Hypothesis / aims of study
Overactive bladder (OAB) is characterized by urinary urgency and is often accompanied by an increased urinary frequency and urinary incontinence; these significantly impact the patients’ quality of life (QOL). The exact mechanisms underlying OAB development are incompletely understood. However, evidence suggests a correlation between OAB and lifestyle-related diseases (such as hypertension and diabetes); these are often attributed to unhealthy dietary habits, including excessive salt consumption. A high salt intake is known to elevate blood pressure and is associated with an increased risk of cardiovascular and kidney diseases.
Our previous study revealed a notable correlation between nocturia and salt consumption [1]; this indicated that limiting the salt intake could serve as a therapeutic approach for nocturia [2]. Furthermore, we found that individuals with an elevated salt intake exhibited exacerbated OAB, particularly urinary urgency; this suggested that moderating the salt intake might alleviate OAB symptoms [3]. Although the precise underlying mechanism remains unclear, excessive and prolonged salt intake impacts not only the systemic water balance, but potentially the bladder function. Nevertheless, a comprehensive examination of the specific threshold of salt intake volume that influences the onset of lower urinary tract symptoms (LUTS), such as OAB, is lacking. Hence, in this study, we aimed to elucidate the relationship between salt intake and LUTS including OAB.
Study design, materials and methods
We performed this cross-sectional, multicenter study on women aged ≥20 years who were newly diagnosed with OAB between January 2023 and February 2024. Women without OAB who agreed to volunteer as participants were included as controls (i.e., non-OAB group).
An OAB diagnosis was established on the basis of the Overactive Bladder Symptom Score (OABSS) questionnaire results: OAB is characterized by scores of ≥2 for question (Q) #3 (urinary urgency) and a total score of ≥3. We estimated the salt intake volume using spot urine samples. Following the establishment of the association between OAB presence or absence and the estimated salt intake, utilizing the cutoff value derived from the receiver operating characteristic (ROC) curve, the participants were categorized into two groups based on their salt intake. Subsequently, a comprehensive comparison of LUTS, including OAB severity, was performed between these two groups.
LUTS were subjectively assessed using the OABSS and the International Prostate Symptom Score (IPSS). In addition, we evaluated the voided volume and maximum flow rate using uroflowmetry and the post-void residual urine volume measured by ultrasound sonography as objective findings. Univariate and multivariate analyses were performed to investigate age, hypertension, diabetes, and salt intake as risk factors for OAB. P-values of <0.05 were considered statistically significant.
Results
Among the 142 participants included in the study, 60 (42.3%) were diagnosed with OAB. The average age was significantly higher in the OAB group than in the non-OAB group (70.2±12.0 years vs. 63.7±14.9 years, P=0.007). Additionally, the salt intake was higher in the OAB group than in the non-OAB group (9.79±2.30 g vs. 8.83±2.74 g; P=0.003).
The salt-intake cutoff value associated with OAB (determined using ROC curve analysis) was 9.40 g (sensitivity: 0.683, specificity: 0.671, area under the curve: 0.702; P=0.013). Based on this, the participants were divided into the excessive salt intake group (group H, salt intake: ≥9.40 g; n=66) and the low salt intake group (group L, salt intake: <9.40 g; n=76). Overall, 40 (60.6%) and 20 (26.3%) participants met the diagnostic criteria for OAB in groups H and L, respectively (P<0.001).
OABSS revealed that compared with group L, group H exhibited significantly higher scores for the following: urinary urgency (Q3; 1.1±1.6 vs. 2.1±1.6; P<0.001), urgency incontinence (Q4; 0.8±1.5 vs. 1.7±1.8; P<0.001), and total score (4.1±3.8 vs. 6.2±3.7; P<0.001). IPSS revealed that the scores for the following voiding symptoms were significantly higher in group H than in group L: incomplete emptying (Q1; 1.3±1.6 vs. 0.7±1.2; P=0.009), intermittency (Q3; 0.9±1.5 vs. 0.5±1.2; P=0.014), and weak stream (Q5; 1.5±1.7 vs. 0.9±1.5; P=0.011). Additionally, a significant impact of excessive salt intake on the participants' QOL was noted, evidenced by the significantly higher IPSS-QOL in group H than in group L (4.0±1.7 vs. 3.2±2.0; P=0.009).
Regarding objective findings, the voided volume was significantly lower in group H than in group L (153±95.3 mL vs. 212.1±89.4 mL; P=0.016). Additionally, the maximum flow rate was significantly lower in group H than in group L (14.3±7.2 mL/s vs. 22.1±8.1 mL/s; P=0.022); furthermore, the post-void residual urine ratio was higher in group H than in group L (15.4±6.8% vs. 11.7±5.3%; P=0.034).
Both univariate and multivariate analyses revealed elevated salt intake (>9.40 g) as an independent risk factor for OAB (odds ratio: 8.53; 95% confidence interval: 3.51–22.96; P<0.001). It persisted as a significant risk factor for OAB (P<0.001) even after adjusting for age, hypertension, and diabetes in propensity score matching.
Interpretation of results
In this study, we elucidated the impact of salt intake on OAB occurrence. The mean age and salt intake were significantly higher in the OAB group than in the non-OAB group. Analysis based on a salt intake cut-off value of 9.40 g showed that both the incidence and severity of OAB were significantly higher, and patients' quality of life (QOL) was significantly worse in group H than in group L. Moreover, salt intake was affirmed as an independent risk factor for OAB in both multivariate analysis and propensity score matching. These findings underscore the importance of dietary guidance in OAB prevention and management. Excessive salt consumption affected not only urinary storage symptoms (such as OAB), but also voiding symptoms (such as residual urine sensation and weak urinary stream). These findings were further corroborated by objective findings. Future research endeavors should delve into a comprehensive investigation of the relationship between voiding symptoms and salt intake.