URINE OSMOLALITY AND CONDUCTIVITY AS INDEXES OF HYDRATION STATUS IN ATHLETES IN THE HEAT

Purpose: The purpose of this study was to determine a quick and easy m ethod for assessment of day-to-day hydration status in athletes in the heat. Methods: Measurement of the osmolality of the first urine sampl e of the day collected after wakening but before breakfast established a standardized collection procedure to allow day-to-day comparisons o f individuals. Results: Laboratory measurements established that a dif ference in osmolality is found when individuals are dehydrated by a mo derate extent In comparison with an euhydrated situation: the osmolali ty of the first morning urine sample of control subjects (N = 11) aver aged over 5 d was 675 (+/- 232) mosmol.kg(-1) (mean +/- SD). For subje cts who were hypohydrated by exercise followed by fluid restriction, m orning urine osmolality was 924 (+/- 99) mosmol.kg(-1) (P < 0.001, N = 11, averaged over 7 d). Field measurements from 29 athletes undertaki ng warm weather training indicated that the athletes could, with appro priate feedback, maintain a satisfactory hydration status. Athletes in weight category sports tended to record a higher morning urine osmola lity, reflecting their attempts to dehydrate: recorded values were 627 +/- 186) mosmol.kg(-1) (nonweight category sports, N = 8), 775 (+/- 2 63) mosmol.kg(-1) (boxers, N = 15) and 777 (+/- 254) mosmol.kg(-1) (wr estlers, N = 6). Results obtained with a hand-held portable conductivi ty were compared with those from measured osmolality. Conclusions: The findings suggest that such an instrument could provide athletes with reliable information as to their hydration status from measurement of the first morning urine of the day and therefore provide a quiet and e asy method for achieving an approximation of hydration status from day -to-day.