Individual variation in urinary sodium excretion among adolescent girls on a fixed intake

Background According to traditional understanding of sodium homesostasis, nearly all of daily sodium intake is excreted in urine, with intra-individual variability attributed to variability in dietary sodium intake and urine collection errors. Objective To analyze the variability of urinary sodium in excretion from a balance study with fixed sodium intakes. Methods Daily 24-h urine collections were assessed for sodium, potassium, and creatinine in 22 black and 13 white adolescent girls (11-15 y, BMI 15-29 kg/m2) in a randomized, crossover design with controlled diets containing either low (57 mmol/d) or high (167 mmol/d) sodium, each fed for three weeks. Results Coefficients of variation (CV) analysis indicated higher variation in urinary sodium excretion about the mean on low (vs high) sodium (40% vs 32%, p=0.02) and in black (vs white) girls (42% vs 30%, p Conclusions The high intra-individual variability in urinary sodium excretion on a fixed diet highlights the potential for substantial error in (a) using a single 24-hr urine collection to estimate an individual’s usual sodium intake and (b) relating sodium excretion from a single 24-hr collection with outcomes. Further research is warranted to understand the causes of such variation.