Introduction

In the general population, obesity is a major cause of morbidity and mortality [1, 2] with an annual estimation of 300,000 premature deaths related to obesity [3]. Also, obesity is associated with high healthcare costs—with at least 92 billion U.S. dollars spent in direct health cost in the USA [3]. The prevalence of obesity is high; 30% of American adults are obese and the prevalence is increasing over time [4].

Obesity is an independent risk factor of serious medical problems such as diabetes [5, 6] and cardiovascular disease [7–9], and affects quality of life adversely, an association that was consistently shown in different populations [10–12]. In addition, obesity is linked to high concentrations of several markers of inflammation such as C-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α); [13, 14] suggesting that there is a pro-inflammatory state in obese patients.

Rheumatoid arthritis (RA) is a common chronic inflammatory disease that affects 0.5% to 1% of the population [15] and is associated with increased mortality rates, physical disability, pain, depression, and impaired health-related quality of life (HRQoL) [16–20]. Patients with RA have decreased quality of life in all domains compared to the scores reported by patients with other chronic diseases [21, 22].

In patients with RA, impaired HRQoL is associated with many disease-related factors such as disease activity, joint damage, extraarticular disease, decreased functional capacity, and depression [16–20, 23, 24]. However, little is known about the relationship between obesity and HRQoL in this patient population. Thus, we set out a study to test the hypothesis that obesity is associated with impaired quality of life and that this association is independent of RA-related factors.

Materials and methods

Patients

Three hundred and fifty-nine consecutive patients, aged 18 years or older who met the 1987 ACR classification criteria for RA [25], and attended the outpatient clinic at the Hospital Nacional Edgardo Rebagliati Martins in Lima (Peru) between January 2005 and April 2006 were enrolled. All patients are part of an ongoing study at the hospital. The Institutional Review Board of the hospital approved the study and all patients gave written informed consent.

Procedures

All patients underwent an interview, physical exam, completed patient questionnaires, and all clinical charts were reviewed. Data collected included age, sex, years of education, duration of disease, presence of extra-articular disease, comorbidities, X-ray erosions, and rheumatoid factor. Disease activity was measured using the disease activity score based on 28 joints [26].

Body mass index (BMI) was calculated as the weight in kilograms divided by the square of height in meters. Based on the BMI, patients were classified as normal (BMI < 25 kg/m2), overweight (BMI = 25–29.9 kg/m2), and obese (BMI ≥ 30 kg/m2).

Health-related quality of life was quantified with the Medical Outcomes Study Short Form 36 (SF-36) [23]. The SF-36 is a widely validated generic patient questionnaire [27] that showed to be sensitive to change in a variety of chronic diseases including symptomatic peripheral arterial, [28] chronic obstructive pulmonary disease, [29] hypertension [30], and systemic lupus erythematosus [31]. It includes 36 items covering 8 health concepts (domains): physical functioning, role limitations due to physical problems, bodily pain, general health, vitality, social functioning, role limitations due to emotional problems, and mental health [32]. Results from each health concept questions are presented in a scale from 0 (poor health) to 100 (good health). Two summary measures were further calculated from the items scores using the procedures recommended by the developers: a physical component score (PCS) and a mental component score (MCS) [33]. Measures of depression were obtained from the (0–3) question included in the multi-dimensional health assessment questionnaire (MDHAQ) [34].

Statistical analysis

Patient characteristics are presented using the means and standard deviations for continuous variables and proportions for categorical variables. Body mass index was correlated with demographic and clinical characteristics with the use of Spearman’s correlations. Health-related quality of life scores were compared among patients with normal weight, overweight, and obese with the use of Kruskal–Wallis tests. Linear regression models were used to examine the independent relationship between BMI categories (independent variable) and total SF-36 score (dependent variable). All covariates (age, sex, disease activity, extra-articular disease, comorbidities, X-ray erosions, presence of rheumatoid factor, depression, education, and disease duration) were chosen a priori based on clinical significance. All analyses used a two-sided significant level of 5% and were performed using the statistical package STATA 9.1.

Results

Patient characteristics

Patients’ mean age was 58 ± 14 years, 87% were female, mean education was 11.4 ± 4.6 years, and mean disease duration was 16.5 ± 10.8 years. Ninety-two percent tested positive for rheumatoid factor. Sixty-one patients had at least 1 comorbid condition, extra-articular disease was reported by 15%, joint damage was observed in 83.2%, their mean DAS28 was 4.9 ± 1.5, and their mean depression score was 2.0 ± 0.9 (Table 1).

Table 1 Demographic and clinical characteristics of patients with rheumatoid arthritis
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One hundred and seventy-two patients (47.9%) had normal BMI, 126 (35.1%) were overweight, and 61 patients (17%) were obese.

BMI and clinical characteristics in patients with RA

BMI was statistically significantly associated with disease activity (rho = 0.13, P = 0.01) and functional capacity (rho = 0.13, P = 0.02), but not with age (P = 0.61), years of education (P = 0.26), pain (P = 0.47), fatigue (P = 0.07), patient global assessment (P = 0.16), and number of comorbidities (P = 0.23).

Quality of life and obesity in patients with RA

Obese patients had lower quality of life (30.8 ± 18.1) than overweight patients (43.3 ± 20.1) and patients with normal weight (43.8 ± 22.2), P < 0.001.

Obese patients also had significant lower scores in both the physical component score (PCS): 29.2 ± 17.2 in obese patients compared to 40.7 ± 20 in overweight patients and 41 ± 21.7 in patients with normal weight, P < 0.001 and the mental component score (MCS): 43.8 ± 22.7 in obese patients compared to 54.6 ± 19.9 in overweight patients and 53.8 ± 22.2 in patients with normal weight, P = 0.004 (Fig. 1).

Fig. 1
figure 1

Relationship between BMI and HRQOL in patients with rheumatoid arthritis. Error bars represent the mean and 95% CI

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Obese patients also had significantly lower scores in the four domains of PCS: physical functioning, physical role, bodily pain, and general health compared to overweight and normal weight patients (Table 2).

Table 2 Association between body mass index (BMI) and health-related quality of life
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Obese patients also had significant lower scores in the three domains of the mental component: social functioning, emotional role, and mental health compared to overweight and normal weight patients (P < 0.001, P = 0.002, P = 0.004). There was no statistically significant association between BMI and the vitality domain (P = 0.10) (Table 2).

The association between obesity and impaired quality of life was confirmed with a linear regression model (Coef = −12.9, P < 0.001) and remained significant after adjustment for age, sex, disease activity, extra-articular disease, comorbidities, X-ray erosions, presence of rheumatoid factor, depression, education, and disease duration (Coef = −5.3, P = 0.039) (Table 3).

Table 3 Unadjusted and adjusted associations between body mass index and health-related quality of life
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Discussion

This study suggests that RA patients who are obese are more likely to experience impaired quality of life than patients with normal weight, independent of the effect of age, sex, disease activity, extra-articular disease, comorbidities, X-ray erosions, presence of rheumatoid factor, depression, educational level, and disease duration.

Our results showed that obese RA patients have impaired of HRQoL compared to patients who are overweight or have normal weight. The mechanisms underlying this association may include higher levels of disease activity and poorer functional capacity. In concordance with this hypothesis, a recent study of patients with early RA followed-up over a period of 5 years, reported that functional capacity (as measured by the HAQ score) and inflammation (as determined by ESR) were able to predict physical health-related quality of life measured with the Arthritis Impact Measurement Scale 2 [24]. However, the fact that in this study the association between obesity and HRQoL remained significant after adjustment for both disease activity and functional capacity is of interest, especially because obesity is modifiable.

In the general population, reducing obesity improves function and decreases pain. Exercise, dietary modification [35], and weight loss [36] are associated with improved health-related quality of life. Lifestyle changes designed to reduce body weight in obese women decreased the concentrations of inflammatory markers including CRP, IL6, and IL-18, [37], and also implicated in the inflammatory mechanisms of RA.

In other populations, obesity affects the physical more than the mental component of HRQoL [38]. Our findings determined that HRQoL is significantly impaired in obese patients with RA, in both the physical and the mental components. Furthermore, all scores of the four PCS domains were significantly lower among obese patients than in the other groups. However, only three of the four mental domains associated with obesity had significantly lower scores (Table 2).

This study has some limitations. Although a temporal sequence is not determined due to the cross-sectional design; the fact that in other populations some interventions to target obesity also modify quality of life, suggest that impaired quality of life may be attributed, at least in part to obesity. Second, our study did not include patients with low BMI, a group associated with higher mortality [39, 40].

In conclusion, obesity is independently associated with impaired quality of life in patients with rheumatoid arthritis. Further studies are needed to determine if reducing obesity improves quality of life in patients with RA.