As your primary fuel source, carbohydrates are important to your body. If you hope to perform to your potential during athletic activity, you must adjust your carb intake to match your energy output -- too many carbs can lead to fat gain, and too few carbs can leave you sluggish and fatigued. That said, it is important to gradually self-adjust until you find the amount that works best for you and your workout.
Most people who exercise do it casually for the health and weight loss benefits. If you exercise up to an hour per day on most days of the week, you don't necessarily need to adjust your carb intake from the normal range, which is 45 to 60 percent of your total calories. This usually translates to between 2.3 g and 3.2 g per pound of bodyweight, so keep a food diary for a few days and record how you feel. If you notice that your carb intake is within the recommended range but you are lagging behind on your exercise routine, try tailoring your intake to your sport.
Weightlifters need protein for muscle repair and growth, but carbohydrates are important too. Any carbohydrates that your body doesn't use for immediate energy become stored as glycogen, which helps to power your workouts. If you don't eat enough carbs, your glycogen stores empty out and your body turns to fat and protein for energy. If you're constantly stressing and trying to build muscle, you want as much protein as possible to go to your muscle tissue, not toward energy production. Most weightlifters do well on a normal carb intake, but if you're feeling slow and "heavy," try increasing to 3.6 g per pound of body weight. That small increase can be enough to help you restock your energy stores. Eating the bulk of your carbs before and after a workout can help the energy be available when you need it, too.
If you're a serious runner, swimmer or cyclist, your workout is fueled almost entirely by carbohydrates. Training for two to four hours daily increases your need to between 3.2 g and 4.5 g per pound of body weight. During especially intense training cycles, as when you're working toward a particular event, your needs can increase to over 5.5 g per pound if you're working more than five hours per day. If you're "hitting the wall" too early in your sessions, try increasing your carb intake incrementally, and don't forget to eat small amounts of carbs every hour or so during your workout.
If you have an event coming up, you can do better than relying on your normal glycogen stores to get you over the finish line. Emptying and re-loading the glycogen can give you a more sustained source of energy, and may help you get that second wind during the last half of the race. Drop your carb intake to about 50 percent of your total daily calories about seven days out, then increase it to around 70 percent of your total three or four days prior to the event. Adjust your protein and fat intake to compensate for the fluctuating carb intake -- it is important you remain at your normal calorie level despite the carb level. Have a small, easily digestible meal the morning of the race, and fly toward the finish.