The study aim was to assess the utility of common hydration
markers to identify mild hypertonic and isotonic hypohydration.
Fifteen males completed three randomized 48-hr trials,
where energy intake was standardized throughout. On day 1,
participants completed a hydration assessment followed by
high-intensity aerobic exercise. After exercise, they began one
of three trials. On the control trial (CON) participants consumed
adequate fluid to maintain euhydration. On the isotonic hypohydration
trial (IH) participants ingested the same fluid as on
CON, but at 0800 hr on day 3 they consumed 0.65 mg/kg of
the diuretic furosemide. On the hypertonic hypohydration trial
(HH), drinking fluids were restricted to 2ml/kg/day for 48 hr
after the exercise on day 1. After 48 hr, the participants completed
a second hydration assessment. Dehydration on HH and
IH was equal to 1.9% and 2.0% of body mass, respectively. Urinary
markers showed the best utility to identify mild hypertonic
hypohydration (Ucol: 2 ± 1, 6 ± 1, 2 ± 1; Usg: 1.008 ± 0.004,
1.028 ± 0.005, 1.011 ± 0.004; Uosm: 260 ± 136, 1043 ± 130,
404 ± 106, CON, HH, and IH, respectively; HH vs CON effect
sizes: d = 5.6, 4.4, 5.6, Ucol, Usg, and Uosm, respectively),
while plasma osmolality (Posm), saliva osmolality (Sosm), and
saliva flow rate (Sfr) showed moderate utility (Posm: 286 ± 4,
296 ± 6, 286 ± 4; Sosm: 55 ± 13, 65 ± 12, 56 ± 12; Sfr: 380 ±
240, 212 ± 173, 431 ± 310, CON, HH, and IH, respectively;
HH vs CON effect sizes: d = 1.8, 0.7, 0.8, Posm, Sosm, and
Sfr, respectively). Isotonic dehydration was poorly identified by
most hydration markers, with the exception of heart rate change
(LSΔHR) from lying to standing (LSΔHR: 14 ± 8, 19 ± 10, 26
± 12, CON, HH, and IH, respectively; HH vs CON effect size:
d = 0.6). Hydration marker utility is dependent on the type of
hypohydration. These results highlight how some commonly
used hydration markers may misdiagnose athletes and patients.