From Sydney West Sports Medicine, NAB RunWest Injury Management specialists; https://swsm.com.au/contact/
Effective hydration starts 2-4 hours before your run with the aim of drinking up to1 litre of water in this time. Gulping down the same amount 10-15 minutes before is much more likely to leave you with an upset stomach, drop your sodium levels increasing your risk of developing hyponatremia (water overload) or leave you needing to take mid run toilet breaks.
An early sign of dehydration is a dry mouth and becoming thirsty. Taking a small sip of water every 15-20 minutes during a run can be very helpful in staying hydrated. It only has to be a small amount just to stop the mouth from getting dry. Many runners these days are carrying small hydration packs (backpacks, belts) which allow you to keep your hands free. The need to run with a water bottle/hydration pack will depend on how long you plan to run for and your level of fitness. Another option is to make sure you pass a playground or oval with a water fountain.
Weighing yourself before and after a run is an effective way at determining how much fluid you have lost whilst exercising. It is normal to lose 1-2% of your body weight following a run however any more than this and you are likely not drinking enough fluid. Make sure you drink 1.5 times the weight difference after a run to rehydrate.
When exercising runners can lose around 1 gram of salt per litre of sweat. Unfortunately for many this can result in cramps. A sports drink, gel or salt tablet can be an effective solution to increase your electrolyte levels. This can be taken before, during or after your run. Sports drinks should not solely replace water but can be used in addition to it.
Dehydration develops over time. When runners collapse it usually means they have been ignoring their body’s warning signs. If you are developing a headache, extreme thirst, muscle cramps or dizziness, STOP and seek help. When rehydrating take small, regular sips of water so as not to overload your stomach. At its worst, dehydration can be life threatening.