(CNN) More than two-thirds of sales attendants in vitamin stores nationwide recommended using a popular sports supplement to a caller who claimed to be a teen athlete, going against expert medical opinion, according to a study published Monday in the journal Pediatrics.

The supplement in question was creatine, a favorite of weight lifters, typically purchased in flavored powders and mixed with liquids. Because this naturally occurring compound is involved in the body's energy production, many athletes believe it helps them develop muscles faster.

The American Academy of Pediatrics and the American College of Sports Medicine recommend (PDF) that those younger than 18 do not use creatine.

"It has been estimated that perhaps upwards of 30% of high school athletes are using Creatine," said Dr. Ruth Milanaik, lead author of the study and a pediatrician at Steven and Alexandra Cohen Children's Medical Center of New York.

"In addition, the majority of teens using Creatine reported that they 'did not read the package insert' and either 'didn't know the dose' they were taking or were purposely 'taking more in order to get quicker results,' " Milanaik wrote in an email.

Recommended by two-thirds of stores

Anecdotal evidence suggests that creatine may impair liver and kidney function and cause muscle cramps, according to the authors. Creatine use may also increase the risk of "compartment syndrome," in which pressure builds in a muscle and prevents blood flow. Finally, the authors warn that dietary workout supplements, such as creatine, are not regulated by the Food and Drug Administration.

"Dietary supplements are regulated by the FDA -- albeit not in the same way as pharmaceutical drugs for example," said Lyndsay Meyer, spokeswoman for the agency. The set of regulations which guide the FDA's oversight of supplements are different from those covering "conventional" foods and drug products, said Meyer.

Still, the authors claim that some supplements are adulterated, in some cases with pharmaceutical drugs, which might lead to adverse effects.

"A young teen patient of mine had serious medical side effects," Milanaik said. "After going over his medical record, on a whim I asked about herbal supplements. He informed me that he was taking Creatine that had been recommended by his local vitamin store to help him 'prepare for the upcoming football season.' "

Her patient ended up in a hospital, diagnosed with dehydration and other issues. Milanaik, with the help of 19-year-old Princeton student researcher Maguire Herriman, decided to look at "whether this was an isolated issue or whether this was widespread."

Were other local vitamin stores routinely recommending off-limit supplements to unsuspecting teens?

Posing as a 15-year-old high school athlete seeking to increase his muscle strength, Herriman called 244 health food stores across the United States and asked the sales attendant what supplements he or she would recommend.

If a sales attendant did not mention creatine or testosterone boosters initially, Herriman would specifically ask about them. He also asked whether a 15-year-old could purchase these products in the store on his own.

"No sales person was coerced into recommendations," Milanaik said. "If a salesperson was willing to answer to our questions it can be assumed that he/she would give similar answers to actual teen customers."

Sales attendants at 164 stores (about 67% of the 244) recommended creatine. Of these, about 38% recommended creatine without prompting, and nearly 29% recommended creatine after being asked specifically about it.

Nearly 10% of sales attendants recommended a testosterone booster to the caller, though medical experts believe this too could be harmful for teens.

Although it is not illegal to sell these products to teens, many have labels that say they are recommended only for adults.

"Not only did we find that health food stores were making recommendations contrary to medical opinion, we found that they were actually ignoring age restriction warning labels on the product packages," Milanaik said. She and her co-authors found that just over 74% of sales attendants stated a 15-year-old was allowed to purchase creatine, while slightly more than 41% said the same-age teen could purchase a testosterone booster.

An individual matter

Richard Kraus, owner of Health Unlimited, a health and natural supplements store based in Atlanta, says he has never encountered this issue with his own customers.

"I talk to a handful of people with athletic teenagers, but that's never come up for that age group," Kraus said. He added that, in general, older teens -- over the age of 17 -- are more likely to purchase supplements or ask for advice. When that happens, his sales associates steer them to Kraus, since he is more knowledgeable.

"I've read things and talked to different trainers," Kraus said. He's been in the business for many years but admits, "I'm not a scientist, not a doctor, so I haven't studied it in a lab or reviewed a lot of studies."

Still, the creatine products on his shelves are sourced from "reputable companies," he said, with labels cautioning "for adults only," or "consult your health care professional before using."

Kraus said he probably wouldn't recommend the product to anyone under the age of 17 who wasn't seriously into athletics.

"In general, a 15-year-old would be too young to benefit from it," he said, adding that teens that age should concentrate on eating a really good diet instead.

Still, Kraus sees supplementation as an individual matter, since some teens are more developed than others and some "demand much more of their bodies and need different things to replenish themselves" compared with the average person, Kraus said. "It's not one-size-fits-all."

'Deception is necessary'

Though the study results serve as an important warning for parents, some might question the ethics of researchers constructing an elaborate ruse in order to gather information.

"Deception in research should be minimized whenever possible," Milanaik said. "However, there are circumstances where deception is necessary, and we believe this is one of them."

She added that she and her colleagues considered an alternative method of a self-report survey of young teens but decided such a survey would not achieve the objective of "focusing on the quality of information provided by employees in stores specializing in sales of vitamins and supplements."

Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital, NYC, said that using an undercover approach to investigate selling supplements to minors is "certainly justified." Glatter had no involvement in the research. "It only makes sense that this would be a rational and logical way to approach such an inquiry."

He added that he could "attest to the dangers of creatine as well as testosterone," because, while working in the ER, he has taken care of teens who have developed kidney and liver problems as a result of using such products.

The American Academy of Pediatrics, which published the study, agreed that no matter its methods, the research was worthwhile. The journal Pediatrics "publishes studies that are relevant to improving the health and wellbeing of children and adolescents. Every manuscript goes through peer-review and an editorial board discussion prior to making a decision as to whether or not to publish. The same process was followed for this study," the academy wrote in an email.

The study raises important issues for many people, Milanaik said.

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Health food stores might want to raise awareness among employees about age-restricted products so they can redirect teens to healthy diets, she said.

Meanwhile, pediatricians might raise awareness of this issue, both among themselves and among parents. Milanaik said she and her co-authors hope their article will help physicians "guide teens in making more educated choices about their health."

Finally, both parents and teens might want to "look at product labels before taking supplements and talk to their physicians if they have questions," Milanaik said. "It should not be assumed that all products in a health food store are 'healthy' for all consumers."