Along with movies, clothes and an iPod mini, dietary supplements compete for your precious cash. So where should you drop your supplement dollars?

Very little research has been conducted specifically on teenagers younger than 18 years of age, so a cautious approach is best. Teens should always discuss their intention of using sports supplements with their parents, and be aware that these useful products are made for active exercisers.

“You should be engaged in intense, serious training when using these supplements,” says Richard Kreider, PhD, director of the exercise and sport nutrition laboratory at Baylor University.

We asked Kreider and two other  noted exercise and nutrition scientists to weigh in on the most popular supplements you may be considering.

Creatine

The line: Creatine helps produce ATP, an important energy source for muscle contraction. Studies show that supplementing delays muscle fatigue in short-duration, high-intensity activities such as sprinting or resistance training, allowing for greater muscle gains.

Consider it? Yes

Know this: If you’re contemplating taking this supplement, you should be an older teen well into puberty (many of the studies on creatine are conducted on 18 and 19 year olds). In general, dosage is 20 g per day (in four doses) for five to six days during the loading phase, then an average of about 0.03 g of creatine per kilogram (or 2.2 pounds) of body weight per day. Creatine is also one of the most tested supplements on the market and has an excellent safety record. Literally thousands of studies have been performed on creatine and no ill side effects have been found.

Safety Guidelines

• Only consider supplements that are legal, have been well-studied and found effective, and appear safe (this is true for adults, too). • Get the approval and opinion of your parents, coaches, physician and/or athletic trainers.   • Practice sound overall nutrition.

Protein Powder

The line: Proteins are located in every cell and tissue of the body, and are important for the maintenance of bone, muscles, connective tissue, skin, internal organs and blood.

Consider it? Yes

MRPs And RTDs

The line: Meal replacement powders (MRPs) and ready-to-drink supplements (RTDs) usually come fortified with vitamins and minerals, and may contain nutrients designed to promote fat loss or boost performance. Protein sources may include whey, casein and soy.

Consider it? Yes

Know this: These supplements offer a convenient alternative to junky snacks, fast food or just skipping meals. They also help to give you carbs, protein and other nutrients before and after exercise when you don’t have time for a sit-down meal. “It’s better to carry around an MRP in your backpack rather than skip a meal,” says Darryn S. Willoughby, PhD, director of the exercise and biochemical nutrition laboratory at Baylor University in Waco, Texas. Just remember that these supplements should not replace a good diet.

Arginine

The line: Arginine contributes to the production of nitric oxide (NO), which promotes blood flow by causing the capillaries and small blood vessels to dilate. In theory, this increases blood flow to exercising muscles, bringing increased oxygen and nutrients.

Consider it? Not at this time

Know this: This supplement is supposed to help you feel a stronger pump when you lift and improve training adaptations, says Kreider. While the researcher has observed some positive results from these arginine-based products, Kreider feels it’s too early to tell if the supplement is effective for the relatively inexperienced teenage athlete. “You’re better off sticking to the things we know about,” he says.

Fat Burners

The line: These thermogenic supplements are designed to stimulate metabolism in order to increase calorie burning and weight loss. Caffeine, sometimes listed as guarana, tea extracts or kola nut, is the main ingredient in many fat burners.

Consider it? No

Know this: Caffeine may be an everyday substance in our society, but that doesn’t change its risk. “Caffeine is a drug, and it’s going to elevate the activ-ity of the heart and the cardiovascular system,” says Willoughby. Those unaccustomed to stimulants, like teenagers, can undergo dramatic increases in the activity of the cardiovascular system, he adds. “It will increase heart rate and blood pressure,” says Willoughby. Bottom line: steer clear of fat burners until you reach adult age.

The line: As the most abundant amino acid found in muscle glutamine plays a role in increasing cell volume as well as protein and glycogen synthesis. Supplementing before and after training may increase gains in muscle and strength. Recent research showed that those who took 5 g glutamine and 3 g of a branched-chain-amino-acid-enriched whey protein enjoyed about 2 pounds more in muscle gain and increased strength than those taking just whey.

Consider it? Yes

Know this: “Glutamine’s a very good amino acid that I like to see in a protein-carbohydrate supplement after a workout,” says Kreider. “Glutamine helps the immune system and with protein synthesis. It alone may not make you stronger or train better, but it may help you not get sick.” As for dosage, 5 g a day can help preserve muscle, while most studies indicate 8–12 g per day is needed to impact the immune system.

Multivitamin

The line: A multivitamin is a sure way to prevent vitamin or mineral deficiencies, while also delivering nutrients in dosages impossible to find in the diet.

Consider it? Yes

Know this: Although few vitamins have shown a direct impact on workouts, some may help you tolerate training by decreasing oxidative damage  or help your immune system during intense training. “When you look at the dietary records of most high school and college athletes, they’re nowhere near appropriate,” says Kreider. “A multivitamin is an insurance policy to make sure that you’re getting at least the minimum that your body needs.”