Long-term variability of urinary salt excretion and blood pressure in hypertensive patients

Abstract

We investigated the long-term trend and variability of urinary salt (sodium chloride) excretion in hypertensive patients. Subjects included 186 hypertensive patients (103 women and 83 men, mean age: 58.5±10.5 years) who underwent 10 successful 24-h home urine collections over a mean observation period of 7.7 years. We measured 24-h urinary salt excretion and blood pressure (BP) sequentially at the time of each collection and monitored the long-term trend and variability of urinary salt excretion. BP significantly decreased from 145±16/85±11 mm Hg to 130±12/70±11 mm Hg and was associated with an increased use of antihypertensive drugs. The 24-h urinary salt excretion also decreased from 9.5±3.6 g per day at the first measurement to 8.5±3.2 g per day at the 10th measurement. Urinary salt excretion during the observation period ranged from a minimum value of 5.2±1.8 g per day to a maximum value of 13.4±3.6 g per day with a coefficient of variation of 29.2±8.1%. When subjects were assigned to a low, medium and high salt group based on the tertiles of the first measurement of urinary salt excretion and the tertiles based on the mean value of 10 measurements during the observation period, only 56.2% remained in the same category, suggesting that a single measurement of urinary salt excretion can only predict long-term urinary salt excretion in approximately half of the individuals. In conclusion, urinary salt excretion shows large variability such that a single measurement may not be sufficient to assess salt intake in individuals.

The Science of Salt: A Systematic Review of Quality Clinical Salt Outcome Studies June 2014 to May 2015

“…Accurately assessing individual salt intake requires several days of complete 24‐hour urine collections while our criteria only required a single 24‐hour period to assess salt intake. Accurate assessment of salt intake relies on complete collections of urine over a 24‐hour period but in many studies 24‐hour urine collections are not complete . The use of adequate criteria for assessing salt intake is important as otherwise inaccurate classification of an individual's salt consumption level will predispose to the null hypothesis or potentially U ‐shaped relationships if those who are noncompliant with 24‐hour urine collection are falsely classified as consuming low amounts of salt .…”

The International Consortium for Quality Research on Dietary Sodium/Salt (TRUE) position statement on the use of 24‐hour, spot, and short duration (<24 hours) timed urine collections to assess dietary sodium intake

“…Forty‐five to fifty percent of respondents switched tertile of sodium intake when a single 24‐hour urine collection was used to estimate long‐term sodium intake vs sequential 24‐hour urine collections in a study from Amsterdam . The required number of 24‐hour urine collections to obtain a stable estimate of usual sodium intake is likely to differ with different dietary patterns, populations, and settings, and has been estimated to be at least 3 non‐consecutive days . Weaver et al found that ten 24‐hour urine collections were required to have a 75% reliability to estimate an individual's sodium intake when on a constant sodium diet and this number is likely higher when there is substantial day‐to‐day variation in sodium intake.…”

“…32 The required number of 24-hour urine collections to obtain a stable estimate of usual sodium intake is likely to differ with different dietary patterns, populations, and settings, and has been estimated to be at least 3 non-consecutive days. 27,29,[31][32][33][34][35][36][37][38][39] Weaver et al 29 found that ten 24-hour urine collections were required to have a 75% reliability to estimate an individual's sodium intake when on a constant sodium diet and this number is likely higher when there is substantial day-to-day variation in sodium intake. The strength of the association between dietary sodium and health outcomes is highly influenced by the number of urine collections.…”

Science of Salt: A regularly updated systematic review of salt and health outcomes studies (April to October 2018)

“…Incomplete urines are particularly of concern as non‐adherence even to placebo is associated with increased mortality as demonstrated in randomized controlled trials . Furthermore, due to large day‐to‐day variability of intake and excretion, a single 24‐hour urine sample is unlikely to give a valid estimate of usual intake, with at least 3 non‐consecutive days required . Additionally, sodium excretion was only measured at baseline; thus, estimates do not reflect any changes in intake over the 9‐year follow‐up period.…”

“…41,42 Furthermore, due to large day-to-day variability of intake and excretion, a single 24-hour urine sample is unlikely to give a valid estimate of usual intake, with at least 3 non-consecutive days required. [43][44][45] Additionally, sodium excretion was only measured at baseline;…”