- no alcohol
- one to four grams per week
- five to nine grams per week
- 10 to 14 grams per week
- 15 to 29 grams per week
- 30 grams per week or more
They also analysed the risk of psoriasis according to how much of each type of drink was consumed (beer, red or white wine and liquor). Possible answers included: none, one-to-three drinks a month, one drink a week, two-to-four drinks a week, or five or more drinks a week.
The calculations took into account the women’s age, BMI, smoking status, energy intake and physical activity (assessed at each follow-up), but not socioeconomic factors or stress.
What were the basic results?
There were 1,150 new cases of patients with psoriasis, 1,069 of whom had also reported their alcohol intake in 1991 and were included in the analysis. There was a 72% increased risk of psoriasis in women who drank an average of 2.3 drinks a week or more compared with women who did not drink alcohol (relative risk 1.72, 95% confidence interval 1.15 to 2.57). There was no association between psoriasis and drinking less than this amount.
For specific types of alcoholic drink, there was a 76% increased risk found for women who drank five or more drinks of non-light beer a week (RR 1.76, 95% CI 1.15 to 2.69). There was no association between psoriasis and light beer, red wine, white wine or liquor.
Although the researchers had reportedly also analysed by total grams of alcohol consumed, these results are not provided in the research paper.
How did the researchers interpret the results?
The researchers conclude that non-light beer intake is associated with an increased risk of developing psoriasis among women.
This study has several strengths, including its large size and the fact that it is reported to have followed up 90% of its participants over an average of 14 years. However, when concluding that alcohol, and non-light beer in particular, is associated with increased risk of psoriasis there are some important points to note:
- There were relatively few new cases of psoriasis: of the 82,869 women responding to the question on psoriasis in 1995, only 1.4% of them had developed psoriasis. Although an association was found with women who drank more than 2.3 alcoholic drinks a week on average, only 28 women who developed psoriasis had consumed this amount. The small number included in the calculations increases the risk of chance findings. Likewise, although an association was found for five or more drinks of non-light beer a week, only 22 women who developed psoriasis had consumed this amount.
- Although the researchers adjusted for some confounders, they did not take into account others that may have affected the results. Family history is an important risk factor for psoriasis, and socioeconomic factors and stress have also been associated. However, these were not taken into account.
- Accurately quantifying alcohol intake in a questionnaire is difficult, as the size of one drink or type of drink may mean different things to different people. Although an alcohol content was assigned to each drink, this is unlikely to be completely accurate unless the women had accurately given the volume of the container they were drinking from and its exact alcohol content. Furthermore, alcohol consumption is likely to vary over time and responses at one time-point may not indicate a lifelong pattern.
- These results cannot be applied to men, and as the study is from the US there may be some differences within the UK, both in alcohol intake and psoriasis incidence. This is demonstrated by the fact that this study considered one drink to be 12.8g of alcohol, which is substantially more than the UK equivalent unit of 8g (about half a pint of weak lager).
Family history is the only clearly established risk factor for psoriasis. However, it is possible that there is an association with excess alcohol intake, and this research is likely to lead to other studies. For now, the advice remains the same: people should not consume more than the recommended daily alcohol intake, which is two-to-three units for women and three-to-four units for men.