The Effect of Hydration on Urine Color Objectively Evaluated in CIE Lab Color Space

Urine color has been shown to be a viable marker of hydration status in healthy adults. Traditionally, urine color has been measured using a subjective color scale. In recent years, tristimulus colorimetry developed by the International Commission on Illumination (CIE L * a * b * ) has been widely adopted as the reference method for color analysis. In the L * a * b * color space, L * indicates lightness ranging from 100 (white) to 0 (black), while a * and b * indicate chromaticity. a * and b * are color directions: –a * is the green axis, +a * is the red axis, –b * is the blue axis, and +b * is the yellow axis. The L * a * b * color space model is only accurately represented in three-dimensional space. Considering the above, the purpose of the current study was to evaluate urine color during different hydration states, with the results expressed in CIE L * a * b * color space. The study included 28 healthy participants (22 males and 6 females) ranging between the age of 20 and 67 years (28.6 ± 11.3 years). One hundred and fifty-one urine samples were collected from the subjects in various stages of hydration, including morning samples after 7–15 h of water deprivation. Osmolality and CIE L * a * b * parameters were measured in each sample. As the urine osmolality increased, a significant linear increase in b * values was observed as the samples became more pronouncedly yellow (τ b = 0.708). An increase in dehydration resulted in darker and significantly more yellow urine, as L * values decreased in lightness and b * values increased along the blue–yellow axis. However, as dehydration increased, a notable polynomial trend in color along the green–red axis was observed as a * values initially decreased, indicating a green hue in slightly dehydrated urine, and then increased as urine became more concentrated and thus more dehydrated. It was determined that 74% of the variance seen in urine osmolality was due to CIE L * a * b * variables. This newfound knowledge about urine color change along with the presented regression model for predicting urine osmolality provides a more detailed and objective perspective on the effect of hydration on urine color, which to our knowledge has not been previously researched.