Quantifying the daily intake of water from morning and spot urine samples; retrospective analysis of a clinical trial in volunteers

Abstract

Background The hydration status can be indicated by biomarkers in the urine. However, the sensitivity and specificity of single measurements of biomarkers in morning urine and spot urine samples to quantify previous and current daily water ingestion is unclear. Methods The water content of food and liquid consumed by 20 volunteers (mean age 42 years) was calculated daily for two weeks. The volunteers increased their consumption of water by approximately 30% during the second week. They measured their excreted urine volume and sampled the morning urine and 24-h collections of urine for analysis of osmolality and creatinine during the first four days of both weeks (N = 157). The same biomarkers of hydration were measured in spot samples taken at every voiding on the other days (N = 762). Receiver operating characteristic (ROC) curves were used to study the ability of pre-specified ranges of biomarkers to quantify the water intake. Results The biomarkers in the morning urine obtained during normal fluid intake quantified the water consumption with an average area under the ROC curve (AUC) of 0.72 for osmolality and 0.66 for creatinine. Spot urine yielded an AUC of 0.74 for osmolality and 0.70 for creatinine. The AUCs obtained for days of increased fluid intake were approximately 10% lower. Large intakes (3–4 L daily) were identified with a sensitivity of 50–80% and low intakes (< 1.5–2 L) with a sensitivity of 20–50%, while false positives occurred in approximately 10%. Conclusion Biomarkers in morning urine and spot urine samples distinguished between large and small daily water intakes. Osmolality was slightly superior to creatinine. The indications were less useful during days of increased fluid intake.

A Study of Fluid Intake, Hydration Status, and Body Composition of Pregnant Women in Their Third Trimester, and Relationships with Their Infant’s Birth Weight in China: A Prospective Cohort Study

“…A total of 573 volunteers in Spain, Germany, and Greece participated in an eight-day study, which showed that TFI was negatively associated with urine specific gravity and color [59]. These basic findings were consistent with research showing that biomarkers in morning urine distinguished between high-level and low-level daily water intakes [60].…”

Characterizing Hydration Practices in Healthy Young Recreationally Active Adults—Is There Utility in First Morning Urine Sampling?

“…Notably, first morning urine (FMU) samples have been effective in differentiating habitual low-volume (LOW) and highvolume (HIGH) drinkers (Hahn, 2023;Perrier, Vergne, et al, 2013). However, whereas FMU is comparatively far less (if at all) detectably impacted by prior dietary solute load and exercise, spot urine samples in general (specific to uOsm and uSG) are not consistently responsive to short-term changes in hydration status (Edwards & Buono, 2022).…”