First it was beer, then it was cigarettes. Finally, researchers have found a vice that's not tied to psoriasis: coffee.
In fact, when Dr. Abrar Qureshi and his team at Brigham and Women's Hospital in Boston first set out to study whether there was a link between the skin disease and java, they thought the anti-inflammatory properties of caffeine might actually protect against psoriasis.
That had been reported by a group of Irani researchers, who applied caffeine directly to the skin of volunteers with psoriasis and found an apparent benefit.
Scientists believe psoriasis is caused by an abnormal immune system attack on the body's own cells, which causes them to form red, scaly patches all over the body that usually itch.
Typical treatments for psoriasis include topical creams, ultraviolet light exposure and systemic drugs that target the immune system.
To see whether consumed caffeine had any influence on whether a person developed psoriasis, Qureshi and his colleagues looked at more than 82,000 participants in the Nurses' Health Study.
All of the participants had filled out questionnaires about their daily food and beverage intake in 1991 and were free of psoriasis at that point.
Over the next 14 years, nearly 1,000 people in the study developed psoriasis, the team reports in the Archives of Dermatology.
Initially, the risk did seem a bit higher among those who got a lot of caffeine in their diet, whether from coffee, tea, soft drinks or chocolate.
But coffee drinkers also smoked more than people with a smaller caffeine intake.
Earlier studies from Qureshi's team have tied psoriasis to both alcohol and tobacco, so when the researchers took the latter into account they found there was no longer any link between caffeine and skin problems.
Although the earlier research doesn't prove that either smoking or drinking causes psoriasis by itself, the findings are another good reason to cut back on unhealthy habits, Qureshi told Reuters Health.
"From a lifestyle point of view," he said, "I'd recommend exercising more, drinking less and quitting smoking."
Dr. Esther Lopez-Garcia, who was not involved in the new work but has studied the health effects of coffee, said there is good evidence that the brew -- at least when filtered -- isn't harmful for healthy people.
"There is also a growing body of evidence suggesting that coffee drinking may decrease the risk of diabetes, stroke and some types of cancer," Lopez-Garcia, of Universidad Autonoma de Madrid in Spain, told Reuters Health in an email.
But she warned that the drink can worsen problems like insomnia, anxiety and high blood pressure.
"Because of these side effects of coffee, it is prudent to recommend moderate coffee consumption," she said.