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-h 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 (hazard ratio (HR) 1.50, 95% CI 1.41-1.59). Associations were stronger in females (HR 1.55, 95% CI 1.44-1.68) than males (HR 1.28, 95% CI 1.17-1.39; p-heterogeneity=0.0011) and in individuals with low eosinophil counts <300 cells·µL 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.