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. 2016 Jul;104(1):49-57.
doi: 10.3945/ajcn.116.132951. Epub 2016 May 25.

Ultra-long-term human salt balance studies reveal interrelations between sodium, potassium, and chloride intake and excretion

Anna Birukov  1 , Natalia Rakova  2 , Kathrin Lerchl  1 , Rik Hg Olde Engberink  3 , Bernd Johannes  4 , Peter Wabel  5 , Ulrich Moissl  5 , Manfred Rauh  6 , Friedrich C Luft  7 , Jens Titze  8
Affiliations

Ultra-long-term human salt balance studies reveal interrelations between sodium, potassium, and chloride intake and excretion

Anna Birukov et al. Am J Clin Nutr. 2016 Jul.

Abstract

Background: The intake of sodium, chloride, and potassium is considered important to healthy nutrition and cardiovascular disease risk. Estimating the intake of these electrolytes is difficult and usually predicated on urine collections, commonly for 24 h, which are considered the gold standard. We reported on data earlier for sodium but not for potassium or chloride.

Objective: We were able to test the value of 24-h urine collections in a unique, ultra-long-term balance study conducted during a simulated trip to Mars.

Design: Four healthy men were observed while ingesting 12 g salt/d, 9 g salt/d, and 6 g salt/d, while their potassium intake was maintained at 4 g/d for 105 d. Six healthy men were studied while ingesting 12 g salt/d, 9 g salt/d, and 6 g salt/d, with a re-exposure of 12 g/d, while their potassium intake was maintained at 4 g/d for 205 d. Food intake and other constituents were recorded every day for each subject. All urine output was collected daily.

Results: Long-term urine recovery rates for all 3 electrolytes were very high. Rather than the expected constant daily excretion related to daily intake, we observed remarkable daily variation in excretion, with a 7-d infradian rhythm at a relatively constant intake. We monitored 24-h aldosterone excretion in these studies and found that aldosterone appeared to be the regulator for all 3 electrolytes. We report Bland-Altman analyses on the value of urine collections to estimate intake.

Conclusions: A single 24-h urine collection cannot predict sodium, potassium, or chloride intake; thus, multiple collections are necessary. This information is important when assessing electrolyte intake in individuals.

Keywords: aldosterone; chloride; diet; electrolyte intake; potassium; salt; sodium; urine.

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Figures

FIGURE 1
FIGURE 1
Sodium, potassium, and aldosterone excretion from a single representative subject studied daily for 205 d (descriptive statistics) in the Mars520 study (A). The panel shows 205 d of relatively stable sodium intake, with the highly variable 24-h UNaV as reported previously (1). The less-stable potassium intake, the highly variable UKV, and the highly variable 24-h UAldoV in the same representative subject are given. Power spectral analysis of the rhythmic change patterns for UNaV, UAldoV, UKV, and urinary Na+:K+ ratio in the same subject (B). UNaV had a dominant 6–8 d rhythmic change pattern, as did potassium intake, UAldoV, UKV, and the urinary Na+:K+ ratio. Day-to-day sodium intake was maintained more constantly during the Mars105 study than during the Mars520 study, whereas potassium intake was more variable during the Mars105 study than during the Mars520 study (data not shown). The data were analyzed with Fourier Transform. †,*,#Significantly different from low-salt diet [ANOVA statistic all P < 0.0001, 12 g re-exposure significantly different from 6 g salt diet (4th and 3rd salt phases, respectively)]. Mars105, simulated 105 d on Mars; Mars520, simulated 205 d on Mars; UAldoV, urinary aldosterone excretion; UKV, urinary potassium excretion; UNaV, urinary sodium excretion. Reproduced from reference 1 with permission.
FIGURE 2
FIGURE 2
Analysis for the detection of rhythmic relations between salt intake, UNaV (A), UKV (B), UClV (C), and UAldoV (A–C) for both the Mars105 and Mars520 studies. Δτ represents the phase shift in days. There was no day on which UNaV, UKV, and UClV reflected sodium, potassium, or chloride intake. In contrast, Δτ shows that UNaV and UAldoV were inversely correlated as reported previously (A). Cross-correlation relations between UKV, UClV, and UAldoV for both the Mars105 and Mars520 studies are shown (B and C); the relation was direct for UKV and inverse for UClV, very similar to the relation for UNaV. CCF, cross-correlation coefficient; Mars105, simulated 105 d on Mars; Mars520, simulated 205 d on Mars; Subj., subject; UAldoV, urinary aldosterone excretion; UClV, urinary chloride excretion; UKV, urinary potassium excretion; UNaV, urinary sodium excretion.
FIGURE 3
FIGURE 3
Although there was a correlation between daily recorded sodium intake and 24-h UNaV, the variability in the latter makes this correlation less robust than expected (A). The potassium intake was flat across the various salt intake levels (B). The correlation between recorded chloride intake and UClV was similar to that observed for sodium (statistic by linear regression) (C). Subj., subject; UClV, urinary chloride excretion; UKV, urinary potassium excretion; UNaV, urinary sodium excretion.
FIGURE 4
FIGURE 4
UNaV was tightly correlated with 24-h UClV at all sodium intake levels (A). Increasing UNaV was associated with a modest rise in UKV (B). UNaV was correlated with UNaV:UKV (C). The correlation between UClV and UKV was similar to that observed for sodium (statistic by linear regression) (D). UClV, urinary chloride excretion; UKV, urinary potassium excretion; UNaV, urinary sodium excretion.
FIGURE 5
FIGURE 5
Bland–Altman plot to test the agreement between single recorded 24-h electrolyte intake levels and single 24-h urine collection for UNaV (A), UKV (B), and UClV (C). The prediction interval to accurately predict intake by excretion is defined as ±25 mmol/d of the mean difference between intake and excretion. With a single collection, the misclassification for sodium is 51%; for potassium, the value is 34%; and for chloride, the value is 52%, similar to sodium. The red solid line indicates the regression line; the gray dotted lines are the upper and lower confidence levels (statistic by Bland–Altman). UClV, urinary chloride excretion; UKV, urinary potassium excretion; UNaV, urinary sodium excretion.
FIGURE 6
FIGURE 6
Analysis of agreement between 3 consecutively recorded sodium, potassium, and chloride intake levels and 3 consecutive UNaV, UKV, and UClV collections. Multiple collections reduce the variability and thereby improve the predictive value of UNaV, UKV, and UClV. The red solid line indicates the regression line; the gray dotted lines are the upper and lower confidence levels (statistic by Bland–Altman). UClV, urinary chloride excretion; UKV, urinary potassium excretion; UNaV, urinary sodium excretion.
FIGURE 7
FIGURE 7
Seven consecutive collections reduce the number of misclassifications of 24-h UNaV (A), UKV (B), and UClV (C) to ≤13%. The red solid line indicates the regression line; the gray dotted lines are the upper and lower confidence levels (statistic by Bland–Altman). UClV, urinary chloride excretion; UKV, urinary potassium excretion; UNaV, urinary sodium excretion.
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