A common practice in sports science is to assess hydration status using the concentration of a single spot urine collection taken at any time of day for comparison against concentration (specific gravity, osmolality, color) thresholds established from first morning voids. There is strong evidence that this practice can be confounded by fluid intake, diet, and exercise, among other factors, leading to false positive/negative assessments. Thus, the purpose of this paper is to provide a simple explanation as to why this practice leads to erroneous conclusions and should be curtailed in favor of consensus hydration assessment recommendations.
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The authors are with the U.S. Army Research Institute of Environmental Medicine, Natick, MA.