Just one energy drink can trigger potentially "life threatening" changes in blood pressure and the heart, warns new research.
The study showed that drinking one 32 ounce can of a commercially available energy drink resulted in more "profound changes" in the heart's electrical activity and blood pressure than drinking the same amount of a control drink containing the same amount of caffeine.
Researchers warned that people who have high blood pressure, underlying cardiac conditions or other health issues might want to avoid energy drinks until more is known about their impact on heart health.
They said that while the US Food and Drug Administration generally considers caffeine in daily doses of less than 400mg as safe, energy drinks often consist of a host of other ingredients.
With more than 500 types of energy drinks on the market, there has been an increase in energy-drink-associated A&E visits and even deaths, prompting questions about their safety.
The study was led by Doctor Emily Fletcher, deputy pharmacy flight commander from David Grant USAF Medical Centre at Travis Air Force Base in California.
Dr Fletcher said: "We decided to study energy drinks' potential heart health impact because previous research has shown 75 per cent of the base's military personnel have consumed an energy drink.
"And nearly 15 per cent of military personnel, in general, drink three cans a day when deployed, which is more than we studied here,"
For the study, published in the Journal of the American Heart Association, 18 young participants were randomly divided into two groups.
The first group received 32 ounces of a commercially-available energy drink containing 108g of sugar, 320mg of caffeine, and various other compounds.
The second group was given a control drink containing 320mg of caffeine, 40ml of lime juice and 140ml of cherry syrup in carbonated water. After a six-day washout period, participants switched drinks.
Researchers measured the electrical activity of the volunteers' hearts by electrocardiogram.
They also measured their peripheral and central blood pressures at the study's start and at one, two, four, six and 24 hours after drink consumption.
Dr Fletcher said: "Peripheral blood pressure is the measurement of the pressure in an outlying artery, typically an upper arm.
"Central blood pressure is the measurement of the pressure in the aorta near the heart.
"Blood pressures at each location are not always affected equally when a substance is introduced, such as medications.
"Central blood pressure is an emerging and potentially superior method to assess health outcomes related to elevated blood pressure."
The researchers found that, when compared to the caffeine group, those in the energy drink group had a corrected QT interval 10-milliseconds higher at two hours.
Dr Fletcher said: "The QT interval is the measurement of the time it takes ventricles in the heart, the lower chambers, to repolarise, or prepare to generate a beat again.
"It's the pause from the end of the electrical impulse generating the heart to beat to the next impulse.
"If this time interval, which is measured in milliseconds, is either too short or too long, it can cause the heart to beat abnormally. The resulting arrhythmia can be life threatening."
She said, to put the 10-millisecond difference in perspective, there are medications that affect the corrected QT interval by six milliseconds and have warnings about the effect on product labels.
While both the energy drink and caffeine-only groups had similar increases in systolic blood pressure, systolic pressures in the caffeine group had almost returned to their original readings after six hours.
Dr Fletcher said: "On the other hand, those who consumed the energy drinks still had a mildly elevated blood pressure after six hours.
"This suggests that ingredients other than caffeine may have some blood pressure altering effects, but this needs further evaluation."
She said, based on this preliminary evidence in young, healthy adults, people who have high blood pressure, underlying cardiac conditions or other health issues might want to avoid or use caution when consuming energy drinks until more is known about their impact on heart health.
Dr Fletcher added: "This is a small study and further studies are needed to confirm these results."
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