Urinary sodium, table salt use, and incidence of late-onset asthma in females and males: evidence from a large prospective cohort

Background The contribution of dietary sodium to asthma development remains unclear, particularly for adult-onset disease. We examined associations between estimated 24-hour urinary sodium excretion, table salt use, and incident asthma in a large prospective cohort. Methods We analysed 420 041 adults aged 40–69 years without prior asthma. Incident asthma was ascertained through linked health records. Urinary sodium was estimated using the INTERSALT equation from spot urine samples. Frequency of adding salt at the table was self-reported. Associations were evaluated using multivariable Cox proportional hazards models with adjustment for sociodemographic, lifestyle, and clinical factors. Results Over a median 10.9-year follow-up, 11 927 participants developed asthma. Compared with the lowest quintile, the highest quintile of estimated urinary sodium was associated with increased asthma incidence (HR=1.50, 95%CI 1.41–1.59). Associations were stronger in females (HR=1.55, 1.44–1.68) than males (1.28, 1.17–1.39; p-heterogeneity=0.0011) and in individuals with low eosinophil counts <300 cells·µL −1 (1.55, 1.43–1.66) compared with those ≥300 cells·µL −1 (1.24, 1.10–1.40; p-heterogeneity=0.0017). Associations were also stronger in never smokers (1.64, 1.51–1.79) than former (1.46, 1.33–1.60) or current smokers (1.16, 1.00–1.36; p-heterogeneity=0.0008). Frequent table salt use (“always” versus “never/rarely”) was associated with higher asthma incidence (HR=1.28, 1.19–1.37), with no meaningful subgroup differences. Conclusions Higher estimated urinary sodium excretion and frequent table salt use were associated with greater asthma incidence. Stronger associations in women and individuals with low eosinophil counts suggest potential phenotype-specific susceptibility. Randomised studies are warranted to test whether sodium reduction lowers asthma risk in these subgroups.