Socio-demographics factors relating to obesity and the development and progression of arthritis in the UK

Obesity, defined as abnormal or excessive fat accumulation, is increasing globally. UK rates of adult obesity have increased by 15% since the mid-1990s and are amongst the worst in western Europe: almost one-third of the adult population now suffers from obesity. Obesity is a complex and multifactorial disease; substantial research has shown that sociodemographic factors, such as a low income, are associated with higher prevalence of obesity.

Arthritis, including osteoarthritis (OA) and rheumatoid arthritis (RA), is estimated to effect up to 10 million people in the UK and is a leading cause of work disability with an estimated cost of £7 billion per year. Similarly to obesity, OA has been linked to several socio-demographic factors, including the type and level of education, income and occupation. On top of that, people with obesity have higher rates of OA and poor disease outcomes, such as joint replacement surgery.

In contrast, studies in RA show a weaker association between socio-demographic factors and disease occurrence. However, this is not the case for smoking; research has shown that people who smoke cigarettes have higher rates of RA, but lower rates of obesity. This demonstrates that obesity may be an important mediator in the relationship between certain social determinants and the occurrence and progression of disease. The association between obesity and RA is less clear; some studies have reported obesity as a risk factor for RA, but the prognosis and severity of RA appears to be better in obese compared to non-obese patients.

The aim of this project is to understand the relationship between social determinants, obesity and arthritis using different cohorts encompassing individuals from a wide range of society, individuals in older age groups and individuals with early inflammatory arthritis. This will be achieved through the following objectives:

- to identify the association between different definitions of obesity and incident arthritis in the English Longitudinal Study of Ageing (ELSA) and Understanding Society (US)
- to define subtypes of arthritis (i.e. osteoarthritis / inflammatory arthritis) in ELSA and US based on questionnaires, available biomarkers and medications
- to investigate the mediating role of socio-demographic factors on the causal pathway between obesity and subtypes of arthritis defined in objective 2 in US and ELSA
- to identify associations between obesity and progression of disease in (i)subtypes of arthritis in US and ELSA defined in objective 2 and (ii)early inflammatory arthritis in the Norfolk Arthritis Register (NOAR)

These objectives will be addressed using different types of datasets, which contain information on socio-demographic factors, obesity and arthritis. This study will therefore use data from two nationally representative survey datasets (US and ELSA) and a longitudinal observational study of patients with inflammatory arthritis (NOAR).