Although smoking was not found to be a causal risk factor for developing psoriasis, it can still be a key trigger for inducing flare-ups in psoriasis.

“We have known for some time that people with psoriasis generally smoke more than those in the general population,” notes Charlotte Näslund-Koch, MD. “In addition, several observational studies have reported an increased risk for developing psoriasis in smokers compared with non-smokers. However, due to the limitations of observational studies, determining a causal relationship between smoking and psoriasis poses a challenge.”

Dr. Näslund-Koch explains that individuals who smoke heavily often have other unhealthy habits such as poor food choices, high alcohol consumption, and stressful lifestyle as well as conditions such as overweight/obesity and diabetes. These confounders, she adds, “could be the real reason for the increased risk for psoriasis. Even though researchers try to adjust for these confounders in their statistical models, there will always be factors for which we cannot adjust. Therefore, we wanted to explore the relationship between smoking and psoriasis using a Mendelian randomization approach, which is less sensitive to confounders and would enable us to determine if there is a causal relationship between smoking and psoriasis.”

Estimating Risk for Psoriasis Based on Smoking Habits

For a study published in Frontiers in Immunology, Dr. Näslund-Koch and colleagues used data from The Copenhagen General Population Study, consisting of approximately 105,000 individuals; 1,240 cases of moderate to severe psoriasis were included to assess the link between smoking and psoriasis. Participants answered a questionnaire about smoking habits and other lifestyle factors and were given a physical examination.

“By linking the personal identification numbers of the participants to the health registries containing information on emigration, hospitalization, and death, we could estimate the risk for psoriasis based on smoking habits,” Dr. Näslund-Koch says. In addition to a physical examination, participants were also genotyped for a specific genetic variation—CHRNA3 rs1051730—which is strongly linked with smoking habits. “Using this genetic variant as a surrogate for smoking habits, we could also assess the causal relationship between smoking and psoriasis. Since our genes are present from birth, they are not affected by lifestyle factors, so by using genetic information, we bypass the limitations seen in conventional observational studies.”

Smokers Have an Increased Risk for Developing Psoriasis

The study team observed that both former and current smokers have an increased risk for developing psoriasis. “This was also the case after adjusting the model for potential confounders such as sex, age, obesity, alcohol consumption, high blood pressure, high cholesterol, physical activity, educational level, and diabetes,” Dr. Näslund-Koch points out. However, using genetic information as a proxy for smoking to avoid confounding, the researchers did not find an increased risk for psoriasis.

“This indicates that smoking does not appear to be a causal risk factor for developing psoriasis,” she says. “The increased risk for psoriasis among smokers is presumably due to other lifestyle factors, which we cannot account for even though we adjust our analyses for several lifestyle factors.” (Figure)

Compared with never smokers, the multivariable adjusted HR of developing moderate to severe psoriasis was 1.64 (95% CI, 1.35-2.00) in ever-smokers with 20 or less pack-years and 2.23 (1.82-2.73) in ever smokers with greater than 20 pack-years, based on observational analyses. The OR of developing moderate to severe psoriasis was 1.05 (0.95-1.16) per CHRNA3 rs10511730 T-allele in ever smokers, based on genetic analyses.

Dr. Näslund-Koch and colleagues agree that their research adds to the understanding of the development of psoriasis. “Although we do not find evidence of smoking as a causal risk factor for developing psoriasis, these results need to be confirmed in other studies. Dr. Näslund-Koch says. “We need to conduct more Mendelian randomization studies, investigating the same issue but within different populations and with additional genetic variants as proxies for smoking, to confirm the results. Even though this study does not find smoking to be a casual risk factor for developing psoriasis, smoking can still be a key trigger for inducing flare-ups in patients with psoriasis.”