With books like Protein Power (Bantam) and Dr. Atkins' New Diet Revolution (Avon) firmly implanted on The New York Times bestseller list, and protein bars and shakes growing in popularity, more people than ever are seeking to tap into the power of protein.

But before you go on an all-out protein-blitz, how can you decide what's best for you?

The Purpose of Protein
No doubt about it, protein performs a variety of roles. First and foremost, it is used to manufacture and repair all of the body's cells and tissues, and forms muscles, skin, bones and hair. Protein makes up the connective tissue that forms the matrix of bones; keratin is a type of protein used to make hair and nails.

It is essential to all metabolic processes; digestive enzymes and metabolism-regulating hormones (such as insulin, which influences blood sugar levels) are all made of protein. This nutrient also intricately takes part in transport functions: Without sufficient protein the body cannot produce adequate hemoglobin, which carries nutrients through the blood. Lipo-proteins are fat-carrying proteins which transport cholesterol through the bloodstream.

Protein helps regulate fluid and electrolyte balance, maintaining proper blood volume. Immunoglobulins and antibodies that ward off diseases are also comprised of protein.

Any protein that you eat that is not utilized for these purposes is stored as fat, although some may be broken down, converted to glucose and burned for energy. This can occur during intensive workouts, or when the body runs out of carbohydrates from the diet or glycogen from its muscle and liver stores.

"Even though the body can depend on the fat it has stored, it still uses muscle protein, unless it is fed protein as food," explain Daniel Gastelu, MS, MFS, and Fred Hatfield, PhD, in their book Dynamic Nutrition for Maximum Performance (Avery). "When dietary circumstances cause the body to use amino acids as a source of energy, it cannot also use these amino acids for building muscle tissue or for performing their other metabolic functions."

One can see why it is so important to eat a sufficient amount of protein daily in food, shakes or bars. Without it, bone tends to break down, the immune system can become impaired, and muscle strength drops as the body uses up muscle protein for energy.

Acid Trip
Proteins are built of chains of amino acids, and 20 different kinds of these building blocks are necessary for protein synthesis within the body. Eleven of them can be manufactured by the body through a process called de novo synthesis; these are referred to as non-essential amino acids. The other nine, which must be obtained from the diet, are known as essential amino acids. (Although some amino acids are called "non-essential," in actuality they are vital: The body needs all 20 amino acids to function properly.)

Some of the more familiar non-essential amino acids include: n Carnitine helps remove fat from the bloodstream n Arginine helps burn sugar Essential amino acids include: n L-tryptophan, a precursor of the neurotransmitter serotonin, helps create calm moods and sleep patterns n L-lysine, required for the metabolism of fats n L-methionine a component of SAM-e (a supplement intended to relieve depression and arthritis, see p. 45)

The body forms and destroys protein from amino acids in a constant cycle of synthesis and degradation. You must consume protein regularly to replace the lost amino acids that are oxidized when protein is broken down and used for fuel. The amount of amino acids lost each day depends on what you eat and how much exercise you do.

Athletes vs. Weekend Warriors
Protein intake in the general population is still adequate, notes Gail Butterfield, PhD, RD, director of Sports Nutrition at Stanford University Medical School. "But we're learning that what is true for the general population may not be true for the athletic population," she says. "With heavy training there is greater protein degradation and you need to increase your intake. Thus, protein requirements are higher for athletes than regular people."

Also, if you diet or restrict your eating in any way, you may also not be getting enough protein.

Certainly, if you work out, eating protein is important. Providing four calories of energy per gram, protein keeps blood sugar steady during exercise. After exercise, it helps replenish and maintain stores of glycogen (stored muscle fuel) and decreases the loss of amino acids, as recent research has shown (J Appl Physiol 81 (5), Nov. 1996: 2095-2104). Lab studies in animals show that protein consumed after you run, lift weights, bike, etc..., helps stimulate muscle growth (Jrnl of Nut 127 [6], June 1997: 1156-1159)

High-protein diets are frequently touted to promote weight loss and increased energy. One of the most influential: the so-called 40-30-30 formula, developed by Barry Sears in his book The Zone: A Dietary Roadmap (HarperCollins), which describes a diet whose calories are 40% carbohydrates, 30% protein and 30% fat. The rationale: when you eat too many carbohydrates, your body uses these starches for energy instead of burning body fat. A high protein diet is supposed to keep your blood sugar balanced and stimulate hormones that burn body fat instead of carbohydrates for energy.

Other fitness experts such as Sherri Kwasnicki, IDEA International Personal Trainer of the Year of 1998, say that while protein is a necessary component of any diet, extreme high-protein plans aren't necessary for recreational fitness buffs. However, she notes that maintaining muscle mass is the key to aging gracefully, and getting enough protein is critical for that.

Protein Sources
Many people today won't eat meat and dairy for ethical reasons, or to avoid the antibiotics and other chemicals in the raising of poultry and cattle. But that doesn't have to prohibit adequate protein intake. All soybean products, including tofu and soymilk, provide complete proteins, which supply ample quantities of all the essential amino acids.

Vegan Power
In the past vegetarians were told to combine particular foods to make sure they consumed all the essential amino acids at each meal. (For example, beans with either brown rice, corn, nuts, seeds or wheat forms "complete" protein.) Today, diet experts aren't so picky. Eating a variety of plant-based foods throughout the day is just as effective as combining them at one meal.

Vegans who avoid all animal products should eat two servings at sometime during the day of plant-based protein sources, such as tofu, soy products, legumes, seeds and nuts.

Protein On-The-Go
The newest sources of protein are bars and shakes, which are growing steadily in popularity. Protein bars now constitute about 12% of the so-called energy bar market, with sales increasing about 38% per year. These bars generally provide at least 20 grams of protein, including soy and whey protein and calcium caseinate (milk protein). The benefits: bars supply protein along with carbohydrates for energy; protein powders, on the other hand, provide quickly digested, easily absorbed amino acids.

Edmund Burke, PhD, author of Optimal Muscle Recovery (Avery), suggests "If you need extra protein, you may benefit from the convenience of a mixed carbohydrate-protein supplement... choose a supplement that's healthy and low in fat."

Amino acid supplements are also growing in popularity, reported to build muscle and burn fat, or improve mood by boosting brain neurotransmitters. The amino acids glutamine, phenylalanine, tyrosine and 5-HTP (a form of tryptophan) are all used to boost spirits and enhance brain function.

And if you still ponder the merits of those high protein diets, do keep in mind that protein may be better at controlling hunger than carbohydrates or fat since it steadies blood sugar, so it may help you stick to a reduced-calorie plan. But excess protein can't be stored as protein in the body: It is either burned for energy or converted to fat. And carbs are still the body's top energy source, so forgoing too many can leave you tired and sluggish.

Still, with so many vital functions-and a variety of sources to choose from-you can't afford to not explore the benefits of protein.