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Athletes and researchers could benefit from a simple and universally accepted technique to determine whether humans are well-hydrated, euhydrated, or hypohydrated. Two laboratory studies (A, B) and one field study (C) were conducted to determine if urine color (${\text{U}}^{\text{col}}$) indicates hydration status accurately and to clarify the interchangeability of ${\text{U}}_{\mathrm{col}}$, urine osmolality (${\text{U}}^{\text{osm}}$), and urine specific gravity (${\text{U}}^{\text{sg}}$) in research. ${\text{U}}_{\mathrm{col}}$, ${\text{U}}_{\mathrm{osm}}$, and ${\text{U}}_{\mathrm{sg}}$ were not significantly correlated with plasma osmolality, plasma sodium, or hemato-crit. This suggested that these hematologic measurements are not as sensitive to mild hypohydration (between days) as the selected urinary indices are. When the data from A, B, and C were combined, ${\text{U}}_{\mathrm{col}}$ was strongly correlated with ${\text{U}}_{\mathrm{hg}}$ and U„_sm. It was concluded that (a) ${\text{U}}_{\mathrm{col}}$ may be used in athletic/industrial settings or field studies, where close estimates of ${\text{U}}_{\mathrm{sg}}$ or ${\text{U}}_{\mathrm{osm}}$ are acceptable, but should not be utilized in laboratories where greater precision and accuracy are required, and (b) ${\text{U}}_{\mathrm{osm}}$ and ${\text{U}}_{\mathrm{sg}}$ may be used interchangeably to determine hydration status.

This study assessed the plasma glucose (PG) and hormonal responses to carbohydrate ingestion, prior to exercise in the heat, in a hypohydrated state versus partial rehydration with intravenous solutions. On separate days, 8 subjects (21.0 ± 1.8 years; 57.3 ± 3.7 ml · kg⁻¹ · min⁻¹) exercised at 50% V̇O_2maxin a 33 °C environment until a 4% body weight loss was achieved. Following this, subjects were rehydrated (25 ml · kg⁻¹) with either: 0.45% IV saline (45IV), 0.9% IV saline (9IV), or no fluid (NF). Subjects then ingested 1 g · kg⁻¹ of carbohydrate and underwent an exercise test (treadmill walking, 50% V̇O_2max, 36 °C) for up to 90 min. Compared to pre-exercise level (294 mg · dl⁻¹), PG increased significantly (>124 mg · dl⁻¹) at 15 min of the exercise test in all trials and remained significantly elevated for 75 min in NF, 30 min more than in the 2 rehydration trials. Although serum Insulin increased significantly at 15 min of exercise in the 45IV trial (7.2 ± 1.2 vs. 23.7 ± 4.7 μIU · ml⁻¹) no significant differences between trials were observed. Peak plasma norepinephrine was significantly higher in NF (640 ± 66 pg · ml⁻¹) compared to the 45IV and 9IV trials (472 ± 55 and 474 ± 52 pg · ml⁻¹, respectively). In conclusion, ingestion of a small solid carbohydrate load prior to exercise in the 4% hypohydration level resulted in prolonged high PG concentration compared to partial IV rehydration.

Twenty (12 male and 8 female) tennis players from two Division I university tennis teams performed three days of round-robin tournament play (i.e., two singles tennis matches followed by one doubles match per day) in a hot environment (32.2 ± $1.5{}^{°}\text{C}$ and 53.9 ± 2.4% rh at 1200 hr), so that fluid-electrolyte balance could be evaluated. During singles play, body weight percentage changes were minimal and were similar for males and females (males -1.3 ± 0.8%, females -0.7 ± 0.8%). Estimated daily losses (mmol · ${\text{day}}^{-1}$) of sweat sodium (Na⁺) and potassium (K⁺) (males, ${\text{Na}}^{+}$ 158.7, ${\text{K}}^{+}$ 31.3; females, ${\text{Na}}^{+}$ 86.5, ${\text{K}}^{+}$ 18.9) were met by the players' daily dietary intakes (mmol · ${\text{day}}^{-1}$) of these electrolytes (males, ${\text{Na}}^{+}$ 279.1 ± 109.4, ${\text{K}}^{+}$ 173.5 ± 57.7; females, ${\text{Na}}^{+}$ 178.9 ± 68.9, ${\text{K}}^{+}$ 116.1 ± 37.5). Daily plasma volume and electrolyte (Na⁺, ${\text{K}}^{+}$) levels were generally conserved, although, plasma [Na⁺] was lower (p < .05) on the morning of Day 4. This study indicated that these athletes generally maintained overall fluid-electrolyte balance, in response to playing multiple tennis matches on 3 successive days in a hot environment, without the occurrence of heat illness.