Pattern of Urinary Sodium Excretion Following Consumption of a Known Quantity of Sodium
Authors: Lobene Andrea J , Martin Berdine R , Macdonald‐Clarke Claire J , Anderson Cheryl A M , McCabe Linda D , McCabe George P , Weaver Connie M 
Published: November 2023

Hypertension is a major risk factor for cardiovascular disease. Reducing sodium intake has been shown to be effective at lowering blood pressure in adults. Sodium intake in research is often assessed using 24‐hour dietary recall, though this is considered inaccurate due to underreporting and difficulties quantifying sodium content in prepared foods. The gold standard for assessing sodium intake is 24‐h urine collection. However, the cost and burden of this method may be prohibitive for both researchers and participants. Previous studies have used spot‐urine samples to predict 24‐h urinary sodium excretion in populations using prediction equations with success. Additionally, analysis of NHANES data showed weakly significant correlations (r ≤ 0.2) between spot urine samples and sodium intake estimated from 24‐h recall. No intervention studies have been conducted to assess the validity of using timed urine collections to predict sodium intake after consuming a known quantity of sodium. The objective of this ancillary study was to explore the pattern of urinary sodium excretion when consuming a known quantity of sodium in food and to assess which timed urine collection may best reflect sodium intake. Thirty‐five healthy, normotensive men and women with mean age 29.7 ± 11.2 years and BMI 24.3 ± 4.4 were fed three meals containing a total of 4.5 g of sodium in one day. All urine was collected in 2‐hour intervals during the testing period and analyzed for sodium content. Total sodium intake during the testing period was 3972 mg, and total mean urinary sodium content was 2206 mg. A repeated measures ANOVA was used to assess differences in mean urinary sodium at different time points relative to when the first meal was consumed. The data were right‐skewed and were log transformed prior to analysis. The results indicate that the quantity of sodium excreted in the urine differed significantly by sample time relative to the first meal (p < .01). A plot of the mean urinary sodium content suggests sodium excretion increases from baseline (fasting) to 4 hours after the first meal, and then remains steady the rest of the day when additional meals are consumed. This is confirmed by the follow‐up tests, which show urinary sodium is significantly higher at 2 hours than baseline (p < .01), and higher at 4 hours than 2 hours (p = .03). Sodium excretion is also significantly higher than baseline at all time points (p < .01). These results indicate that urinary sodium excretion peaks approximately 4 hours after eating a sodium‐containing meal after fasting. This suggests that sodium content in a spot urine sample collected 4 hours after a morning meal may best represent the sodium content of that meal. Further studies are needed to assess the utility of this time point for estimating sodium intake. Future studies should confirm these findings in different populations, such as children, and under a range of sodium intakes.

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