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After drinking a 16-ounce can of "Rockstar Punched," young adults had a 74 percent increase in blood levels of the "fight-or-flight" hormone norepinephrine, said lead researcher Dr. Anna Svatikova, a cardiologist at the Mayo Clinic in Rochester, Minn.
That's more than double an average 30 percent increase in norepinephrine the same participants experienced when they consumed a fake energy drink, Svatikova said.
The sham energy drink contained the same amount of sugar and nearly the same calories, but did not include natural stimulants found in the Rockstar drink, she said. The stimulants in the real energy drink include caffeine, taurine, guarana, ginseng and milk thistle extract.
The young adults also experienced a significant increase in their blood pressure after consuming the energy drink, the study found.
"The worry is that if these responses are seen in healthy young people, perhaps the effects of energy drinks may be more pronounced in people who already have high blood pressure or arrhythmias," leading to more heart attacks and strokes, Svatikova said.
The findings were to be presented Sunday at the American Heart Association's annual meeting in Orlando, and the results will be published simultaneously in the Journal of the American Medical Association.
Emergency room visits involving energy drinks doubled between 2007 and 2011, rising from about 10,000 to nearly 21,000, SAMHSA said.
Previous studies have shown that energy drink users experience a dramatic rise in their blood pressure, Svatikova said. Researchers conducted this study to see if that blood pressure spike is caused by a change in the person's hormone levels.
The study included 25 healthy young adults. They were between the ages of 26 and 31, and had no known heart risk factors. Each person drank one 16-ounce can of Rockstar Punched or the sham drink in random order on two separate days.
Researchers measured participants' blood pressure and blood levels of norepinephrine before and 30 minutes after they chugged each drink.
The body usually releases norepinephrine in fight-or-flight situations of extreme stress, Svatikova said. The hormone increases blood pressure and the heart's ability to contract, and increases arousal and alertness in the brain.
Participants' norepinephrine levels increased more than twice as much when compared to when they consumed the sham drink, researchers found.
Blood pressure also spiked due to energy drinks. For example, mean blood pressure increased by 6.4 percent after energy drink consumption, compared with a 1 percent increase when the young adults downed the fake drink, the study found.
"They are not benign, these products, and there's no limit to how these products are consumed by individuals," said Linda Van Horn, a professor of preventive medicine and nutrition at Northwestern University Feinberg School of Medicine and a spokeswoman for the American Heart Association.
Due to the small size of this study, "it's probably premature to launch any kind of specific concerns or warnings about these products," Van Horn added.
But both Van Horn and Svatikova said doctors should take note of this possible effect, and consider asking heart patients to cut energy drinks out of their diet.
"As physicians, we should perhaps ask people about energy drink intake, and factor this in as we interpret their vital signs in emergency settings," Svatikova said. "For the consumers, they should use caution when consuming energy drinks, because these drinks may increase their risk of sudden heart problems, even among young people."
The American Beverage Association took issue with the findings.
"There is nothing unique about the caffeine in mainstream energy drinks, which is about half that of a similar-sized cup of coffeehouse coffee," the association said in a statement. "According to a study published in the International Journal of Cardiology, drinking coffee would produce similar results to the findings of this abstract."
"The safety of energy drinks has been established by scientific research, as well as regulatory agencies around the globe," the association said. "Just this year, the European Food Safety Authority confirmed the safety of energy drinks and their ingredients after an extensive review."
Leading energy drink manufacturers in the United States display total caffeine content on their packaging, along with warnings that say energy drinks aren't recommended for children, pregnant or nursing women or anyone who is sensitive to caffeine, the association added.
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