Urinary Excretion of Sodium, Potassium, and Chloride, but Not Iodine, Varies by Timing of Collection in a 24-Hour Calibration Study
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Because of the logistic complexity, excessive respondent burden, and high cost of conducting 24-h urine collections in a national survey, alternative strategies to monitor sodium intake at the population level need to be evaluated. We conducted a calibration study to assess the ability to characterize sodium intake from timed-spot urine samples calibrated to a 24-h urine collection. In this report, we described the overall design and basic results of the study. Adults aged 18–39 y were recruited to collect urine for a 24-h period, placing each void in a separate container. Four timed-spot specimens (morning, afternoon, evening, and overnight) and the 24-h collection were analyzed for sodium, potassium, chloride, creatinine, and iodine. Of 481 eligible persons, 407 (54% female, 48% black) completed a 24-h urine collection. A subsample (n = 133) collected a second 24-h urine 4–11 d later. Mean sodium excretion was 3.54 6 1.51 g/d for males and 3.09 6 1.26 g/d for females. Sensitivity analysis excluding those who did not meet the expected creatinine excretion criterion showed the same results. Day-to-day variability for sodium, potassium, chloride, and iodine was observed among those collecting two 24-h urine samples (CV = 16–29% for 24-h urine samples and 21–41% for timed-spot specimens). Among all race-gender groups, overnight specimens had larger volumes (P < 0.01) and lower sodium (P < 0.01 to P 5 0.26), potassium (P < 0.01), and chloride (P < 0.01) concentrations compared with other timed-spot urine samples, although the differences were not always significant. Urine creatinine and iodine concentrations did not differ by the timing of collection. The observed day-to-day and diurnal variations in sodium excretion illustrate the importance of accounting for these factors when developing calibration equations from this study.
This article is from Journal of Nutrition 143 (2013): 1276, doi: 10.3945/jn.113.175927.