24-h hydration status: parameters, epidemiology and recommendations

Area of Science:

• Physiology
• Human Hydration
• Nutritional Science

Background:

• Urine osmolality (Uosm) is a key indicator of hydration status.
• Significant ontogenetic, individual, and cultural variations in Uosm exist.
• Uosm differs markedly between fetuses/infants and children/adults.

Purpose of the Study:

• To review factors influencing Uosm in different populations.
• To introduce the concept of 'free-water reserve' for quantifying 24-hour euhydration.
• To analyze hydration status in children using Uosm and free-water reserve.

Main Methods:

• Review of existing data on Uosm in infants, children, and adults.
• Analysis of urine volume and Uosm in 189 boys (DONALD Study).
• Calculation of free-water reserve based on Uosm and water intake.

Main Results:

• Intercultural differences in mean 24-h Uosm are substantial (e.g., Germany, USA, Poland).
• Gender differences in Uosm are common in industrialized countries.
• In studied boys, median free-water reserve was 11 ml/24-h, indicating adequate hydration.

Conclusions:

• Uosm and free-water reserve are valuable metrics for assessing hydration.
• Understanding variations in Uosm is crucial for public health and epidemiology.
• Further global data on Uosm and free-water reserve can inform health studies.