Abstract

Background Observational studies have found U-shaped associations between body mass index (BMI) and infections. While people with type 2 diabetes (T2D) are generally more likely to live with obesity and be at higher risk of infections, it is unknown whether BMI has the same impact on infection risk in people with and without diabetes, and whether this varies by type of infection. Methods 516,935 people with T2D and 751,909 people without diabetes, aged 18–90 and alive on 1/1/2015 with BMI measured during 2011–14 matched on age, sex and ethnicity in the Clinical Practice Research Datalink. Infections during 2015–19 were collated from primary care and linked hospitalisation records. Poisson regression estimated incidence rate ratios (IRR) for infection, across 12 BMI categories (from ≤19 kg/m2 to >48 kgm2) using a reference group of >24–≤26 kg/m2. Separate models for T2D and non-diabetes were used, adjusting for age, sex, ethnicity, deprivation, smoking and co-morbidity. Additional analyses explored associations by common infection types. Results People with T2D were at higher infection risk than people without diabetes at all BMI levels, however the pattern observed at different BMI levels was similar in both groups (e.g. BMI >48 compared to BMI >24–≤26, T2D IRR = 2.35, 95%CI 2.26–2.44, non-diabetes IRR = 2.52, 95%CI 2.30–2.75 for hospitalisation-related infections). Hospitalisation-related infections showed a U-shaped association with BMI not explained by age, smoking or co-morbidity, whereas primary care infections were predominately associated with higher BMI levels only. Cellulitis showed the strongest trends in relation to high BMI levels, whilst lower respiratory tract infection and sepsis were related to both high and low BMI levels. Conclusions Infection risks are consistently higher for people with T2D but the association with increasing levels of BMI appears similar both in patients with and without diabetes. Additionally, being underweight is also associated with increased risk of infections requiring hospitalisation.