How to Hydrate Before a Half Marathon

Dehydration causes increased heart rate and limits the body's ability to cool itself. To perform your best in a half-marathon, you will need to be properly hydrated. This starts days before the race. You will need to consume the right amount of fluids while avoiding alcohol and being sure not to over-hydrate yourself.

Make sure that you are well hydrated in the days leading up to your half-marathon. You don't want to have to take in excess fluids on the day of the race, so it is important to maintain your hydration levels beforehand. The Institute of Medicine recommends that men consume approximately 3 liters of water per day and that women consume 2.2 liters of water. If you run in the days leading up to your race, weigh yourself before and after each run; consume an extra 16 ounces of fluid for each pound that you lose during the run.

Susan Powell of Runners World recommends drinking 16 to 24 ounces of fluids -- either water or a mixture of water and sports drinks -- two to three hours before a half-marathon. This will give you enough time for your body to process the water, so you shouldn't need to go to the bathroom during the race. About 30 minutes before the race begins, consume another 5 to 7 ounces of liquid. This should be your last intake of water before the race.

Alcohol is a diuretic, meaning that it increases the level of water that the body excretes through urination. This can leave you dehydrated, so some nutritionists advise you not to consume alcohol at all in the 48 hours prior to a half-marathon. If you do drink alcohol in the days leading up to the race, Kristen Wolfe Bieler of Runner's World advises that you consume 8 ounces of water for each alcoholic beverage you consume.

It's sometimes recommended that runners should drink water whenever they're thirsty, but according to the "Journal of the American Academy of Physician Assistants" this can lead to over-hydration. Over-hydration can be lethal if your blood sodium drops below healthy levels, a condition called hyponatremia. The "Journal of the American Academy of Physician Assistants" notes a case in which a woman died after consuming excessive amounts of water before and during a half-marathon.