Ambient Solar Radiation Predicts Psoriasis Treatment Intensity

Observations of seasonal variation in psoriasis severity (worsening in winter and improving in summer) suggest a beneficial effect of ambient solar radiation [1, 2]. Additionally, “climate therapy” using intentional sunlight exposure reduces psoriasis severity [3]. Interestingly, an analysis of psoriasis clinical trial participants by Okun and Okun [4] found no relationship between psoriasis severity and the ambient ultraviolet radiation at investigator sites. Also, Jacobson et al. [5] compared the psoriasis prevalence in extant studies by the mean country latitude and found no substantial correlation. Using a diverse sample of the United States, our study assessed the association of annual local solar radiation with psoriasis treatment intensity. We analyzed de-identified participant data from the National Institutes of Health's All of Us Research Program version 7, which includes data from 2017 to 2022. Seven thousand seven hundred forty eight participants with a diagnosis of psoriasis were identified. To reduce the likelihood of patients requiring systemic immunosuppressive treatments for other conditions, we excluded participants with diagnoses of psoriatic arthritis, rheumatoid arthritis, systemic lupus erythematosus, atopic dermatitis, uveitis, Crohn's disease, and ulcerative colitis. Our final cohort included 5179 participants after excluding participants with missing values for 3-digit ZIP code or body mass index (BMI) as obtained from the participant's electronic health record. This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guidelines. Treatment intensity was assessed by the presence or absence of a systemic psoriasis treatment or phototherapy (Table S1). Mean annual global horizontal irradiance (GHI) data (1998–2016) for the contiguous United States by ZIP code was obtained from the National Renewable Energy Laboratory [6]. Deprivation index (a composite score calculated by the All of Us Program using measures of poverty, education, housing, and lack of health insurance at the census tract level) was included to control for confounding related to differences in access to care. Descriptive statistics of mean and standard deviation or frequency and percent were calculated. GHI was divided into quartiles: under 3.97 kWh/m2/day (Quartile 1, e.g. Seattle, Washington), 3.97 to 4.05 (Quartile 2, e.g. Chicago, Illinois), 4.05 to 4.92 (Quartile 3, e.g. Atlanta, Georgia), and over 4.92 (Quartile 4, e.g. Los Angeles, California). Multivariable logistic regression compared treatment odds by participant ZIP code GHI, adjusting for deprivation index, age, BMI, sex assigned at birth (hereafter “sex”), race, and ethnicity. Significance was set at two-tailed p < 0.05, and analysis was performed using SAS Studio in the All of Us Researcher Workbench. Of the 5179 participants with psoriasis in the study, 1028 received systemic treatment or phototherapy (Table 1). Lower GHI of the participants' ZIP code was associated with higher odds of receiving systemic treatment or phototherapy (Figure 1). Obesity and female sex were also associated with higher odds of systemic treatment or phototherapy. Age over 75 was associated with lower odds of systemic therapy or phototherapy. We did not find an association between deprivation index, race, or ethnicity and treatment intensity. Our study shows that lower levels of ambient solar radiation are associated with higher odds of receiving systemic therapy or phototherapy for psoriasis, with these treatments serving as possible indicators of more severe disease. The lack of association between severity and ultraviolet radiation seen by Okun and Okun [4] may have been due to clinical trial participants having stable moderate-to-severe psoriasis and being instructed to limit their environmental ultraviolet radiation exposure. Of note, the study by Jacobson et al. [5] of latitude and psoriasis prevalence excluded the United States and China due to the range of latitudes spanned by these countries. Solar irradiance also varies with factors beyond latitude, such as altitude and cloud coverage [6]. The associations of obesity, sex, and age with treatment seen in our study align with prior research. Obesity is associated with psoriasis severity, and weight loss leads to a reduction in the Psoriasis Area and Severity Index (PASI) [7]. Although male sex is associated with higher PASI scores, female sex is associated with a greater likelihood of receiving biologic treatment [8]. Increasing age is associated with a lower intensity of psoriasis treatment [9]. No significant association between the deprivation index and treatment status is encouraging from a health equity perspective. Limitations include the All of Us Program being a volunteer-based sample of only the United States. Also, we cannot account for people's movement to areas of different solar irradiance or outdoor activities that increase ultraviolet radiation exposure. Clothing coverage is another determinant of personal solar radiation exposure [10] for which we could not account. Nonetheless, we add to the literature on environmental components of psoriasis by suggesting that lower ambient solar radiation is associated with more frequent systemic psoriasis treatment. B.R.W. and A.B.F. authors were involved in conceptualization, methodology, formal analysis, writing, and editing. B.R.W. performed the data curation and prepared the original draft. We gratefully acknowledge All of Us participants for their contributions, without whom this research would not have been possible. We also thank the National Institutes of Health's All of Us Research Program for making available the participant data examined in this study. The All of Us Research Program is supported by the NIH, Office of the Director: Regional Medical Centers: 1 OT2 OD026549; 1 OT2 OD026554; 1 OT2 OD026557; 1 OT2 OD026556; 1 OT2 OD026550; 1 OT2 OD026552; 1 OT2 OD026553; 1 OT2 OD026548; 1 OT2 OD026551; 1 OT2 OD026555; IAA number: AOD 16037; Federally Qualified Health Centers: HHSN 263201600085 U; Data and Research Center: 5 U2C OD023196; Biobank: 1 U24 OD023121; The Participant Center: U24 OD023176; Participant Technology Systems Center: 1 U24 OD023163; Communications and Engagement: 3 OT2 OD023205; 3 OT2 OD023206; and Community Partners: 1 OT2 OD025277; 3 OT2 OD025315; 1 OT2 OD025337; 1 OT2 OD025276. No ethical approval was needed for this study because participant data are deidentified. A.B.F. is a consultant for Bluefin and Incyte (fees). He is an investigator for Amgen, Bayer, Biogen, CellDex, Galderma, Incyte, Leo, and UCB (research support). He is a speaker for Imedic Healthcare Solutions (Hyderabad India). B.R.W. declares no conflicts of interest. This study used the All of Us Research Program's Controlled Tier Dataset version 7, available to authorized users on the Researcher Workbench. Global horizontal irradiance data were obtained from the National Renewable Energy Laboratory at https://www.nrel.gov/gis/solar-resource-maps.html. Table S1 Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.