Physicians' Academy for Cardiovascular Education

Single 24-hour urine collection does not suffice to estimate an individual’s sodium intake

Use of a Single Baseline Versus Multi-Year 24-Hour Urine Collections for Estimation of Long-Term Sodium Intake and Associated Cardiovascular and Renal Risk

Olde Engberink RHG, van den Hoek TC, van Noordenne ND, et al., - Circulation. 2017; Originally published June 27, 2017. https://doi.org/10.1161/CIRCULATIONAHA.117.029028

Background

It is well-established that high sodium intake increases blood pressure (BP). Since BP lowering is known to reduce CV events and kidney disease progression, it is expected that the BP lowering via a reduction in sodium intake will have CV and renal protective effects. Recent prospective cohort studies show, however, inconsistent results about the optimal sodium intake for CV and renal protection.

No adequately powered randomized controlled studies have assessed the effects of sodium reduction on CV outcomes. Observations from prospective cohort studies should be interpreted with caution, as reverse causality, residual confounding and incorrect estimation of dietary sodium intake may affect the observed associations [1,2].

Most cohort studies rely on a single collection of morning or 24-hour urine sample to estimate long-term sodium intake. While a single 24-hour urine sample is generally considered the best way to assess sodium intake in clinical practice, it may be an inaccurate representation of long-term sodium intake, due to day-to-day variation in intake, susceptibility to collection errors and potential influence of medication. Moreover, 24-hour sodium excretion has been described to differ from 24-hour sodium intake during fixed sodium intake [3,4].

This retrospective cohort study compared the value of using a single baseline 24-hour samples with using at least 1 follow-up samples for estimation of sodium intake, as well as the impact of sodium intake on long-term outcome in subjects with preserved kidney function (eGFR >60 ml/1.73m2/min). 574 Subjects were eligible and 401 had at least one follow-up measurement in the first year following inclusion and 529 had an additional measurement within 5 years. Multiple measurements in a person were averaged per time period. During a median follow-up of 12.7 years, 9776 24-hour urine collections were analyzed.

Main results

Conclusion

This study shows that, on a population level, a single baseline measurement and repetitive follow-up 24-hour urine sodium measurements lead to similar estimates of sodium intake. Estimations of individual sodium intake, however, differ significantly. This affected the association between sodium intake and long-term outcomes.

These data indicate that a single 24-hour urine sodium collection can be used to estimate average sodium intake in a population, but is imprecise for estimation of long-term individual sodium intake, and thus inadequate to study the relation between sodium intake and long-term CV or renal outcome.

Imprecise estimation of sodium intake at baseline has consequences for classification of subjects as low, moderate or high sodium consumers. Only when 1- and 5-year estimates were used, a positive correlation between sodium intake and long term CV risks was seen.

References

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Find this article online at Circulation