Nocturnal polyuria is the most common cause of nocturia, but its pathology is not yet understood and its treatment is often difficult. Previous studies have reported that renal dysfunction is one of the factors associated with nocturnal polyuria. However, the cause and effect relationship between renal dysfunction and nocturnal polyuria remains to be elucidated. We hypothesized that a decrease in renal function leads to a carryover of salt excretion into the night time and causes an increase in night-time urine output. The aims of this study were to determine the effect of decreased renal function on circadian changes in salt excretion and urine output, and to determine the cause and effect relationship between renal dysfunction and nocturnal polyuria.

Thirty-nine patients who underwent nephrectomy at our hospital between December 2018 and March 2020 were included in the study. Two days before and seven days after surgery, blood tests and 24-hour urine storage tests were performed to assess renal function, salt excretion, and urine output. Urine was collected every 12 hours from 10 o’clock to 22 o’clock (day-time urine) and from 22 o’clock to 10 o’clock (night-time urine), and salt excretion and urine volume were measured during the day time and night time, respectively. According to each parameter, night-time urine output rate (night-time urine volume/daily urine volume) and night-time salt excretion rate (night-time salt excretion/daily salt excretion) were calculated. Renal function, day-time and night-time salt excretion, and night-time urine rate were compared before and after nephrectomy.

The median age was 65 years, and the genders were 21 males and 18 females. The primary disease was a living kidney transplant donor in 27 cases, renal cancer in 10 cases, and renal pelvic cancer in 2 cases. Comorbidities were hypertension in 15 cases, dyslipidemia in 5 cases, and diabetes mellitus in 3 cases. The pre- and post-operative eGFR (mL/min/1.73m2) were 71.6 ± 2.27 vs. 46.3 ± 1.78 (P <0.0001), daily salt excretion (g/day) were 7.50 ± 0.45 vs. 6.75 ± 0.35 (P = 0.091), day-time salt excretion (g) were 4.65 ± 0.36 vs. 3.53 ± 0.24 (P = 0.0010), night-time salt excretion (g) were 3.00 ± 0.26 vs. 3.33 ± 0.21 (P = 0.229), night-time salt excretion rate (%) were 39.3 ± 2.57 vs. 49.7 ± 2.01 (P = 0.0009), and night-time urine rate (%) were 40.3 ± 2.37 vs. 46.4 ± 1.96 (P = 0.042) (Figure 1, 2) (means ± SEM, pre-operation vs. post-operation, paired t-test).

As a result of the nephrectomy, renal function was reduced by approximately 35%. With decreased renal function, day-time salt excretion decreased, night-time salt excretion rate increased, and night-time urine rate increased significantly. These findings suggest that a decrease in renal function is associated with a decrease in day-time salt excretion and an increase in night-time salt excretion rate, resulting in nocturnal polyuria.

Renal dysfunction increases night-time urine rate via increased night-time salt excretion rate.