The Hydration Sweet Spot: Importance of Aquaporins

Hydration Status in Men Working in Different Thermal Environments: A Pilot Study

“…When studying the effect of the time of day and season of the year on urine indices of hydration status, it is important to note that urine concentration (osmolality, specific gravity, color) is influenced by many factors, e.g., fluid intake, diet, exercise, ambient temperature, and the time from the previous voiding [41,45]. It should also be stressed that urinary indices of hydration status reflect the renal response to fluid homeostasis (i.e., maintenance of osmolality and plasma volume) and do not reflect real-time hydration status at a cellular level with regard to osmoregulation and volume status [41,46]. Therefore, it is suggested that, based on urine hydration markers, the most accurate assessment of hydration status can be made by examining hydration indices in the first morning urine [45].…”

Reviewing the current methods of assessing hydration in athletes

“…There are several considerations when implementing urinary hydration assessments; i) a urine sample reflects all urine in the bladder since the previous void, thus the timing since the last void will influence results [ 26 ], ii) ingesting hypotonic fluids results in water being excreted before the intracellular and extracellular fluids equilibrate, this can result in erroneous urine results indicating euhydration [ 2 , 52 ], iii) when assessing acute dehydration and rehydration, urine variables poorly correlate to more robust measures such as P OSM due to hormonal changes during rehydration influencing the reabsorption of water and electrolytes [ 11 , 26 , 53 ], iv) previous research investigating the accuracy of urinary assessments is mixed, with research reporting it to be robust [ 30 , 52 , 54 ] while other research indicates the opposite [ 15 , 55 – 58 ], v) as with P OSM, there is a biological variation between individuals and hence, use of single cut-off points as opposed to comparisons with baseline measures is likely to produce erroneous results [ 5 , 28 ], vi) previous research has suggested the use of single spot measures be excluded entirely due to the large degree of potential confounding factors and questionable normative values [ 59 ], vii) the urinary excretion rate has identified as a potential confounder of concentration-based assessments which should be accounted for where possible, and, viii) urinary measures represent the renal response to fluid homeostasis and not real-time hydration status at a cellular level [ 60 ].…”

“…The process of estimating whole body sweat loss via body mass is far more complicated than one may expect. Indeed, body mass assessments may be confounded by time of day, food/fluid consumption, sweat composition, respiratory water loss, exercise-induced substrate utilisation and metabolic water production [ 22 , 60 , 77 , 78 ]. The longer the period of time between measures, the greater the difficulty in maintaining appropriate controls to ensure body mass changes relate predominantly to changes in hydration.…”