A consistent log-linear relationship between tuberculosis incidence and body mass index

BACKGROUND: Low weight for height is an established risk factor for tuberculosis (TB), and recent studies suggest that overweight is a protective factor. No previous systematic review has been done to explore the consistency and establish the gradient of this apparent 'dose-response' relationship. METHODS: A systematic literature review was carried out to identify cohort studies that collected data on weight and height at baseline and that used a diagnosis of active TB as the study outcome. Weight-for-height measures used in the original studies were transformed into body mass index (BMI). Exponential trend lines were fitted to each data set. RESULTS: Six studies were included. In all of them, there was a log-linear inverse relationship between TB incidence and BMI, within the BMI range 18.5-30 kg/m(2). The average slope gave a reduction in TB incidence of 13.8% 95% confidence interval 13.4-14.2 per unit increase in BMI. The dose-response relationship was less certain at BMI 30 kg/m(2). CONCLUSION: There is a strong and consistent log-linear relationship between TB incidence and BMI across a variety of settings with different levels of TB burden. More research is required to test the relationship at very low and very high BMI levels, to establish the biological mechanism linking BMI with risk of TB and to establish the potential impact on the global TB epidemic of changing nutritional status of populations.