This is a secondary analysis of a monocentric, randomized, double-blind, placebo-controlled cross-over trial in healthy adults investigating oral urea as a copeptin stimulation test for polyuria-polydipsia syndrome [10].

The study was conducted at the University Hospital Basel, Switzerland (June 2023-June 2024), approved by the local ethics committee (EKNZ 2023 − 00751), registered at ClinicalTrials.gov (NCT05890690), and conducted according to the Declaration of Helsinki. All participants gave written informed consent prior to any study procedures.

Twenty-two healthy individuals aged ≥ 18 years using no medication except hormonal contraception were included.

Exclusion criteria were evident disordered drinking habits and diuresis (polydipsia > 3 L/24 h, polyuria > 40-50mL/kg body weight/24 h), pregnancy, breastfeeding, allergies to components of the study drink, or recent investigational drug trial participation (within 30 days).

Subjects attended two morning study visits after overnight fasting and two-hours of fluid restriction to receive urea or placebo in random order, with a wash-out of at least 3 days. Participants were asked to abstain from heavy exercise, consumption of alcohol and nicotine in preparation for the study visit.

One weight-adapted dose of urea (0.5 g urea/kg body weight, 30–45 g) was dissolved in 200 ml water with lemon-lime flavored powder (0.4 g per g urea, containing maltodextrin and citric acid) to improve the bitter taste of urea. Both urea and lemon-lime flavor powder were manufactured by OMANDA AG, Switzerland.

The placebo drink, designed to mimic the taste of urea, contained bitter-tasting nutritional supplements (20 ml Ergytonyl®, 1 ml Carmol®, 1 ml Bitter Liebe®) and lemon-lime flavor in 200 ml water.

Unblinded staff members, otherwise not involved in the study prepared the study drinks, ensuring blinding of participants and investigators.

Baseline blood samples were collected ≥ 20 min after venous catheter insertion. Subjects then received the study drink to consume within 10 min, followed by 50 ml orange juice to mask the taste of urea.

Blood samples were taken 30, 60, 90, 120 and 150 min after study drink ingestion.

GH serum levels were measured at baseline, 60 and 120 min based on previously conducted studies, demonstrating GH peaks usually occur between 60 and 120 min and last about 60 minutes [2].

The central laboratory of the University Hospital Basel performed all laboratory analyses. Urea and glucose were measured immediately; samples for GH measurement were stored at − 80 °C until batch analysis and measured by electrochemiluminescence immunoassay (ECLIA, intra-assay variability coefficient: <1%, inter-assay variability coefficient: <4% [11] , Cobas8000, Roche Diagnostics GmbH, Germany).

Laboratory parameters over time were evaluated with summary statistics and visual representation of the data (box plots). Numeric data is presented as median with interquartile range (IQR), categorical data as frequency with percentage.

The main outcome was the absolute GH level difference after placebo and urea. P-values were calculated using Wilcoxon signed-rank test for paired samples to assess GH changes between urea and placebo intake. Statistical analyses were performed in R v4.4.3.