If you are eager to learn more about how to best fuel your body for top performance, you might enjoy muscling through three pounds and 557 pages of Sports Nutrition: A Practice Manual for Professionals. The new fourth edition of this in-depth resource was recently released by SCAN, the sports nutrition practice group of the American Dietetic Association. (It is available at Shop Online.)

Although this resource book is written for primarily sports dietitians, strength coaches, athletic trainers, and other health professionals who influence an athlete's eating practices, serious athletes might also like to feast on this hard-core (but well written and relatively easy to read) book that will answer all sports nutrition questions from A to Z, including alcohol, carbs, calories, fats, fluids, protein, vegetarian diet, weight gain, zinc, and more!

To give you a taste of the information in the book, here are a few sports nutrition nuggets that might be of interest.

* The average 150-pound athlete has only 1,000 to 2,000 calories of stored carbohydrates (glycogen), but more than 80,000 to 120,000 calories of stored fat. Most of the fat is deposited in adipose tissue under the skin, but a little bit is also stored directly in the muscles and is an important source of fuel, especially during prolonged exercise. * Don't try to eat a fat-free diet! The recommended intake for athletes is about 0.5 gram of fat per lb. of body weight per day. This equates to 60 to 80 grams per day of dietary fat for athletes who weigh 120 to 160 pounds. That's 15 to 20 teaspoons of butter! Preferably, the fat comes from healthful sources: nuts, peanut butter, olive and canola oil, and avocado.

* While some fat is good, excess calories of fat are fattening. Your body easily stores excess dietary fat as body fat. That's why you want to carefully carbo-load on pasta and breads, not fat-load on Alfredo sauce, butter, cheese, or chips.

* Your body stores carbohydrates in the muscles in the form of glycogen (1,200 to 1,600 calories) and also in the liver (300 to 400 calories.); this feeds into the bloodstream (100 calories.) and fuels your brain. During hard training that depletes your muscle glycogen, you enhance your body's ability to store even more glycogen; which in turn enhances your ability to exercise for longer before "hitting the wall."

* Athletes should eat at least 2 grams of carbohydrates per pound of body weight per day. That's a minimum of 240 grams of carbs (about 1,000 calories) per day for a 120 pound woman and equates to ten pieces of fruit or five cups of cooked pasta. Athletes in hard training actually should eat four to five grams of carbs per pound. No Atkins diet here!

* Adult athletes require about 0.5 to 0.75 gram of protein per pound (1.2 to 1.7 grams of protein per kilogram). Scientific evidence suggests if you eat more than 0.8 gram of protein per pound (1.8 gm pro/kg), you'll burn the excess protein for energy. In other words, eating a very high protein diet does not result in greater muscle gain, even with intense resistance training. To bulk up, eat more overall calories so you'll have abundant energy to build muscles.

* Because eating before exercise can enhance performance, you should target: 0.5 gram carb/lb body weight 1 hour pre-exercise 1.0 gram carb/lb 2 hours pre- exercise 1.5 gram carb/lb 3 hours pre- exercise 2.0 gram carb/lb 4 hours pre- exercise.

This means, if you weigh 150 pounds, you need about 75 grams of carbohydrates--about 300 calories--of carbs one hour pre-exercise, and 1,200 calories four hours out. This tends to be far more than most athletes consume. Experiment to learn how much your body can tolerate and try to build up to this target if you currently eat less than this.

* Consuming carbs during endurance exercise can delay fatigue by 30 to 60 minutes. Target about 1 gram of carbohydrate per minute of exercise--equivalent to 240 calories of carbs per hour if you weigh about 150 pounds. That's about one quart of Powerade per hour.

* Consuming carbs as soon as tolerable after hard exercise enhances muscle glycogen replacement because-- 1) The blood flow to the muscles is faster immediately after exercise, so carbs are carried to the muscles faster; 2) The muscles are better able to take up the carbs because of increased sensitivity to insulin, the hormone that helps transport carbs into muscles. Plan to have a banana, fruit yogurt, fruit smoothie, and/or fig bars readily available.

* Both liquid and solid carbs refuel the muscles equally well, so take your choice: chocolate milk or a pasta dinner.

* While many athletes believe "thinner is better," don't try to get your body fat below five percent (men) or twelve percent (women). Each athlete has a fat percentage and body weight at which he or she performs best. Hence, you should listen to your body and take note of how you feel and perform, as opposed to forcing your body to achieve a self-selected number.

* Warning: Body fat measurements--even under research conditions-- can be plus or minus three to four percent. If you are told your body fat is sixteen percent, it might be thirteen percent or nineteen percent. Just having a different person measure your body fat can significantly alter the measurement. Use body fat measurements only as a guide and give yourself a body fat range.

* At rest, your body burns approximately 0.45 calorie per pound per hour. If you weigh 150 pounds, you burn about 70 calories per hour of bed rest, or about 1,700 calories per 24 hours of doing nothing except staying alive;

about 375 calories per hour of moderate exercise, such as brisk walking at a pace of 15 minutes per mile;

about 1,200 calories per hour of high intensity exercise, such as running at a pace of 5.5 minutes per mile;

Clearly, the harder you exercise, the more you can eat!

But take heed: hard workouts followed by naps reduce your daily calorie needs. Athletes who turn into post-exercise couch potatoes commonly reward themselves with too much food and fail to attain their desired weight goals.

Sports dietitian Nancy Clark, MS, RD, teaches active people how to eat to support their hard training. She has a private practice at Healthworks (617-383-6100), the premier fitness center in Chestnut Hill, MA.

Her best selling Sports Nutrition Guidebook ($23), Food Guide for Marathoners: Tips for Everyday Champions ($20) and Cyclist's Food Guide ($20) are available via www.nancyclarkrd.com.