Seasonal Variation in the Daily Urinary Sodium Excretion in Outpatients from the Morioka Region of Northern Japan

Objective Although the daily urinary sodium excretion (UNaV) is considered to provide the most reliable estimate of the daily sodium intake, it may be affected by salt loss due to sweating in summer. However, the seasonal variation in the daily UNaV associated with a normal lifestyle is unknown. Methods This study was performed in 348 outpatients from the Morioka region during three seasons: summer (summer 1), winter, and the following summer (summer 2). The daily UNaV (g salt/day) was estimated by the second morning urine method three times during each season. Seasonal variation was defined as a significant trend across the three seasons together with a significant difference between winter and both summers. Results In women, the daily UNaV was higher in winter (11.8 +/- 3.0 g salt/day) than in summer 1 (11.2 +/- 2.9 g salt/day) or summer 2 (11.0 +/- 2.9 g salt/day). In contrast, there was no marked seasonal variation in men. An analysis stratified by age (4 quartiles) identified seasonal variation in the older 2 quartiles of women (aged >= 68 years). In these women, the mean seasonal difference in the daily UNaV was 0.9 g of salt/day for both winter vs. summer 1 and winter vs. summer 2, while it was 0.1-0.8 g of salt/day in the other groups. Conclusion Seasonal variation in the daily UNaV only occurred in older female patients and was relatively small. This is evidence for restricting salt intake throughout the year and should reassure patients who are anxious about salt loss due to sweating in summer.