The amount of blood your heart pumps with each beat decreases
Less oxygen-rich blood reaches your working muscles
You produce less energy aerobically
You are doomed to run at a slower pace
How much to drink?
Baseline fluid needs are approximately 4 pints per day. When you are training, you must factor in the fluid losses. By weighing yourself before and after you run, you can calculate how much to drink with the objective of bringing your weight back up.
You usually don’t retain all the fluid that you drink. To become fully hydrated you must typically drink 1.5 times the amount of weight you lost. If you lost 3 pounds during a run, you must drink 4.5 pounds of fluid during the next several hours to become fully hydrated. When you include the baseline amount of 4 pints, then your total fluid requirement becomes 8.5 pints.
During a run, runners’ stomachs can only empty about 6-7 ounces of fluid every 15 minutes. This represents about 24-28 ounces per hour. It is important to keep fluid levels steady. Over the course of a marathon, runners lose about 3.2 percent of their body weight. A loss of more than 3 percent of body weight leads to a loss of more than 9 percent in performance. The effects of progressive dehydration can have an enormous impact in a training run or a race.
Carbohydrates can help performance during training sessions lasting more than one hour. Replace carbohydrates immediately after a workout. Your body can store glycogen at a faster rate during the first hour after a training session, so it is important to have a carbohydrate drink immediately after exercise.
Alcohol causes dehydration. One drink or two may be a nice way to reduce tension or stress. It is important to take extra (non-alcoholic) fluid to make up for the dehydrating effects of alcohol. Drink an extra ounce of fluid for each ounce of beer and an extra 3 ounces of fluid for each ounce of wine.
Less blood returns to your heart