The engine can't run without fuel. The same goes for your body during strenuous physical activity.
Proper fueling for workouts and races is vital for optimal performance; it can mean the difference between great and mediocre results. Here are some nutrition and hydration guidelines to help you before, during and after your workouts.
Under 60 minutes
For workouts and races less than 60 minutes, there's little need for calorie replacement. Use water or a no-calorie electrolyte drink such as Propel or Nuun. Using a sports drink is not necessary for shorter workouts or races because your body has enough stored energy to sustain the effort.
Hydration needs depend on an athlete's height, weight, sweat rate, outside temperature and relative humidity. Aim to drink 16 to 24 ounces an hour (up to 32 ounces an hour for some athletes). See below on how to calculate sweat rate.
60 to 90 minutes
For workouts and races longer than 60 minutes, electrolyte replacement becomes necessary. Use a sports drink like Gatorade or Powerade during your workout to replace lost electrolytes. This is especially important for longer workout and for hot or humid environments.
Again, aim to drink 16 to 24 ounces an hour. A good rule is to drink every 10 minutes or so. Take small sips frequently during your workout instead of large gulps. Try using a handheld water bottle, FuelBelt or Camelback to carry fluids with you.
Over 90 minutes
Once your workout hits the 90-minute mark, you need to start replacing the calories being burned to fuel your body. Just like fluid needs, calorie requirements vary from athlete to athlete. Consume a total of 150 to 300 calories an hour, or 30 to 60 grams of carbohydrate an hour. These calorie counts include both what you get from food and drinks.
Trial and error is the best way to figure out your per-hour calorie needs. Experiment while training and eventually you'll learn if you have enough fuel to power your workout or if you should increase your caloric intake. There are many different food options to fuel workouts: gels, chews, bars, real food etc. Try different options and flavors to find out what works best for you.