Simple SummaryEngaging in physical exertion in an elevated temperature environment leads to water–electrolyte disturbances as well as to the disruption of thermal balance and internal homeostasis, which enhances the physiological–biochemical changes associated with the physical work. In order to maintain the body’s exercise capacity, it is essential not only to hydrate the body but also to provide it with an adequate amount and quality of carbohydrates. Disturbances of thermal and fluid balance homeostasis pose a health risk, exercise performance may be reduced in the heat, and athletes are therefore likely to attempt to reduce this performance loss. The aim of this study was to evaluate the effect of different hydration strategies (isotonic, water, and no hydration) on young men’s hematological indicators, plasma volume, and blood lactate levels during prolonged physical exercise in a high ambient temperature. Water and isotonic hydration strategies allow for a better maintenance of water–electrolyte homeostasis during physical activity in a high-temperature environment, but consumption of isotonic drinks has a greater effect on the hydration of extracellular spaces with minimal changes in hematological indices.AbstractBackground: Increased internal body temperature during dehydration can be accompanied by water–electrolyte imbalances, higher levels of lactate during and after physical exertion, and changes in blood volume. Adequate hydration with carbohydrate–electrolyte fluids during physical activity can prevent dehydration and delay the onset of fatigue, allowing for proper biochemical and hematological reactions during exertion. A suitable drinking plan should consider the pre-exercise hydration level as well as the requirements for fluids, electrolytes, and substrates before, during, and after exercise. The objective of this study was to assess the impact of different hydration strategies (isotonic, water, and no hydration) on hematological indicators (hemoglobin concentration, hematocrit number, erythrocyte count, leukocyte count, and mean corpuscular volume) and lactate concentration during prolonged physical exertion in a high-temperature environment in young men. Methods: The research method was quasi-experimental. The study involved 12 healthy men aged 20.6 ± 0.9 years, who were characterized by a body height (BH) of 177.2 ± 4.8 cm, a body mass (BM) of 74.4 ± 7.6 kg, a lean body mass (LBM) of 61.1 ± 6.1 kg, and a body mass index (BMI) of 23.60 ± 0.48. Measurements were taken of body composition and hematological and biochemical indicators. The main tests consisted of three series of tests separated by a one-week break. During the tests, the men performed a 120 min exercise with an intensity of 110 W on a cycle ergometer in a thermo-climatic chamber at an ambient temperature of 31 ± 2 °C. During exertion, the participants consumed isotonic fluids or water in an amount of 120–150% of the lost water every 15 min. The participants who exercised without hydration did not consume any fluids. Results: Significant differences in serum volume were observed between the use of isotonic beverage and no hydration (p= 0.002) and between the use of isotonic beverage and water (p= 0.046). Immediately after the experimental exercise, hemoglobin values were significantly higher with no hydration than with water (p= 0.002). An even stronger significance of differences in hemoglobin was observed between no hydration and isotonic beverage consumption (p< 0.001). There was a statistically significant difference in the number of leukocytes between the consumption of isotonic beverage and no hydration (p= 0.006). Conclusions: Each active hydration strategy allows for a better maintenance of water–electrolyte homeostasis during physical exertion in a high-temperature environment, and isotonic beverage consumption had a greater impact on hydrating extracellular spaces with the smallest changes in hematological indicators.