The link between tuberculosis and body mass index.

The evidence behind the strong correlation between reactivation of tuberculosis (TB) and low body mass index (BMI) was reviewed. This strong association between TB and BMI occurred only with pulmonary TB and not extra-pulmonary TB, indicating that a low-BMI body build may in some way predispose to TB reactivation in the lungs. A possible explanation may be the congenital apical lung bullae that occur in 15% of the population and are likely to enlarge in young low-BMI males since biomechanical modeling of pleural stress has shown a massive 40x increase in apical pleural stress in low antero-posterior (AP) diameter chests associated with a low-BMI build. This suggests that pre-existing lung cavities may predispose to TB reactivation instead of current thinking that cavitation occurs after reactivation. Supporting this hypothesis is the relatively common incidence of both TB and primary spontaneous pneumothorax (PSP) occurring simultaneously. Furthermore, this hypothesis also gives a potential explanation for the apical location of secondary TB in the lower lobe as the conventional explanations of high apical oxygen levels and gravity appear to be invalid for the lower lobe. This opens up the possibility of a clinical trial that low-dose computerized tomography (CT) may be used to screen for the presence of subclinical apical bullae in low-BMI, high TB risk cohorts.