Rheumatoid arthritis (RA) is a chronic inflammatory disease of unknown etiology that results in destruction of articular tissues and joint deformities. Free oxygen radicals have been implicated as mediators of tissue damage in inflammatory diseases of joints 1; thus, it is reasonable to hypothesize that antioxidants, such as selenium and alpha-tocopherol, may provide protection against RA. During the 1970s, the mean serum selenium concentration in Finland was very low, 50 μg/liter. 2,3 In the early 1980s, it rose owing to consumption of imported high-selenium grain; later, the mean levels doubled owing to supplementation of fertilizers with sodium selenate. 4 A 40% decline in rheumatoid factor (RF)-negative RA incidence was observed in the late 1980s. 5 Furthermore, in a preliminary nested case-control study, based on a small number of RA cases, we found a suggestively elevated risk for RA at lower levels of serum selenium and antioxidant vitamins. 6 Our findings were recently replicated in a similar small study in the United States. 7
The present nested case-control study is based on a larger number of subjects diagnosed with RA during 17 years of follow-up. In this study, we evaluated the predictive effect of serum selenium and serum alpha-tocopherol levels on the incidence of RF-positive and RF-negative RA.
Subjects and Methods
During 1973 and 1977, the Social Insurance Institution’s Mobile Clinic Unit carried out multiphasic health examinations in 12 municipalities in four regions of Finland. 8 In each of the four geographic regions, all inhabitants or a random sample of inhabitants of a rural municipality and an urban or semiurban municipality, as well as the employees of one factory, were invited to attend the examination. A total of 19,518 men and women who were 20 years of age or older (83% of those invited) participated in the examinations. The mean age of the subjects was 46 years (age range, 20–98 years).
A questionnaire regarding medical history and smoking habits was sent to the subjects along with the invitation to attend the medical examination. A specially trained nurse interviewed the participants at the Mobile Clinic and verified the answers to this self-administered questionnaire, administering it, if necessary. Venous blood samples were taken, and serum total cholesterol was determined using an autoanalyzer modification of the Lieberman-Burchard method. 8
The population at risk (9,423 men and 9,286 women) consisted of participants who had no previous history of arthritis or other rheumatoid disease, according to either the Social Insurance Institution’s population register or the questionnaire administered at the baseline examinations. 9 All examinees were followed continuously to assess morbidity and mortality. Participants in the survey who later developed chronic arthritis were identified by linking the survey data with the Social Insurance Institution’s population register through the end of 1989, using the unique code assigned to each Finnish citizen. In Finland, drug costs are reimbursed for certain chronic diseases, including chronic inflammatory rheumatic diseases. Glucocorticoids, nonsteroidal antiinflammatory, and disease-modifying antirheumatic drugs are reimbursed to patients with rheumatic diseases. Eligibility requires a comprehensive medical certificate written by the attending physician and approved by a specially trained physician who serves as an expert adviser. The sensitivity of drug reimbursement as an inclusion criterion is about 95%; nearly 80% of the cases considered to have RA or a related disease (for example, chronic polyarthritis) on the reimbursement certificate met the American College of Rheumatology 1987 Classification criteria for RA. 5
We reviewed information on RF status of all cases, as well as radiographs of hands and feet of all but a few, to identify incident cases of RA. We obtained sufficient information concerning diagnosis, onset of the disease, and RF status on 122 cases (88 RF-positive and 34 RF-negative). 9 We selected three controls per case by individual matching, using sex, age, and municipality as matching factors. In some instances, serum from only two controls was available; thus, the total number of controls was 357. We also controlled for both the time of baseline examination and the duration of serum sample storage by matching for municipality.
Serum samples were drawn at the baseline examination and kept frozen at –20°C until thawed in 1994. Serum alpha-tocopherol and retinol concentrations were determined simultaneously using high-pressure liquid chromatography. 10 The serum selenium concentrations were determined with electrothermal graphite furnace atomic absorption spectrometry. 11 The coefficients of variation for analytical precision were 4.3% for serum selenium and 3.9% and 2.3% for serum alpha-tocopherol and serum retinol, respectively. The intraclass correlation coefficient for long-term reproducibility (4–7 years) was 0.39 for serum selenium and 0.59 for serum alpha-tocopherol. 12
We used conditional logistic regression 13 to estimate the strength of association between serum micronutrient levels and the risk for RA. The relative risks, estimated as odds ratios, were computed for tertiles of serum selenium and alpha-tocopherol concentrations. Smoking and serum total cholesterol, which are related to serum antioxidant status 14,15 and occurrence of RA, 16,17 were included as confounding variables and effect-modifying factors in the models.
The mean levels of serum selenium or alpha-tocopherol did not differ notably between all of the future incident cases of RA combined and their matched controls, except for serum selenium levels among RF-negative RA cases and their controls (Table 1). Individuals with a low serum selenium level also had lower serum alpha-tocopherol level (Table 2). Both micronutrients were positively associated with serum retinol and serum total cholesterol.
Serum selenium was associated with subsequent development of RA among subjects who had RF-negative RA (Table 3). The smoking- and cholesterol-adjusted relative risk of the disease between individuals in the highest and lowest tertiles of serum selenium was 0.16 [95% confidence interval (CI) = 0.04–0.69]. The association was strongest during the first 10 years of follow-up. The relative risk was 0.06 (95% CI = 0.01–0.45) for that time interval and 0.55 (95% CI = 0.04–7.43) for longer follow-up periods.
Serum alpha-tocopherol, adjusted for smoking and serum total cholesterol, was associated with RF-negative RA during the total follow-up period (Table 4). During the first 10 years of follow-up, however, individuals in the highest tertile of serum alpha-tocopherol as compared with those in the lowest tertile showed a reduced risk for both RF-positive and RF-negative RA. Nevertheless, an opposite effect was observed if the first 10 years of follow-up were excluded.
We observed an inverse relation between serum selenium concentration and subsequent risk for RF-negative RA. The association was stronger during the first 10 years of follow-up to the mid-1980s, a time when serum selenium values were very low in Finland. 2,3 The weaker predictive value of serum selenium for longer follow-up periods may also be due to the weak reproducibility of serum selenium measurements. 12 Thus, the reduced risk associated with higher serum selenium levels in the present study is apparently a conservative estimate.
RF-negative RA is a heterogeneous group of disease conditions. It includes subsets such as juvenile idiopathic arthritis of adult onset, “hidden” psoriatic arthritis, and chronic reactive (HLA B27-associated) arthritis without any clear evidence of triggering infection. RF status in our patient series was based on the examination when the diagnosis had been reached. Some of the patients may have converted to RF-positive at a later phase. Analysis of the RF-negative cases according to subgroups (men vs women, erosive vs nonerosive at the moment of examination, age at onset of disease) did not reveal any differences (data not shown). Testing for RF had been performed in several different laboratories. Matching for municipality in the present study, however, simultaneously matched for possible systematic shifts in RF status.
The sensitized sheep cell agglutination (Waaler-Rose) test for RF was positive in 27 of the pre-illness specimens from subjects who developed RF-positive RA and in none of the subjects who developed RF-negative RA. 9 This result indicates that the distinction between RF-positive and RF-negative cases was relatively accurate.
In line with suggestions from previous studies, 6,7 we found that a higher serum alpha-tocopherol level predicted a decreased risk for RA. This association was observed for both types of RA but only for the shorter follow-up period. Ocke et al18 observed that the concentration of alpha-tocopherol declines during storage of serum samples at –20°C. We found that although serum alpha-tocopherol concentrations were reduced by about 20% during storage at –20°C, the reliability was still satisfactory. 12
The risk factors for RA are mainly unknown. Recent epidemiologic studies have suggested that smoking is a risk factor for RF-positive RA in particular. 16,19–21 Serum total cholesterol has also been reported to predict the occurrence of RA. 17 The adjustment for smoking and serum cholesterol performed in our study may be unsatisfactory owing to confounding effects caused by other factors, however. For example, some studies have reported associations between dietary factors and RA. 22,23 We had no dietary data available in the present study. Thus, it cannot be ruled out that the associations reported are due to some dietary factors or some other unknown confounding factors.
In conclusion, low selenium status may predict occurrence of RF-negative RA. The results also lend support for a similar association between serum alpha-tocopherol and overall RA risk. Epidemiologic studies alone may be insufficient to identify the true mechanisms underlying the association between poor antioxidant status and RA, because antioxidants per se may not account for all of the benefits associated with the dietary pattern leading to high serum levels of the antioxidants.
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