When you lose body fluids during exercise (through sweat, respiration, etc.), your blood plasma volume decreases. Thicker blood is harder for the heart to pump, which leads to two things: reduced blood flow to your working muscles and less heat released through your skin.
Dehydration can result in lower cardiac output, less blood flow to your skin, and decreased sweat production, all of which contribute to a rise in core body temperature! As you might guess, exercising in the heat exacerbates dehydration, which puts even more strain on your body. Hotter temperatures can even shift your body’s fuel preference to burn more carbohydrates, leading to early fatigue if muscle glycogen stores become depleted.
These negative effects can be triggered by as little as 1-2% body weight loss! To make matters worse, exercise also feels harder when you’re dehydrated, as cognition and mood can be negatively affected by even a modest amount of dehydration.
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