Salt intake moderates the association between BMI and cardiometabolic risk: evidence from an occupational cohort in Beijing

Objective This study aims to examine whether daily salt intake (DSI) modifies the association between body mass index (BMI) and cardiometabolic indicators in a Beijing occupational population. Methods In this cross-sectional study, 1,866 adults undergoing occupational health examinations were included. DSI was estimated from fasting morning spot urine using the Sun_C equation. Based on correlation screening, seven cardiometabolic indicators were selected for further analysis: HDL-C, uric acid (UA), ApoA1, creatinine (Cr), ALT, ApoB/ApoA1, and diastolic blood pressure (DBP). Associations were assessed using multiple linear regression and restricted cubic spline models adjusted for age and sex. Effect modification by DSI was evaluated using interaction terms and the Johnson–Neyman technique. Results Mean DSI was 9.50 ± 2.18 g. RCS analysis revealed significant non-linear associations between BMI and markers, including HDL-C, ApoA1, and UA (all P-non-linear < 0.05), with inflection points concentrated at 24.95 kg/m 2 . DSI significantly moderated the relationships between BMI and HDL-C and ApoA1, whereas the interaction for UA was suggestive but did not reach statistical significance. In the young and middle-aged population (<60 years), these effects followed distinct sex-specific threshold patterns. Specifically, the lower DSI moderating thresholds for men ranged from 13.4 g to 16.2 g (encompassing HDL-C, UA, and ApoA1), while thresholds for women were notably lower, ranging between 11.2 g and 12.4 g. When salt intake remained below these limits, the adverse effects of obesity on metabolic indicators were significant and robust. Among elderly participants, all interaction terms were non-significant ( p > 0.2). Conclusion The moderating influence of daily salt intake on cardiometabolic associations with BMI varied by age and was more evident in younger adults. The thresholds identified in this study differed by sex, at approximately 13–16 g/d in men and 11–12 g/d in women, and were higher than the WHO-recommended ceiling of 5 g/d. In practical terms, although most participants remained below these sex-specific thresholds, the metabolic burden of excess body weight was evident across the working-age adult population regardless of current sodium intake. These findings suggest that a single sodium restriction target may not fully reflect differences in metabolic context, although the present results should be interpreted cautiously in light of the cross-sectional design.