Body Mass Index and Risks of More Than 40 Cause-Specific Mortality in Chinese Women: A Prospective Cohort Study

The health consequences of being underweight are less well-studied than obesity. We aimed to examine the association between body mass index (BMI) and mortality from a wide range of diseases across the full BMI spectrum, with particular focus on how low BMI may affect premature and nonpremature death.

After excluding ever-smokers and those with pre-existing major diseases, this study included 262 704 women aged 30-79 from the China Kadoorie Biobank. The baseline BMI (kg/m²) was calculated using measured height and weight and classified into six groups: < 18.5, 18.5-19.9, 20.0-22.4, 22.5-23.9 (reference), 24.0-27.9 and ≥ 28.0. Long-term follow-up was conducted by linking to the local death registry. Cox regression was used to estimate the hazard ratios (HRs) of BMI with all-cause and cause-specific mortality, after adjusting for waist circumference and potential confounders.

At baseline, 3.9%, 7.8% and 11.4% of participants had BMIs (kg/m²) of < 18.5, 18.5-19.9 and ≥ 28.0, respectively. During a median follow-up of 17.1 years, 29 531 deaths were recorded, including 11 455 premature deaths (under the age of 70) and 18 076 nonpremature deaths. The risk of all-cause mortality significantly increased at the lower extreme of BMI, reached a nadir around 23.5 kg/m² estimated from restricted cubic splines and showed a modest increase at the upper extreme. Participants with BMI < 18.5, 18.5-19.9 and 20.0-22.4 had higher risks of premature death (HRs) of 1.91 (1.73-2.10), 1.24 (1.14-1.34) and 1.16 (1.11-1.21), respectively. The corresponding HRs for nonpremature death were 1.46 (1.36-1.56), 1.17 (1.10-1.23) and 1.04 (1.01-1.08). The analysis of premature death involved 33 diseases from eight ICD-10 chapters, and nonpremature death involved 40 diseases from nine chapters. Underweight was linked to an increased risk of premature death in seven chapters and 10 diseases, including neoplasms (HR = 1.36, 95% CI: 1.16-1.59), endocrine-metabolic (6.03, 4.13-8.82), circulatory (1.85, 1.56-2.20), respiratory (5.85, 3.89-8.80), digestive (5.64, 3.13-10.18), genitourinary (2.61, 1.20-5.69) and external causes (2.06, 1.55-2.72). Underweight was also associated with an increased risk of nonpremature death in four chapters and seven diseases. In contrast, obesity was only associated with increased risks of premature and nonpremature death in two and one chapters, respectively.

Among Chinese female healthy never-smokers, underweight was an important risk factor for all-cause and multiple cause-specific mortality, especially the risk of premature death. While addressing the global obesity epidemic, the negative health consequences of low weight should not be ignored.