Impact of Female Sex on the Susceptibility to Hypernatremia Among Older Community-Dwelling Individuals in Japan

Purpose: Older individuals are at high risk for hypernatremia. However, actual data on serum sodium levels and differences between the sexes remain unclear in the older Japanese population. This study aimed to describe the data regarding serum sodium level and hypernatremia prevalence and to investigate whether female sex is associated with an increased risk of hypernatremia. Patients and Methods: We retrospectively analyzed the data of adults aged ≥65 years without severely reduced kidney function who underwent an annual health checkup in 2019. Serum sodium levels were investigated as the outcome and corrected for glucose, if necessary. Clinical characteristics were compared between women and men. Results: In the 903 participants consisting of 273 women and 630 men who were enrolled in this study, the overall prevalence of hypernatremia, defined as a serum sodium level ≥145 mmol/L, was 12.5%. Female participants showed significantly more frequent hypernatremia than male participants (17.6% vs 10.3%, p = 0.003) and higher serum sodium levels (median [interquartile range]; 143.0 [142.0, 144.0] vs 142.4 [141.5, 144.0], p <0.001). Serum creatinine (sCr), but not estimated glomerular filtration rate (eGFR), was correlated with serum sodium levels (r_s = −0.108, p = 0.001). In the binary logistic regression analysis, female sex was significantly associated with hypernatremia (odds ratio, 1.89; 95% confidence interval, 1.23–2.89; p = 0.004) even after adjusting for age, alcohol use, antihypertensive agent use, body mass index, and winter season. The association between female sex was reduced and no longer significant after adjusting for sCr, although the association remained unchanged after adjustment for eGFR. Conclusion: One-eighth of the older community dwellers in Japan exhibits hypernatremia after an overnight fast, and female sex is a significant risk factor. Since sCr is a surrogate of muscle mass, smaller muscle mass possibly mediates the association between female sex and hypernatremia.