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Why are fluids important?

Fluids, especially water, serve as the medium for a cell’s biochemical reactions and facilitates a constant internal temperature. Water comprises about 55 to 65% of the entire body mass and 80% to 84% of kidney, lung, and skeletal muscle tissues. Water must be consumed by humans because:

  1. The body constantly loses water from the lungs, skin, and kidneys. Also, fluid constantly moves into and out of the blood and cells.
  2. Strenuous exercise, labor, or living in hot environments increase sweat losses to the point that daily water requirement can increase 2 to 6 times.
  3. If fluid consumption is insufficient or excessive, changes of cell volume affect metabolism, nerve conduction, transport of materials, hormone release, as well as cell growth and other functions.
  4. Increasing your dietary intake of sodium chloride (salt) or protein may increase the water requirement for
    whole-body fluid and electrolyte balance.
  5. Mild dehydration (e.g., 2% of body weight) reduces exercise performance, cognitive function, and alertness.

The Science of Urine Color

Attempting to simplify the analysis of urine, Professor Armstrong oversaw a series of experiments, beginning in 1994, testing the validity of a numbered urine color chart. The logic underlying the first study (1) proposed that virtually anyone could determine her/his hydration state, if urine color were directly proportional to the gain or loss of body water. The initial laboratory study involved developing a numbered scale of colors ranging from very pale yellow (number 1) to brownish green (number 8). This research demonstrated that urine color likely would be useful and effective during daily activities, exercise, and heavy labor.

A few years later, a second investigation (2) evaluated the effects of a large water loss (4% of body weight) on urine biomarkers. Nine subjects performed strenuous exercise in a 98°F laboratory environment, then undertook a 21-hour period of oral rehydration. Interestingly, urine color tracked dehydration as effectively as urine specific gravity, urine osmolality, plasma osmolality, plasma total protein concentration, or plasma sodium concentration.

A third investigation (3) observed hydration biomarkers of women, as they undertook 6 weeks of physical training in a hot laboratory environment. Again, urine color proved to be strongly related (i.e., as determined by statistical correlation) to hydration state. When urine color was pale yellow or straw colored, these women were well hydrated. The above research does not mean, however, that urine (or any body fluid) offers a perfect index of hydration, every hour of the day. No body fluid (urine, blood, saliva) perfectly reveals hydration status perfectly in all situations (4), because the brain responds each day to many dynamic challenges to water balance.

Take Away for You

The above investigations, research conducted in California among elderly adults (5, 6,7), and a published review of 12 potential hydration biomarkers (4), show that urine color provides a simple and valid method to monitor hydration in real time, during the course of daily activities or physical exercise. Urine color is not a substitute for sophisticated laboratory techniques, in hospitals or laboratories, but it offers anyone an opportunity to check their own body water balance, at any time of the day.


  1. Armstrong L.E., Maresh C.M., Castellani J., Bergeron M., Kenefick, R. W., LaGasse, K.E., Riebe, D. Urinary indices of hydration status. International Journal of Sport Nutrition 4:265-279, 1994.
  2. Armstrong L.E., Herrera Soto J.A., Hacker F.T., Casa D.J., Kavouras S.A., Maresh C.M. Urinary indices during dehydration, exercise, and rehydration. International Journal of Sport Nutrition 8:345-355, 1998.
  3. Armstrong L.E. Assessing hydration status: The elusive gold standard. Journal of the American College of Nutrition 26(5):575S-584S, 2007.
  4. Mentes JC, Wakefield B, Culp K. Use of a urine color chart to monitor hydration status in nursing home residents. Biological Research for Nursing 7: 197 - 203, 2006.
  5. Perrier ET, Johnson EC, McKenzie AL, Ellis LA, Armstrong LE. Assessing Hydration in Children: From Science to Practice. Annals of Nutrition and Metabolism 3:5-9, 2015.
  6. Perrier ET, Buendia-Jimenez I, Vecchio M, Armstrong LE, Tack I, Klein A. (2015, 3). Twenty-four-hour urine osmolality as a physiological index of adequate water intake. Disease Markers Epub, 2015.
  7. Guelinckx I, Frémont-Marquis AS, Eon E, Kavouras SA, Armstrong LE. (2015, 8). Urine colour change as an indicator of change in daily water intake: a quantitative analysis. European Journal of Nutrition 8:1-7, 2015.