The findings of our study suggest an increased prevalence of overweight and obesity among primary care patients with OA. Furthermore, the burden of OA increased with the BMI and thus confirmed our hypothesis that QoL of OA patients is inversely correlated with the BMI. QoL of patients with OA is mainly determined by pain and physical disability. As our results show, pain as well as physical disability increased with patients' weight. In respect to QoL, patients with OA can be compared to primary care patients in general, as Sach et al. assessed health related quality of life (HRQL) with three different instruments, the EQ-5D, the EQ-VAS and the SF-6D, and also found obesity to be associated with lower HRQL .
Bramlage et al. found a prevalence of 37.9% of overweight persons and 19.4% of obese persons among all primary care attendees in Germany . Rates of overweight/obesity increased steadily with the number of comorbid conditions and were highest in patients with diabetes (43.6/36.7%) and hypertension (46.1/31.3%), followed by patients with cardiovascular disorders. With 41.1% overweight and 33.1% obese patients, the prevalence rates we found for patients with OA are significantly higher. Similar results in a cross-sectional study were found by Wannamethee et al. who showed that the prevalence of CV risk factors and morbidity, disability and medication use increased significantly with increasing overweight.
Obese patients were more likely to be referred to a specialist and received significantly more x-rays than non-obese OA patients. Regarding encounters with GPs, the initially significant difference disappeared after adjusting for the number of comorbidities. It can be discussed if this adjustment is appropriate since many of the comorbidities were associated with obesity. However, the focus of this study was OA-related HSU. Nevertheless, the revealed HSU patterns are in line with other findings showing that increase in body weight is associated with increase in medical care costs compared to weight maintenance .
The positive effects of PA on the QoL and wellbeing but also on the course as well as on the symptoms of OA has been shown in multiple studies . Especially for patients with OA in the knee, strengthening the musculus quadriceps femoris can reduce pain and slow down the progress of OA most probably mediated by increased stability to the joint [29, 30]. Even though a causality can not be assessed due to the cross-sectional design of the study, our results, showing that obese OA patients have a highly significantly reduced physical activity, emphasize the need for life style counselling .
Obese patients in our study were significantly more limited in functional disability than non-obese patients. This finding may be due to two different reasons: First of all, the findings regarding perceived pain suggest that these patients simply suffer from more pain that limits functional ability. Secondly, muscle strength, especially the m. quadriceps femoris has been shown to be of great importance for stability of the knee and the incidence, progress and symptoms of OA. As Zoico et al. could show, a high BMI and high body fat were associated with greater probability of functional limitation . The skeletal muscle index (SMI) was the strongest predictor for functional disability of patients (without OA).
Prior research has shown that the prevalence of depression and depressive mood among OA patients is increased compared to the normal population of the same age . Physical limitation (especially to the lower body), pain and social contacts were revealed as most important predictors for a clinically relevant depressive disorder (minor or major depression). Interestingly, overweight was not associated with a higher PHQ-9 score (compared to normal weight) but obesity was. This is an important finding since prior research showed that there is some kind of bidirectional relationship between functional disability and depression: although functional disability can lead to depression, depression has a detrimental effect on physical mobility .
The association between obesity and depression has been assessed in a number of studies, including longitudinal studies. Results suggest that obesity predicts later depression . Our data, showing that obese patients have significantly higher PHQ-9 scores, are in line with these findings.
Some weaknesses of our study have to be acknowledged: Our data are not able to assess causality of the association between QoL, physical activity and the BMI. But they confirm the relationship in a large sample of primary care patients and emphasize the influence of obesity on QoL, PA and HSU. Despite the study's weaknesses, to our knowledge, it is the largest study so far assessing the association of BMI, QoL and HSU in primary care patients with OA.