Severity of underweight affects the development of nontuberculous mycobacterial pulmonary disease; a nationwide longitudinal study

Regarding to known association between underweight and non-tuberculous mycobacterial pulmonary disease (NTM-PD), the underweight was simply categorized as body mass index (BMI) less than 18.5 kg/m(2), mainly because of its low prevalence. We aimed to better define the impact of BMI severity on NTM-PD development. We analysed health data from 4,332,529 individuals examined in 2009 and followed up until December 2017 to determine the incidence of NTM-PD. Based on the BMI in kg/m(2), the population was categorized into mild (17.00–18.49), moderate (16.00–16.99), and severe underweight (< 16.00) groups. Using Cox proportional-hazards analyses, hazard ratios for NTM-PD were calculated according to the severity of underweight in reference to normal BMI (18.50–22.99). Over a median follow-up of 5.6 ± 1.2 years, 6223 participants developed NTM-PD. Risk of NTM-PD increased significantly with the severity of underweight: multivariate adjusted hazard ratios (95% confidence intervals) for mild, moderate, and severe underweight were 2.34 (2.17–2.52), 3.50 (3.07–3.99), and 4.35 (3.63–5.21), respectively. In subgroup analysis, being younger (< 65 years old) or male exacerbated the effect of severe underweight on the risk of NTM-PD. This study proved that as underweight categories became more severe, the risk of NTM-PD increased proportionally.