Abstract MP37: Estimating Sodium Intake Using Timed Urine Collections From a Controlled Feeding Study

Abstract

Introduction: Hypertension is a major risk factor for CVD affecting 34% of American adults. High sodium (Na) intake is identified as a modifiable risk factor for high blood pressure, but our ability to assess Na intake is poor. The gold standard is to measure Na excretion in a 24h urine collection, but this is burdensome. An alternative method is to predict 24h Na excretion from a spot urine sample. However, we do not know how well predicted 24h Na excretion reflects intake, the true outcome of interest, or the ideal time to collect a spot urine sample. These gaps can only be filled using a controlled feeding study in which intake is known and timing of urine collections is controlled. Hypothesis: We hypothesized that, with currently available equations, 24h Na excretion predicted from spot urine samples will not accurately reflect intake. Methods: This is a secondary analysis of a randomized, multi-phase, crossover, full-feeding trial in healthy adults ( n = 39, aged 29.7 ± 11.2 y). Data from the two control phases were used in which dietary intake and testing conditions were identical. Participants ate three meals in a clinic setting and spot urine samples were collected every 2 hours as part of a 24h collection. Urine samples were analyzed for sodium, potassium, and creatinine. The INTERSALT equations were used to predict 24h Na excretion from each of the 9 spot urine samples collected. Predicted 24h Na excretion values were compared with observed 24h excretion and known dietary intake using repeated measures ANOVA. Intra-class correlations for each timed spot urine sample were calculated to determine the variability in average spot urine Na excretion between the two control phases. Results: Predicted 24h Na excretion using the spot urines at hour 10, 12, overnight, and early morning collection were not significantly different from observed 24h Na excretion (all p ≥ 0.21). However, predicted 24h Na excretion underestimates observed 24h Na excretion by about >600 mg. Importantly, all predicted 24h excretions as well as observed 24h excretion were significantly lower than intake by >1000 mg (all p < 0.001). Intraclass correlations were significant at hour 0, 2, 8, 12, and evening/overnight (all p < 0.05), suggesting low day-to-day variability in Na excretion at these times. Conclusions: Regardless of time of spot urine collection, predicted 24h Na excretion is lower than Na intake, suggesting currently-available equations do not provide adequate estimates of Na intake. The next step is to develop equations to accurately predict Na intake from a controlled feeding study model using spot urine samples collected at a time when Na excretion is consistent from day to day. These improved equations could be used to implement effective dietary interventions to reduce blood pressure and CVD risk.