I compare it to fueling your car for a drive:
- Gas tank 1/4 full: Drive to store 5 miles roundtrip = enough gas to get me there and back
- Gas tank 1/4 full: Drive to job 25 miles roundtrip = need $20 worth of gas to safely commute
- Gas tank 1/4 full: Drive to visit friend 100 miles one-way = must fill gas tank before trip
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Carbohydrates
As a runner, carbohydrates should make up about 60 - 65% of your total calorie intake. Without a doubt, carbs are the best source of energy for athletes. Research has shown that for both quick and long-lasting energy, our bodies work more efficiently with carbs than they do with proteins or fats. Whole grain pasta, steamed or boiled rice, potatoes, fruits, starchy vegetables, and whole grain breads are good carb sources.
- Protein
Protein is used for some energy and to repair tissue damaged during training. In addition to being an essential nutrient, protein keeps you feeling full longer, which helps if you're trying to lose weight. Protein should make up about 15% - 20% of your daily intake. Runners, especially those running long distances, should consume .5 to .75 grams of protein per pound of body weight. Try to concentrate on protein sources that are low in fat and cholesterol such as lean meats, fish, low-fat dairy products, poultry, whole grains, and beans.
More: Sports Nutrition Mistake: Not Getting Enough Protein
Fat
A high fat diet can quickly pack on the pounds, so try to make sure that no more than 20 - 25% of your total diet comes from fats. Stick to foods low in saturated fats and cholesterol. Foods such as nuts, oils, and cold-water fish provide essential fats called omega-3s, which are vital for good health and can help prevent certain diseases. Most experts recommend getting about 3,000 mg of omega-3 fat a day.
(The information above, and more, can be found here.)
Remember that many things can affect your run: weather, how much sleep you've had (or lacked!), your workload, personal situations, illness or injury, etc., so be sure to keep this in mind when you are fueling.
For example, if you're due for an early morning speed workout on Thursday and you had a great sleep, the weather is ideal and you feel great, you may not need much to fuel your tank. But the following Thursday, a similar speed workout is on your schedule, but the weather is unseasonably hot, you had a rough sleep the night prior, and you just don't feel up for it. Maybe you'll need a banana, or a slice of whole grain bread with honey...(or skip the workout all together to avoid injury)...but you see, depending on what your engine needs determines how you fuel it for optimal performance.
Have a great, safe run!
http://running.about.com/od/nutritionforrunners/Nutrition_for_Runners.htm