Cramps can be a horrid side effect for longer distance running in especially the heat. Cramps are not pleasant, your muscles rapidly contract and over shorten. Low potassium levels and low blood sodium are major causes of cramps, usually caused by major dehydration.
The problem while running in the heat is that you're sweating out precious fluid and also electrolytes and it is impossible to replace those fluids quickly enough while you are running. If you drink too much, your body will not be able to absorb liquid and you will often develop stitch or even worse hyponatremia where too much water dilutes the sodium in the body.
To tackle the problem of low blood sodium levels and so avoid the dreaded cramp, you need to make sure you are fully hydrated before you start to run with a healthy drink of water. A pretty simple test of your hydration level is to check if your urine is clear, then you'll know you are fully hydrated, but if it is dark then you are very dehydrated. Drink plenty in the days leading up to a long run in the heat and avoid caffeine and alcohol as these will surely delay your hydration and could even affect your performance.
During the long run take regular sips from a water bottle or the water provided at a drinks station if you are in a race, but do not gulp it down as you will only give yourself a stitch and a bloated stomach. Also it is advisable to take on board a little energy with your water; so a sports drink would be a good choice to replace those sips of water where necessary.
After a long run you will also need to start to rehydrate yourself immediately. You will also be low on electrolytes at this stage so it is advisable to consume a sports drink that contains electrolytes as well as the usual carbohydrates.
You may be one of those people that experience cramps later on in the day or at night. A good solution to this while also helping you rehydrate is to drink a large glass of tonic water before going to bed. Tonic water contains a plant extract quinine that acts as a type of muscle relaxant, making it more likely those cramps will leave you in peace. There are 27 mg of quinine per 235ml of tonic water. You can mix the tonic water with a flavor of your choice like orange juice or lemon.
A lot of health-conscious people pay attention to their diets and what foods they consume, but do not spend much time on the fluids they drink. A good health drink should be an important part of the daily nutritional needs.