Impact of baseline body mass index on asthma incidence in middle-aged and elderly populations: A prospective analysis from China

<b>Background:</b> Asthma prevalence is rising globally, with China reporting 2.1%-5.6% rates, particularly in aging populations. Body mass index (BMI), a key measure of weight status (kg/m²), is linked to chronic diseases, yet its bidirectional role in asthma remains unclear. This study evaluated baseline BMI and the asthma risk in Chinese adults ages ≥ 45 years by focusing on underweight and obesity as dual risk factors. <b>Methods:</b> A prospective cohort of 7135 adults ages ≥ 45 years without baseline asthma was derived from the China Health and Retirement Longitudinal Study (CHARLS). Participants were categorized into BMI groups: underweight (<18.5 kg/m²), normal weight (18.5-24.9 kg/m²), overweight (25.0-29.9 kg/m²), and obese (≥30.0 kg/m²). Kaplan-Meier curves estimated the cumulative asthma incidence. Multivariable Cox regression and restricted cubic spline analyses evaluated associations. <b>Results:</b> Over 10 years, 420 participants (5.9%) developed asthma. Incidence rates increased significantly with BMI extremes: underweight (10.9%) and obese (7.3%) groups exhibited higher risks versus the normal weight (5.6%) group. Adjusted models revealed a U-shaped association: individuals who were underweight had a 74% elevated risk (hazard ratio [HR] 1.74 [95% confidence interval {CI}, 1.24-2.42]; p < 0.001), and the participants who were obese had a 39% increase (HR 1.39 [95% CI, 1.01-1.91]; p = 0.039). The overweight status showed no association (HR 0.92; p = 0.519). Restricted cubic spline confirmed nonlinearity (p < 0.05), with risks that escalated at low and high BMIs. <b>Conclusion:</b> We demonstrated a U-shaped association between BMI and incident asthma risk, with both underweight and obesity increasing the risk of asthma development. For elderly Chinese people, being underweight is a more dangerous risk factor for asthma.