Psoriasis, a chronic and immune-mediated inflammatory skin disorder with no cure, is characterized by red, scaly plaques. These plaques most frequently appear on the elbows, knees, scalp, and lower back, but can affect any skin surface (1). It impacts millions worldwide, with a notably higher prevalence in Western countries characterized by older populations and higher income levels (2). The rate of psoriasis varies significantly by region, from 0.14% in East Asia to 1.99% in Australasia, with also substantial prevalence in Western Europe (1.92%), Central Europe (1.83%), and North America (1.50%) (1). Considered now as a serious non-communicable systemic disease, psoriasis is linked with a wide range of co-morbidities, including psychological, metabolic, arthritic, and cardiovascular issues (1). Prior research has indicated that psoriasis can be independently linked to a higher risk of mortality (3), presenting complex challenges for both dermatological and systemic health care, especially in individuals with more severe forms of the disease (1, 2).

The relationship between psoriasis and obesity has been a subject of increasing interest within the medical research community (4–6), given the growing evidence suggesting that obesity, as a prevalent and potentially modifiable risk factor, can exacerbate the clinical trajectory of psoriasis and vice versa (4, 6, 7). It was found that the inflammatory pathways shared between obesity and psoriasis, which involve cytokines such as TNF-α and IL-6, may suggest a biological plausibility for their bidirectional correlations (8–10). Both conditions are associated with a heightened risk of mortality (3, 11).

Body mass index (BMI) serves as a convenient and reliable approach for evaluating obesity in most people most of the time (12). Aune et al. identified a dose–response relationship between BMI and the risk of psoriasis onset, noting the lowest risk at a BMI of 20 and a significant risk increase starting from a BMI of 22.5–24 (13, 14). However, the association between BMI and mortality remains elusive. A US retrospective cohort study, spanning recent two decades, revealed a U-shaped relationship relating weight to mortality, indicating highest death rates at two extreme sides of BMI. Interestingly, being overweight (BMI 25–30) did not correlate with increased mortality (15). Contrarily, a meta-analysis suggested that both overweight and obesity are linked to a higher risk of all-cause mortality (16). Tobias et al. provided additional evidence to support a strong association between escalating BMI and increased mortality starting from a BMI of 25 kg/m² (17). Despite the well-documented relationship between psoriasis, obesity, and increased risks of morbidity and mortality, the role of BMI, especially in the categories considered underweight or overweight, in affecting mortality is still unclear. The mediating effects of factors such as smoking, income and educational level can further complicate this association (18, 19).

A study suggested that higher BMIs are associated with more severe psoriasis among individuals who are lean or overweight (20). Another study showed that more severe psoriasis could be related to an increased risk of mortality (21). Based on these prior findings, one may speculate that, among non-obese individuals with psoriasis, a lower BMI can be associated with a reduced psoriasis severity, and thus a reduced risk of mortality. However, no current study provides direct evidence about this. Additionally, the influence of potentially confounding factors such as sex, age, previous smoking history, and comorbidities on such relationship is uncertain.

The National Health and Nutrition Examination Survey (NHANES) can provide invaluable resource for studying these associations, based on a large, nationally representative sample of the U.S. population. Therefore, through analyzing its data, we aimed to provide evidence about the impact of BMI on the risk of all-cause mortality in individuals with a history of psoriasis. We also extended our investigation to ascertain if this link persists in those without psoriasis. In addition, we aimed scrutinize the influence of various factors, such as age, sex, prior smoking history, on this critical association, aiming to inform clinical management and intervention strategies effectively.

A total of 22,876 individuals, with or without a history of psoriasis, aged 20 years old and above were included in the final analysis. 618 (2.7%) were identified as having a history of psoriasis (see Figure 1 for participant selection details). The lowest recorded baseline BMI among all participants was 16.56, and the highest was 63. Among individuals with histories of psoriasis, only 3 participants had a baseline BMI below 18.5, and 26 participants had a baseline BMI exceeding 45.

To the best of our knowledge, the present study was the first to explore the relationship between BMI and the risk of all-cause mortality among individuals with psoriasis. More specifically, we analyzed data from the NHANES database collected between 2003 and 2006, as well as between 2009 and 2014, encompassing a total of 22,876 individuals aged 20 years and above, with or without a history of psoriasis. Our primary finding suggested a positive correlation between BMI and the risk of all-cause mortality among individuals (≥20) with a history of psoriasis. After adjusting for multiple potential confounders, BMI was found to independently increase all-cause mortality risk in this population, showing a trend where mortality risk increased by 5.5% for each unit rise in BMI. These results remained consistent even when utilizing a propensity score matching (PSM) approach. However, similar associations were not observed among individuals without histories of psoriasis in our study.

Despite these novel findings, our results from the restricted cubic splines (RCS) analysis and Multivariate Cox Regression model revealed nuanced results within the population of individuals with psoriasis. The RCS analysis indicated a potential dose–response relationship between baseline BMI and the risk of all-cause mortality. For BMIs ranging from 20 to 28.1, no significant correlation with mortality risk was identified, partially aligning with the results of our stratified analysis when considering BMI as a categorical variable. This stratified analysis suggested no difference in mortality risk between the lean and overweight groups. However, notably, from a BMI of 28.1 onwards, an evident increase in mortality risk was observed in the RCS analysis as BMI values climbed, suggesting a potential linear relationship within this higher BMI spectrum. By contrast, the Multivariate Cox Regression analysis indicated a continuous elevation in mortality risk, with a hazard ratio (HR) of 1.055 per unit increase in BMI across the entire BMI spectrum. This discrepancy between analyses may be attributed to methodological disparities. Compared to the RCS approach, which allowed for exploring such relationship across distinct BMI segments and offered insights into the behavior of this association at a more granular level, the Cox regression model provided an average effect size across the entire BMI range. This approach potentially overshadows subtleties within individual segments. Therefore, although we suggested a positive association between BMI and all-cause mortality risk, evidenced by significant results even after propensity score matching, the interaction between a BMI below 28.1 and mortality risk may be more complex than initially indicated. Further evidence is needed to investigate the impact of a BMI below the obesity threshold on mortality risk. Furthermore, where possible, future prospective studies are encouraged to include larger populations with psoriasis and conduct extended follow-ups to document dynamic changes in BMI, thereby providing comprehensive evidence to better explain and understand the current findings.

Although the impact of overweight on mortality risk is still debated, factors such as a BMI exceeding 30 (i.e., obesity), cigarette smoking, and being male consistently emerge as risk factors for higher all-cause mortality, irrespective of participants having psoriasis or not (16, 30–32). These associations persist in our study even after adjusting for multiple potential confounding covariates, including race/ethnicity, education level, family income-to-poverty ratio, and comorbidities, highlighting the protective effects of being non-obese, non-smoking, and female. Furthermore, in line with previous studies (30, 33), our results also indicated that overweight in older adults aged over 60 was not associated with increased mortality compared to a BMI < 25 group. This finding could be attributed to many elderly individuals having a relatively lower BMI due to unintentional weight loss from illness, potentially exacerbating illness due to sarcopenia (34). Conversely, higher weight increases the risk of developing multiple conditions such as diabetes, arthritis, fatty liver, hypertension, heart failure, and kidney disease (17).

Excessive adiposity (body fatness), a well-established risk factor for premature mortality, is primarily linked to various chronic diseases (14, 16). However, excess weight reflected by a higher BMI could not directly reflect excess body fat. It was suggested that BMI is a relatively poor indicator of body fat percentage and does not account for fat mass distribution across different body sites (35). The observation that there was no change in mortality within the BMI range of 22.5 to 30 in a previous study and below 28.1 in our study may be due to the weak association between BMI and adiposity within this range (15, 35). For example, prior research has shown that among individuals with a BMI of 25 kg/m², body fat percentage in men ranged from 14 to 35%, and in women, it ranged from 26 to 43% (35), suggesting the instability of utilizing BMI to reflect one’s body fat.

Previous studies suggested an indirect correlation between BMI and mortality among non-obese individuals with psoriasis, indicating that a higher BMI may lead to a higher risk of severe psoriasis and subsequently to a higher risk of all-cause mortality (20, 21). In our study, we did observe a notably positive correlation between BMI and the risk of more severe psoriasis in non-obese group. However, we did not find a significant difference in mortality rate between lean individuals with mild psoriasis and overweight individuals with moderate-to-severe psoriasis. These results may indicate that the relationship between BMI and mortality between lean and overweight groups of individuals with psoriasis is far more complicated than what could be inferred indirectly based on prior findings (20, 21). For future studies, exploring alternative metrics to assess body fat mass and their impacts on these associations is encouraged, especially between lean weight and overweight groups. Moreover, in clinical practice, it is advisable to comprehensively consider the specific condition of a person with psoriasis when assessing their all-cause mortality risk based on BMI.

Overall, our study has several strengths. Firstly, the nationally representative sample size is commendable. Secondly, we adjusted for multiple potential confounding factors using the propensity score matching (PSM). Before PSM, the overweight group had the highest percentages of male participants, non-Hispanic Whites, and individuals with the highest Poverty Income Ratio (PIR), with significant differences noted. The post-PSM comparison increased the validity of our analysis by compensating for these baseline differences, thus ensuring a more balanced and unbiased assessment across the groups. Interestingly, the results remained consistent with those obtained before PSM, enhancing the reliability and stability of our findings. Thirdly, while prior findings indirectly suggest a positive association between BMI and all-cause mortality in non-obese psoriatic individuals, our study provided direct evidence that did not support this relationship. Lastly, our study contributed additional evidence regarding the insignificant association between BMI and all-cause mortality among the general population, consistent with findings from previous studies.

However, there are several limitations to our study. First, we relied on single baseline measures of BMI, precluding evaluation of the time-varying association of interest. Second, the questionnaire format of the psoriasis profile may introduce recall bias. Third, our results are based solely on U.S. adults, limiting generalizability to other populations. Finally, due to the limitations of using BMI to reflect body fat mass and the possibility of residual and unknown confounding factors, our results should be interpreted with caution.

In a nationally representative sample of U.S. adults, we observed a trend wherein a higher BMI was linked to an increased risk of all-cause mortality among individuals with psoriasis, but not among those without the condition. However, the definitive impact of overweight compared to normal weight or underweight remains inadequately explored. Further studies are needed to provide additional evidence on these relationships. Additionally, exploring alternative metrics for assessing body fat to investigate these associations is also urgent.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by National Health and Nutrition Examination Survey. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

ZW: Writing – original draft, Conceptualization, Data curation, Formal analysis, Methodology. GN: Writing – original draft, Conceptualization, Data curation, Investigation, Methodology. CS: Writing – review & editing, Conceptualization, Formal analysis, Methodology. DS: Writing – review & editing, Conceptualization, Investigation, Methodology, Project administration, Resources, Supervision, Visualization, Writing – original draft.