''Party like a rock star.'' ''For energy that lasts.'' ''Xapp your mind, Xapp your muscles.'' ''Vitalizes body and mind.'' Amp, Monster, Adrenaline Rush, Red Bull, XAPP: Are we talking about heavy metal bands here?

No, these are the latest entries in the exploding energy-drink market, replete with fan clubs. What their devotees are cheering about are the caffeinated sodas with added vitamins, caffeine, herb extracts, supplements, and a large dose of hype.

No matter that they have a chemical sweetness and an aftertaste. Red Bull tastes like liquid bubble gum; Xapp, a ''protein energy drink'' whose second ingredient is whey protein powder (which is hard to disguise), is barely palatable. Popular among teenagers and ''20-somethings,'' these citrusy and lightly carbonated drinks are sometimes consumed before a workout, but also before tests, at parties, and even in bars (sometimes mixed with alcohol).

These companies know their target audience. A call to the company that produces Monster Energy Drink to ask about the ingredient listed as ''energy blend'' was met with this message: ''Hey Dudes, we're down in the lab mixing up the wicked brew. Leave your name and number and we'll call back. Party on!'' For the record, we called twice and they didn't call back.

A lot goes into this legal quick-fix. Besides caffeine, other ingredients include guarana extract, a natural caffeine-like stimulant derived from a Brazilian plant; taurine, an amino acid that aids the body in times of stress; and B vitamins such as niacin, pyridoxine, and pantothenic acid, which are also found in fruit and help produce energy.

But is the ''buzz'' about the power drinks really due to these added stimulants?

Joyce Dendy of Watertown, a dietitian and certified personal trainer, believes the value of these ingredients has not been proven scientifically. She wonders aloud if the drinks' success is more a matter of ''the placebo effect'' and marketing. Simply put, is it mind over matter?

Red Bull was distributed to a team of 12-year-old boys during halftime at a soccer game recently. Someone must have read the package about increased endurance and improved performance. With 80 milligrams of caffeine in an 8-ounce serving, Bull has less caffeine than an 8-ounce cup of coffee(135-250 milligrams, depending on the brand) but more than twice as much as colas, which contain about 45 milligrams of caffeine in 12 ounces. Caffeine is a diuretic that can contribute to dehydration, clearly not desirable during a sporting event.

When water went out of fashion in the mid-1960s, Gatorade was the first sports drink on the market. Propel and Powerade have since joined a long list of similar hydrating drinks that contain no caffeine. These sports drinks provide the athlete with energy and replace electrolytes lost during strenuous activity. Dendy feels there is a place for these drinks.

''Glucose and/or sucrose are often the second ingredient after water,'' she said. ''These are added to give the body the type of sugar that can be absorbed to help provide energy. Both of these sugars are absorbed quickly through the lining of the stomach and into the blood stream. It is sugars like fructose and corn syrup that have a slower absorption rate and thus stay in the stomach longer. If consumed during exercise, they can cause cramping.''

Loss of electrolytes like sodium, potassium, calcium, and magnesium can be an issue for endurance athletes. Electrolytes are important as they carry nerve impulses and aid in muscle contractions. According to Joan Buchbinder of Brookline, however, a dietitian and sports nutritionist, the average person doing a 60-minute workout in the gym is not in danger.

''Take some water and add a splash of your favorite juice,'' Buchbinder said. ''This will keep your blood sugar up.'' When asked if adding a little salt would be helpful, she responded that it is unnecessary as Americans already have too much salt in their diets.

Now that water is back in fashion, even juice and water companies have jumped in with their own enhanced drinks. Water sold with vitamins in products like Vitaminwater and in Fruit2O Plus contains fruit juice, electrolytes, vitamins, and calories. When asked about the value of these vitamin waters, Dr. Robert Stacks, head of pediatrics at Faulkner Hospital in Boston, chuckled, then asked: ''What ever happened to water?''

He explained the body needs only a certain amount of these vitamins. When it gets what it needs, it expels the rest. ''I bet a lot of people have some very strong urine,'' he said. ''Now, if only the urine could pump the iron.''

Speaking of pumping iron, a whole slew of other drinks is sold at places like GNC and The Vitamin Shoppe. Rip Force and Speed Stacks are designed to be taken before a workout and are sold as performance enhancers. Luke Thompson, a 23-year-old college student from Arlington, says these help him have a better workout. This was echoed by several buff sales guys at these two shops. One did acknowledge that the effect lasts just a short while, ''and then you crash.''

Drinks like Rip Force and Speed Stack also contain mah huang, a Chinese herb known commonly as ephedra, the dietary supplement used as a stimulant in weight loss and energy improvement that recently was banned by minor league baseball. Ephedra, which increases the heart rate and is also a diuretic, is sold legally only to those 18 or older. Sales clerks at a GNC store in the Burlington Mall said the company is beginning to take these drinks off the market due to recent bad press surrounding ephedra.

Dendy insists one cannot underestimate the importance of good nutrition, adequate hydration, and rest in performance and recovery.

''If you are not sure about a certain ingredient, find out about it on the Internet,'' she said. ''You can also consult with a dietitian.''

Kevin Russo, wrestling coach at Watertown High School, agrees. He said Americans are looking for a ''quick fix,'' when fitness is really a matter of hard work and good nutrition.

The beverage industry has a website, www.bevnet.com, that rates each of these drinks and gives nutrition information.