Body mass index and the risk of infections in institutionalised geriatric patients.

The objective was to examine the effect of BMI on the incidence of various infectious diseases in institutionalised, geriatric subjects. In a retrospective cohort study we analysed medical records of 619 patients aged 75 years and older (mean age 87.6 (sd 6.4) years) who were treated in a geriatric hospital in Vienna, Austria. The total incidence rate of infection in this population was 0.80 per person-year. The most frequent infections were urinary tract infections (0.30 per person-year), followed by infections of the lower respiratory tract (0.19 per person-year), diarrhoea (0.12 per person-year) and other infections (0.20 per person-year). Incidence risk ratios were obtained by a multiplicative Poisson regression model. There was a J-shaped curve in the incidence of infections recorded by BMI with a nadir at 27-28 kg/m2. Compared with the reference group with a BMI of 24-27.9 kg/m2, subjects with a lower BMI had a higher incidence rate of infections. The incidence risk ratios, adjusted for sex, age and chronic diseases, were 1.62 (95 % CI 1.21, 2.17) for those with a BMI of < 20 kg/m2 and 1.84 (95 % CI 1.40, 2.42) for those with a BMI of 20-23.9 kg/m2. However, also patients with a BMI of 28 kg/m2 and above had a higher incidence rate of infections, with an incidence risk ratio of 1.54 (95 % CI 1.07, 2.22). These results show that both underweight and obesity are associated with a higher risk of infections in institutionalised geriatric patients.