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Three independent studies examine the impact of obesity on rheumatoid arthritis (RA), either as a risk factor for the development of disease or as a prognostic factor for the severity of disease.1–3 Interestingly, these studies have investigated the role of obesity at different stages of the disease, ranging from the ‘preclinical phase’,1 to the early RA period,2 to the established disease stage.3 This is important, as the role played by environmental factors may vary according to disease stage.4 An example of the evolving impact of environmental factors during the course of the disease is provided by tobacco smoke, which is the best established risk factor for RA disease development5 and associated with the development of severe extra-articular manifestations,6 but which may have a protective effect on the progression of joint damage in later stages of the disease.7
Obesity is a growing global health problem8 and has been associated with increased risk for a number of chronic diseases. There have been conflicting reports on the impact of obesity on the risk of RA, but the majority of studies indicate a positive association in women.9–12 As always in the study of lifestyle factors and disease risk, there are methodological issues related to the direction of causality, to recall bias in retrospective investigations and to selection bias. Therefore, prospective population-based studies are needed to formally establish the causal role of obesity in RA.
Lu et al1 report on the effect of overweight or obesity on the development of RA in the Nurses’ Health Survey (NHS, enrolment from 1976) and the subsequent NHS II (enrolment from 1989)—two large prospective studies of female registered nurses, which have been used extensively for epidemiological research. Body mass index (BMI) was based on self-reported height and weight, …
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