Associate Professor, Obstetrics and Gynecology, Division of Female Pelvic Medicine and Reconstructive Surgery, Washington University School of Medicine, St. Louis

Dr. Ghetti reports no financial relationships relevant to this field of study.

SYNOPSIS: Researchers suggest that excessive salt intake can contribute to urinary frequency and nocturia.

SOURCE: Matsuo T, Miyata Y, Sakai H. Daily salt intake is an independent risk factor for pollakiuria and nocturia. Int J Urol 2017;24:384-389.

The objective of this study was to identify the relationship between daily salt intake and lower urinary tract symptoms. This was a cross-sectional study of patients with lower urinary tract symptoms admitted to Nagasaki University Hospital in Japan for nonurinary diagnoses. Patients with any condition that affects urinary function were excluded, including patients with a history of pelvic surgery, obvious bladder overactivity, benign prostatic hyperplasia, urethral stricture, pelvic organ prolapse, urological malignancy, and neurogenic bladder, as well as patients with end-stage renal disease or acute urinary tract infection. The main outcome was daily salt intake estimated by calculating sodium and creatinine concentrations of spot urine samples. Lower urinary tract symptoms were measured by the core lower urinary tract symptom score (CLSS) questionnaire, and average urinary volume and frequency was evaluated using a three-day frequency volume chart. Subjects included 728 participants (229 men and 499 women). Subjects were divided into a low salt intake group (L-salt group) and a high salt intake group (H-salt group) based on the median salt intake. Subjects in the L-salt group ingested < 9.2 g/day and subjects in the H-salt group ingested > 9.2 g/day. Subjects in the H-salt group were older, had higher body mass index, and were more likely to be hypertensive, but were less likely to have hyperlipidemia. Daytime and nighttime frequency, as well as diurnal and nocturnal urine volumes as measured by voiding charts, were significantly higher in the H-salt group compared to the L-salt group and were correlated with daily salt intake. Daytime frequency and nocturia as measured by CLSS were significantly higher in the H-salt group. On multivariate analyses, salt intake and hypertension were independent factors for the daytime frequency and nocturia.

COMMENTARY

Hashim et al summarized International Continence Society (ICS) recommendations on nocturia and reviewed definitions and guidelines regarding the evaluation of nocturia. The symptom of nocturia refers to a patient’s report of waking at night to pass urine. The sign of nocturia is indicated by the number of times a person awakens.

In reading the recommendations, I was intrigued by the authors’ brief summary of urine production and output volume. The ICS recommendations outlined that urine production rates increase when the following conditions exist: diuresis, natriuresis, conditions in which large quantities of products are present in the glomerular filtrate (such as poorly controlled diabetes mellitus), or renal tubule dysfunction.¹ Although my own evaluation of patients with frequency and nocturia frequently involves considerations of diuresis, fluid intake, glucosuria, or diabetic control as well as renal function, it rarely has focused on natriuresis. Natriuresis is the process of sodium excretion in the urine. Surplus salt increases sodium excretion and urine production.

To date, little data exist on the relationship between salt intake, urinary frequency, and nocturia. The study by Matsuo et al is one of the first reporting a positive relationship. The Centers for Disease Control and Prevention’s 2015-2020 Dietary Guidelines for Americans recommend that Americans consume < 2,300 mg of sodium daily and state that, on average, most Americans consume at least 3,400 mg each day.² Sodium can add up quickly, since many foods, especially processed foods such as canned goods, salad dressings, deli meats, and snacks, contribute to high amounts of sodium. Interestingly in this study, the estimated daily sodium intake was 9,200 mg daily, with the median intake 7,400 mg in the L-salt group and 11,400 mg in the H-salt group. Unlike prior studies, Matsuo et al did not rely on self-report to determine salt intake but instead used a validated calculation of salt intake based on spot urine samples. It is unclear whether U.S. estimates of salt consumption are accurate or whether the relationships found in this study hold true in patients with lower salt intake. This study was limited in being able to fully explain the relationships between voiding and salt intake. Numerous confounding factors may affect these relationships, including the relationship between hypertension and salt intake, medical comorbidities (including sleep apnea, congestive heart failure, lower extremity edema), sleep quality, and diuretic use. In addition, the authors did not collect data regarding type and quantity of fluid consumption.

The ICS recommendations regarding nocturia center around identifying patients’ symptoms and distinguishing these from bother, as well as evaluating signs of nocturia by physical examination (including post-void residual) and bladder diary. Together, these allow the distinction between 24-hour polyuria, which can result from diabetes, diabetes insipidus, or salt loss; nocturnal polyuria aggravated by sleep apnea and peripheral edema; lower urinary tract dysfunction; or sleep disturbance. Although additional studies are needed to determine the complex relationships between salt intake and daytime and nighttime voiding, when evaluating urinary frequency and nocturia, we might consider inquiring about our patients’ salt consumption alongside their fluid intake, medications, and medical comorbidities.

REFERENCES

1. Hashim H, Drake MJ. Basic concepts in nocturia, based on International Continence Society standards in nocturnal lower urinary tract function. Neurourol Urodyn 2018; Aug 2. doi: 10.1002/nau.23781. [Epub ahead of print].
2. Centers for Disease Control and Prevention. Get the Facts: Sodium and the Dietary Guidelines. Available at: https://www.cdc.gov/salt/pdfs/sodium_dietary_guidelines.pdf. Accessed Aug. 22, 2018.