Protein bars and shakes are no longer just the tools of the iron-pumping trade. They have become the official snack foods of the fitness-conscious. The question is, do you need them? After water, protein is the second most abundant substance in the human body. When we digest dietary protein, enzymes break it down into little sub-units called amino acids. They are used to build and maintain body tissues (muscles, skin, organs, etc.), which are constantly breaking down under normal physiological wear and tear.

"About half of the total amount of protein in muscle tissue is broken down and replaced every 150 days," explains nutritionist Susan Kleiner, Ph.D., R.D., who works with both body builders and professional basketball players.

During exercise, you cause microscopic tears in your muscle fibers (evident in the soreness you feel the next day). Your body uses the amino acids derived from dietary protein to synthesize new muscle protein to "glue" the muscle fibers back together. "This makes the muscle fibers increase in diameter, get stronger and contract more powerfully," explains Kleiner.

So how much protein do you need if you're active?

The official Recommended Dietary Allowance (RDA) of daily protein intake for a healthy adult is 0.8 grams of protein per kilogram—or .36 grams per pound—of body weight. That works out to approximately 50 grams a day for the average (139 lb.) woman and 63 grams for the average (175 lb.) man. (To give you an idea of how those totals translate into food, a 3-ounce sirloin steak contains 26 grams of protein and an 8-ounce glass of milk contains 8 grams.)

It has been estimated that athletes such as endurance runners and serious weightlifters require somewhere between .45 grams to .73 grams of protein daily per pound of body weight. Kleiner recommends devoted weight-lifters get 1.2 grams of protein per kilogram of bodyweight (.55 grams per pound of bodyweight) and that the most intensive endurance and power-lifting athletes consume 2 grams of protein per kilogram—or .91 grams per pound—of body weight.

Of course, these high-end protein totals are not for the casual exerciser. Regularly taking in more protein than your body needs can be hard on your kidneys and the excess grams you consume each day won't get you more muscle, just more calories