Hydration in events exceeding on hour or in very hot locations is very important. Hydration is similar to nutrition in that you want to consume small amounts frequently. Water is lost through sweat and respiration (breathing) and must be replaced. A decrease in body fluid can have disastrous consequences. We lose water constantly through sweat and respiration as well as through excretion of waste products like urine and bowel movements. Water must constantly be replaced for survival and optimal performance.
Hydration is a measure of the water contained in your blood stream. As you get dehydrated your blood volume drops and your effective percentage of red blood cells (in that blood volume) increases. An increase in red blood cell percentage due to a decrease in overall blood plasma volume causes the blood to become thicker and harder for the heart to pump. A typical reaction to dehydration is that your heart rate will increase in order to provide the blood required for the desired effort. Core body temperature also increases when we are dehydrated which effectively increases the stress and perceived effort on the body.
A good rule of thumb is to consume between 800ml and 1200ml of fluid per hour depending on the weather conditions and your body size and sweat rate.
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