About an hour before the workout you should have about 20 ounces of liquid.
-Leslie Bonci quoted in a NY Times article.
MY COMMENT: Unless you are preparing for a marathon or getting ready for a very long (2 hours or longer) training run, I don't know anybody who really does this.
Read the full article, then see my comments below:
First, I’d be very uncomfortable drinking 20 oz of water one hour prior to exercise, and that much certainly isn’t necessary for a 30 minute workout. Second, I’d be curious what the definition of “gulping” and what frequency is recommended. Better advice would be to weigh yourself before and after every workout and using trial and error target not losing more than 3% of your body weight. Some experts agree that blanket guidelines for hydration are not a good idea- this much water for a 100 lb female with a low sweat rate running on a cool day in fact may be very dangerous. Third, I’d be careful what type of carbohydrate you eat 1 hour prior to exercise. If I eat something high on the glycemic index prior to exercise often times my blood sugar drops so low after start my run that I experience light-headedness