Dehydration

Even slight dehydration can cause performance to plummet, as water is lost from the bloodstream.  As your blood volume decreases, your heart has to work harder. 

Dehydration causes your blood volume to drop which in turn lowers your body’s ability to transfer heat and forces your heart to beat faster, making it difficult for your body to meet its aerobic demands.

Drink 225ml to 400ml one to two hours before a run. 

Didn’t plan ahead then drink 115ml to 225ml of fluid 15 to 30mins before you run.

Its’ also been shown that drinking icy cold fluid can help you exercise longer as its lowers body temperature and perceived effort.  So before a hot run, have a slushy made of crushed ice and perhaps your favorite sports drink.  To keep drinks chilled when you run, fill a bottle halfway, freeze it and top it off with fluid before starting.

Drink frequently – drinking smaller amounts at regular intervals can help you to absorb fluid more effectively (and avoid stomach sloshing) Set your watch to beep every 15 minutes as a reminder to consider your thirst.

which in turn lowers your body’s ability to transfer heat and forces