After six days, the participants switched drinks.
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The team noted the people drinking energy drinks had a significant 10-milliseconds higher QT interval (the time it takes the heart’s ventricles to prepare to beat again) than those drinking the caffeinated control drinks.
According to Time, irregularities in the QT interval can lead to abnormal heart beats, or arrhythmia. And some medications that affect intervals by just six milliseconds, four less than the disparity from the two drinks, carry warning labels.
Researchers also found that when people drank the energy drink, their blood pressure increased by five points after drinking the beverage and remained at mildly elevated levels after six hours.
The systolic pressures in the control group, however, increased by under one point after drinking the beverage and returned to their original levels by six hours time.
“This suggests that ingredients other than caffeine may have some blood pressure altering effects,” Fletcher said, but added that further studies are needed to confirm the findings.
"The energy drink industry claims that their products are safe because they have no more caffeine than a premium coffee house coffee," Jennifer L. Harris from University of Connecticut's Rudd Center for Food Policy and Obesity told NBC News.
“However, energy drinks also contain a proprietary 'energy blend,' which typically consists of stimulants and other additives. Some of these ingredients (including taurine and guarana) have not been FDA-approved as safe in the food supply, and few studies have tested the effects of caffeine consumption together with these 'novelty' ingredients,” she said.
While the research isn’t particularly worrisome for healthy individuals, though moderate or limited consumption is still recommended until more studies show the drinks’ impact on heart health, certain individuals with risk factors associated with heart issues should be cautious when it comes to consuming energy drinks.
Researchers also recommend moderate consumption during exercise or sports or any activity that increases individuals’ blood pressure and heart rate.
Read the full study.