Variation of all-cause mortality with Fat-free Mass Index（FFMI）and Fat Mass Index（FMI）in individuals with Asthma: results from the NHANES database prospective cohort study

Abstract Background The relationship between FFMI, FMI, and mortality in patients with asthma remains unknown. This study aimed to examine the associations between FFMI and FMI and all-cause mortality in a cohort of American adults diagnosed with asthma. Methods This study included 15200 adults from NHANES. To assess mortality, we linked participant records to the National Death Index. Survival differences across quintiles of FFMI and FMI were explored using Kaplan-Meier plots and log-rank tests. The study investigated the nonlinear relationships between FFMI, FMI, and mortality. Additionally, stratified analyses were undertaken to pinpoint subgroups of individuals who exhibited an elevated risk of mortality. Results After a median follow-up, 13360 were presumed alive and 1840 had deceased. Kaplan-Meier plots revealed significant differences in all-cause mortality in patients with asthma among the FFMI quintiles and FMI quintiles. Specifically, individuals in the lowest FFMI quintile (Q1, 10.4-16.0) exhibited a markedly increased risk of all-cause mortality (HR: 4.63; 95% CI: 1.59,13.5; p< 0.01). Similarly, increased all-cause mortality risks were observed in the upper three quintiles of FMI-Q3 (4.8-6.1) with HR: 2.9 (1.20,7.00, p<0.05), Q4 (6.2-8.3) with HR: 3.37 (1.41,8.03, p<0.01), and Q5 (8.4-22.8) with HR: 4.6 (1.31,16.2, p<0.05). Moreover, the risk of all-cause mortality increased with increasing FMI and decreasing FFMI (p for non-linearity<0.001 in both cases) respectively. Subgroup analyses further elucidated these associations across different categories. Conclusions All-cause mortality is decreased in patients with asthma when FMI is less than 5.4 kg/m², and increased when FFMI is less than 18.6 kg/m².