Runners often struggle with dehydration, and it is a difficult factor to get right as it is heavily dependent on constantly changing intrinsic and extrinsic factors. Many will know how dehydration can affect their performance, from muscle cramping and fatigue, to heat exhaustion.
Just a 2% body mass loss of water through exercise puts a marked physiological strain on the body. During exercising in the heat, core body temperature increases by 0.12°C - 0.25°C, and heart rate increases by 3 - 5 beats per minute for every 1% of body mass lost through water. As core temperature increases and water deficit ensues, the body is making the necessary physiological adjustments to maintain heat loss when sweat rate and skin blood flow are reduced.
Greater physiological strain includes the following factors:-
increased heart rate
a decreased heart stroke volume
decreased filling time for the heart
a greater perception of effort and anticipatory regulation of pace
reduced blood volume
Increased blood concentration of salts and glucose
Given all the above factors, the body will expel more energy, become less efficient and find it hard to maintain the current level of intensity.
If you are running for more than hour, it is recommended you take on between 700ml - 1 litre of water per hour. Be also sure to replace lost electrolytes through sweat regularly, to avoid muscle cramping and fatigue.