Sodium balance study in participants with chronic kidney disease

Backgroud
High sodium intake is associated with worse cardiovascular and renal outcomes, whereas the contrary is observed when potassium rich diets are consumed. Because of this, patients with kidney and cardiovascular disease are advised to limit sodium intake to 2 g/d. To monitor sodium intake, patients collect 24-hour urine in which sodium excretion is measured. This method is based on the assumption that 24-hour sodium excretion equals 24-hour sodium intake. Recent studies demonstrated that this assumption is false and that sodium can be stored in and released from a newly discovered skin compartment. The use of 24-hour potassium excretion for estimation of potassium intake, although not commonly used, has also shown to be inaccurate. As a result, dietary advices to patients based on 24-hour urine collections are inadequate. We need improved urine-based methods for estimation of dietary sodium and potassium intake, preferably with limited patient burden.

Design
This is an observational study. Participants with chronic kidney disease will receive a 14-day standardized diet, containing a fixed amount of sodium (157 mmol/day = 3600 mg/day) and potassium (85 mmol/day = 3300 mg/day). During the study all urine will be collected.

Objectives
The primary aim of this study is to assess whether repeated morning, daytime or pre-night spot urine sampling can accurately estimate dietary sodium intake and to determine the number of spot urine collections that are needed. We will assess whether using repeated spot urine collection is superior to using a single 24-hour urine collection in estimating dietary sodium intake. We will also explore this approach for potassium intake. The secondary objective is to define whether the dietary sodium-to-potassium (Na/K) ratio can be more accurately predicted than dietary sodium or potassium intake separately, by measuring the urinary Na/K ratio. If so, we will determine the number of spot and 24-hour urine collections that are needed for accurate estimation of the dietary Na/K ratio. Further, we will assess whether sodium or potassium intake estimation by 24-hour urine collection can be improved when sodium or potassium excretion is corrected for aldosterone and cortisol excretion.

We hypothesize that repeated spot urine sampling is non-inferior to a single 24-hour urine collection to estimate 24-hour dietary sodium and potassium intake.

Day -7, 0, 3, 6, 9, 12, 15, 18; however, study visits are flexible and will be scheduled based on the availability of participants

n/a