Abstract 30: High Intra-Individual Variation in Urinary Sodium Excretion in Teenage Girls on a Fixed Sodium Intake

Abstract

Introduction: According to traditional understanding of sodium homeostasis, nearly 100% of daily sodium (Na) intake is excreted in urine, with intra-individual variability attributed to variability in dietary Na intake and non-adherence with urine collection procedures. However, a recent report from a Russian space flight simulation documented unexpectedly high day-to-day variability in urine Na excretion on a fixed intake. Objective: To analyze daily urinary Na excretion from a balance study that was conducted in black and white girls on both low and high levels of dietary Na. Methods: Sodium balance was assessed in 22 black and 13 white adolescent girls, (11-15 y, BMI 15-29 kg/m 2 ) in a randomized, crossover design with controlled diets containing either low (57 mmol/d) or high (167 mmol/d) Na, each for three weeks. Participants collected all urine in acid washed containers. Urine was pooled as 24-h samples and analyzed for sodium by atomic absorption spectrophotometry and creatinine by automated colorimetric method. Coefficients of variation (CV) for urinary Na were calculated for each teen. A mixed model was used to describe the effects of dietary Na and race. Results: The figures below give an example of the Na variability of one black teen on both the high (left) and low (right) Na diets. The horizontal line represents the Na intake. The CV analysis indicated higher variation about the mean on low (vs high) Na (40% vs 32%, p=0.02) and in black (vs white) girls (42% vs 30%, p<0.001). There was no diet x race interaction. Excretion of 50 mmol/d or less was documented on 6.6% of the days during the high Na dietary period. Conclusions: The high intra-individual variability in urinary Na excretion on a fixed diet highlights the potential for substantial error in (a) using a single 24 hour urine collection to estimate usual Na intake and (b) relating Na excretion from a single 24 hour collection with outcomes.