Prevalence of rheumatoid arthritis in relation to serum cadmium concentrations: cross-sectional study using Korean National Health and Nutrition Examination Survey (KNHANES) data (2024)

Strengths and limitations of this study

• The largest study to date of serum and urinary heavy metal concentrations in a cohort of patientswith rheumatoid arthritis (RA)and healthy controls.

• The analysis for the first time of heavy metal levels and RA prevalence included important socioeconomic parameters relevant to heavy metal exposure and corrected for these factors in the cohorts studied.

• Detailed clinical characteristics of patients with RA such as rheumatoid factor, anti-cyclic citrullinated peptide antibody and disease activity were unavailable.

• This study as with any cross-sectional study may have potential unknown inherent biases and confounding factors.

Data source and ethical considerations

The KNHANES is a national health and nutrition survey to identify health-vulnerable groups for policy prioritisation by identifying current statuses and trends of the public health and nutrition status. It is a national statistical project that has been carried out since 1998 based on the National Health Promotion Act. The Korean CDC (KCDC) provides raw data of KNHANES at its homepage (https://knhanes.cdc.go.kr). It also provided analysis guidelines so that weighting process can be appropriately performed for each variable. Written informed consent was obtained from all participants since 1998. Since 2008, data have been collected to assessthe effects of exposure to heavy metals in the environment. The data source was deidentified and opened to the public. The study was approved by Seoul National University Hospital and College of Medicine in Seoul National University (Institutional Review BoardNo: 1803-010-925).

Participants

All participants included in the survey with heavy metal or mineral measurement were included for analysis. We defined patients with RA as having a physician diagnosis of RA in interview results of KNHANES data. Control subjects were defined as individuals who denied the diagnosis of RA made by a physician in the questionnaire. Heavy metals or minerals were measured in different time periods of the survey programme which resulted in three different data sets for analysis: Cd, Hg and Pb from 2008 to 2012 survey (RA=693 vs control=31 522; group 1, n=45 811); serum Mn and urine As from 2008 to 2009 survey (RA=330 vs control=13 998; group 2, n=20 277); and serum Zn from 2010 survey (RA=133 vs control=6119; group 3, n=8958).

Measurements

Serum Cd, Pb and Mn levels were measured by graphite furnace atomic absorption spectrometry (PerkinElmer Analyst 600, PerkinElmer, Finland). Serum Hg levels were measured by gold amalgamation (DMA-80, Milestone, Italy). Urine As levels were measured by atomic absorption spectrometry adjusted by creatinine.²¹ Serum Zn levels were measured by inductively coupled plasma mass spectrometry (ICP/MS; PerkinElmer ICP-MS, USA).

Smoking status was determined based on two definitions. First, based on the WHO definition, smokers are those who have smoked 100 cigarettes or more in their whole life. Second, current smokers are considered for those who are currently smoking and have smoked 100 cigarettes or more in their whole life. We used the WHO definition of smokers to evaluate theeffects of serums Cd, Pb, Hg, Mn and Zn. The second definition of smokers was used for urine As due to the short half-life of As.

Statistical analysis

We investigated the characteristics of three KNHANES survey populations with regard to heavy metal measurements. The prevalence and 95% CI of RA were calculated for each population. Associations between serum heavy metal or mineral levels and prevalence of RA were evaluated using a general linear model considering complex sample design after adjusting for age, sex and smoking status. We included missing data values in the model as a categorical variable to calculate adjusted ORs as stated in the analysis guideline provided by the KCDC. Using a logistic regression model, we calculated ORs and 95% CIs for increases of 1 µg/L in serum level of Cd and Hg, 1 µg/dL in serum level of Pb and Mn, and 1 µg/L creatinine urine As contributing to prevalent RA after adjusting for age, sex and smoking status. All statistical analyses were twosided and p value <0.05 was considered statistically significant. All analyses were performed using IBM SPSS V.22 (IBM) or Stata software V.13.0 (StataCorp).

Patient and public involvement

The KNHANES data were released after anonymisation. The study population was not involved in the design of this study.

Table 1

Baseline characteristics of three populations of Korean National Health and Nutrition Examination Survey (KNHANES) in heavy metal measurement

Table 2

Demographic features of patients with RA and controls analysed by complex sample design

Table 3

Estimated means of serum and urine heavy metal levels between patients with RA and controls analysed by complex sample design

Principal findings of the study

Our study showed that serum Cd levels were increased in patients with RA compared with those in the control group (1.30±0.07 vs 1.17±0.01 µg/L, p<0.01). Our results also revealed that the risk of RA prevalence was exponential. The OR (95% CI)of RA prevalence according to serum Cd increase of 1 µg/L was 1.28(95% CI1.03 to 1.61) (figure 1). We described changes in OR according to serum Cd levels using cubic spline curve (figure 2). The cubic spline curve provided an intuitive understanding of the linear increase in RA risk with a dose–response relationship.

The geometric mean of serum Cd levels in the third KNHANES performed in 2005 has been reported to be 1.53 µg/L (95% CI 1.48 to 1.58).²⁸ Blood Cd levels can change with demographic and lifestyle factors including smoking. However, geometric means of blood Cd levels for non-smoking general population did not exceed 2 µg/L.²⁹Figure 2 demonstrated that the slope was steeper at Cd levels less than 1.5 µg/L where serum levels of the healthy general population would have been better represented. Furthermore, ORs for RA prevalence increased according to low serum Cd concentrations throughout the toxic range of serum Cd levels. It can be concluded that high levels of Cd can increase the risk of RA appreciably in women independent of smoking. These results can be reconciled with animal data suggesting that low levels of Cd could be protective^{8 9} while high levels exacerbate disease.⁸ However, the OR of RA prevalence according to an increase in serum Cd level of 1 µg/L showed a significant increased risk of RA prevalence only in female patients (male: 0.86(95% CI0.41 to 1.81), female: 1.33(95% CI1.06 to 1.66)). In this study, Cd did not appear to be associated with RA prevalence in men. Diminished statistical power due to low prevalence of male RA (23%) in group 1 RA cohort with only 159 RA men might have influenced results reported here. Additionally. RA men might not necessarily have high serum Cd levels to initiate their disease. Instead, they might have high lung Cd levels as a recent review of male patients with RA has highlighted that RA men have enhanced exposure to various dusts that act as absorbers of Cd derived from cigarette smoke and that the Cd is trapped in the lung to initiate RA.³⁰

Unanswered questions and future research

Further studies with much larger cohorts of RA men taking occupation of male patients with RA into account are needed. It is also important to study those unexposed to various dusts to determine if they have raised serum Cd levels in the future. In the study reported here, we were not aware of occupations of patients with RA or controls.

Strengths and weaknesses of this study

We believe that the RA population studied here is representative of Korean RA population reported in the literature attending secondary care rheumatology outpatient clinics in terms of gender, age and smoking history.^{31 32} The autoantibody status of the patients with RA studied here is not known. However, given the potential for Cd to trigger antigen PTMs, it is possible that Cd is mainly associated with seropositive RA. It was noteworthy that the geometric mean of blood Cd level in the Korean population studied here was 1.17±0.01 µg/L. This is much higher than blood Cd levels reported in Sweden (median blood Cd in men and women: 0.24 µg/L and 0.27 µg/L, respectively).³³ The prevalence of ACPA positivity is also different between RA cohorts in Korea (87%)³⁴ and Sweden (63%).¹² Whether there are significant differences in blood Cd levels in the general populations of these two countries remains unclear. Further studies specifically investigating the relationship between seropositive RA and blood Cd will be of importance.

This study is unable to fully address the reasons why serum Cd levels are significantly increased in patients with RA. Although our results were adjusted for personal history of cigarette smoking, exposure to involuntary cigarette smoking was not accounted for here. A recent study has demonstrated an association between involuntary smoking exposure with urine cotinine level and blood Cd level in female Korean non-smoking population.³⁵ It is noteworthy that female non-smokers with exposure to secondhand cigarette smoke have significantly higher RA disease activity than female non-smokers without exposure to secondhand smoke.³⁶ However, the studies investigating whether exposure to involuntary cigarette smoke is associated with RA have not been reported yet.

Strengths and weaknesses in relation to other studies

We were able to demonstrate a strikingly significant increase in the prevalence of RA in women with increasing quartiles of serum Cd levels, showing 19-fold difference in female RA prevalence between individuals in the lowest quartile of serum Cd level and those in the highest quartile (0.18% vs 3.42%). Our study is the first study to account for environmental risk factors such as smoking, socioeconomic status, gender and age for variations in serum Cd levels when examining the relationship between raised serum Cd levels and RA prevalence. The prevalence of RA is somewhat higher than expected. We defined RA as a physician diagnosis of RA recorded as positive by the patients in their questionnaire. It is conceivable that patients with osteoarthritis or patients with RA with very mild disease (seronegative and not taking disease-modifying drugs) were included, however, this is likely to have weakened the association between RA prevalence and raised serum Cd levels given the plausible association between Cd and seropositive RA.

Our findings could not be explained by an increased smoking frequency in female patients with RA as smoking prevalence (ever having smoked) in the RA cohort was 27%, which was strikingly lower than that in the respective controls (42.6%). Furthermore, serum Cd levels were not significantly higher in female RA smokers compared with those in female controls (data not shown). Therefore, the risk factors other than smoking might account for the increased prevalence of RA in those females with serum Cd levels in the highest quartile.

Possible explanations and implications for clinicians and policymakers

A recent study on Malaysian RA women who rarely smoke has demonstrated an increased risk of RA (OR 2.8, 95% CI1.6 to 5.2) associated with working in the textile industry.³⁷ However, this finding might have been confounded by exposure to Cd as the textile industry has used Cd in dyes to colour textiles.⁵ Most other occupations associated with RA^{38 39} also have the potential to predispose individuals to Cd inhalation and raise serum Cd levels. However, most of these occupations are relevant to men. Recently, a case–control study has demonstrated an increased risk of male ACPA-positive RA among bricklayers and concrete workers (OR 2.9, 95% CI1.4 to 5.7) and this association is independent of confounders such as smoking or silica exposure.⁴⁰ Cement dust exposure is associated with significantly raised serum Cd levels in Nigerian cement workers (twofold increase) compared with controls. However, female and male residents living in close proximity to the cement factory have 90-fold higher serum Cd levels than controls, exemplifying the fact that women can have markedly raised Cd levels with indirect exposure to toxic substances usually associated with male orientated occupations.⁴⁰

Increased serum Cd levels can occur as the consequence of diet. In a Japanese study, Cd levels in diets are significantly (p<0.01) correlated with blood Cd levels when results from examinees of the same survey region are pooled and compared in terms of geometric means.⁴¹ Furthermore, Cd absorption is enhanced with iron deficiency.⁴² It is well known that patients with RA commonly have iron deficiency.^{43 44} Rambod et al have demonstrated a number of significant differences between the diets of patients with RA and control subjects and it is possible that the significant differences in Cd levels reported here in the patients with RA could represent differences in diets and this would be subject to further research.⁴⁵

Unanswered questions and future research

In view of the above, further studies are needed to determine if occupations, exposure to secondhand cigarette smoke, area of residency (living in close proximity to relevant industries and main roads), diet and iron deficiency are associated with raised serum Cd levels in patients with RA. In this study, there was no significant association between RA and serum levels of metals (Pb, Hg, Mn and Zn) or urine As level. Therefore, metals per se do not appear to be associated with RA. Cd might have very specific pathological properties that predispose individuals to RA when they are exposed to this metal. Metal Cu has consistently been observed to be associated with RA. A meta-analysis has demonstrated that serum levels of Cu in Caucasian patients with RA are significantly higher than those in Asian patients with RA.⁴⁶ RA disease activity has been observed in those living in places where farm soils contain high levels of Cu.⁴⁷ Recently, it has been demonstrated that the induction of protein citrullination and ACPA generation in an animal model exposed to Ni nanomaterials.⁴⁸ Further studies are needed to determine if other metals are also associated with RA.

In conclusion, our results showed that serum Cd level was significantly increased in patients with RA and OR of female RA prevalence was increased according to serum Cd concentration. These findings add substantial evidence to the concern that increased exposure to Cd is an RA risk factor in the general population.

Reviewer comments:

Author's manuscript:

Patient consent for publication: Not required.

Contributors: SHJ, DH, JYL and YWS designed this study and wrote the manuscript. YWS obtained funding. SHJ and JYL analysed the data. All authors have full access to all data (including statistical reports and tables) used in the study and take responsibility for the integrity of the data and accuracy of the data analysis. YWS acts as the guarantor.

Funding: This study was supported by a grant (HI14C1277) from the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI) funded by the Ministry of Health and Welfare, Republic of Korea. It was also supported by a grant (NRF-2015M3A9B6052011) from the National Research Foundation funded by the Ministry of Science, ICT and Future Planning, Republic of Korea. Cornwall Arthrits Trust also provided funding.

Disclaimer: The funders played no role in the study design, data collection, data analysis, data interpretation, writing of this report or the decision to submit this article for publication.

Competing interests: None declared.

Ethics approval: Institutional Review Board of Seoul National University Hospital and College of Medicine in Seoul National University (IRB No 1803-010-925).

Provenance and peer review: Not commissioned; externally peer reviewed.

Data sharing statement: No additional data available.

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