Physical workload is associated with increased risk of rheumatoid arthritis: results from a Swedish population-based case-control study.
OBJECTIVE
This study investigated: (1) the association of physical workload (PW) and risk of rheumatoid arthritis (RA); (2) the potential interactions between PW and the genes in the human leucocyte antigen (HLA) region.


METHODS
A population-based case-control study involving incident cases of RA (3150 cases and 5130 controls) was performed using data from the Swedish Epidemiological Investigation of Rheumatoid Arthritis. Information on 7 types of self-reported PW exposure andHLA-DRB1genotypes of cases and controls were gathered. Anticitrullinated protein antibody (ACPA) status of cases was identified. For each PW exposures, exposed participants were compared with unexposed participants. ORs with 95% CIs of RA (overall), ACPA-positive RA and ACPA-negative RA associated with different PWs were estimated using logistic regression. HLA-PW interactions were estimated using the principle of departure from additivity of effects by calculating attributable proportion (AP) due to interaction.


RESULTS
ORs of developing RA associated with 6 various PW exposures ranging from 1.3 (95% CI 1.1 to 1.4) to 1.8 (95% CI 1.6 to 2.0) were observed. Exposure to more types of PW was associated with increasing risk for RA (p<0.0001). No major difference in the ORs between ACPA-positive and ACPA-negative RA was found. For some exposures, we found evidence of interactions between PW and theHLA-DRB1shared epitope genes, regarding risk of ACPA-positive RA (AP: from 0.3 (95% CI 0.1 to 0.5) to 0.4 (95% CI 0.2 to 0.6)).


CONCLUSIONS
PW is associated with the risk of ACPA-positive and ACPA-negative RA. Interactions between PW and theHLA-DRB1shared epitope were found in ACPA-positive RA.
Institute of Environmental Medicine, Karolinska Institute, Stockholm, Sweden; Rheumatology Unit, Department of Medicine at Karolinska University Hospital, Karolinska Institute, Stockholm, Sweden.
Pingling
P
Zeng
Rheumatology Unit, Department of Medicine at Karolinska University Hospital , Karolinska Institute , Stockholm , Sweden.
Lars
L
Klareskog
Institute of Environmental Medicine, Karolinska Institute, Stockholm, Sweden; Center for Occupational and Environmental Medicine, Stockholm County Council, Stockholm, Sweden.
Lars
L
Alfredsson
Institute of Environmental Medicine, Karolinska Institute , Stockholm , Sweden.
Camilla
C
Bengtsson
Ant-CCP; Epidemiology; Rheumatoid Arthritis