Nutrition has a big impact on exercise performance—not having enough of the right foods and fluids can compromise your performance. First, the basics. When we’re talking about food for working out, there are a few components that need to be considered:

• Carbohydrates: Carbohydrates are the main fuel source for your body. They are stored as glycogen in your muscles and liver and used during exercise. Sports drinks that contain electrolytes and carbs can help to keep your energy up — but this is if you’re doing seriously intense or prolonged exercise, like running a marathon.
  
  
• Electrolytes: Electrolytes, and especially sodium and potassium, get lost through sweat. Replenishing them is important in helping you stay properly hydrated.
  
  
• Water: Water is the number one most important thing to guzzle when you’re working out. Water is necessary for every bodily function, including exercise, and you lose it when you sweat. You’ll lose even more water (and electrolytes — more on that below) if you're exercising when it's hot and humid. For the most accurate way to know you're drinking enough water, weigh yourself before and after exercise — drink 16 to 24 ounces of fluid for every pound you lost.
  
  
• Calories: Some people exercise to lose weight, some to stay in shape, some because they love it (and maybe a combination of these). If exercise is part of your weight-loss plan, you want to make sure you don’t overcompensate for a workout by splurging on more calories than you burned. On the other hand, if you’re not trying to lose weight, you want to make sure you are taking in enough calories to maintain your weight and prevent muscle loss.
  
  
• Protein: Protein is needed for muscle building and repair. It’s particularly important post-workout (more on that below).
  
  

Now for your game plan. Here’s a breakdown of what to eat before, during and after exercise:

Before: Start off your workout hydrated by drinking 2 to 3 cups of water in the three to four hours before you exercise and keep sipping as you sweat (if you work out first thing in the morning, just stay as hydrated as you can the day before and drink some water when you wake up). If you’re working out for less than an hour, water may be all you need throughout and after your workout to replenish what you’ve lost. If you’re dehydrated, you won’t perform as well. For a longer workout (or if you just don’t do well exercising on a totally empty stomach), eat something easy to digest — a meal (three to four hours beforehand) or a snack (one to two hours beforehand) that’s low in fat, low in fiber, moderate in protein and high in carbs.

Pre-workout ideas: Consider low-fat cottage cheese and blueberries, nonfat Greek yogurt with sliced banana, or a turkey sandwich in a pita pocket with cucumber, lettuce, tomato and mustard).

During exercise: For a regular (less than one hour) workout, you’ll just need water during exercise to replenish the fluids you’re losing. If your workout is longer than an hour, get some carbs (30 to 60 grams per hour) to keep your blood sugar levels up. This is especially important for endurance athletes and extreme weather conditions (heat, cold, altitude). Sports drinks, which contain electrolytes and carbs, can be helpful for these longer workouts.

After exercise: Once you’re done, you need to replace fluid and electrolytes, energy (that means calories), and carbs to replace glycogen—the reserves of energy that are stored in your muscles. The College of Sports Medicine recommends you need 0.5 to 0.7 grams of carbohydrates per pound of body weight (that’s 75 to 105 grams for a 150-pound person) during the first 30 minutes following intense, long-lasting exercise and every two hours for the next four to six hours. You also need some protein for muscle building and repair. If you’re just going for a regular, shorter workout, your next meal can give you the replenishment you need. Choose a healthy mix of carbs, protein and liquid.