Here’s more evidence for why you should limit your consumption of caffeinated energy drinks, especially if you have elevated cardiovascular risk: Drinking large volumes of them not only raises blood pressure but can also prolong the heart’s QT interval, a measure of electrical activity that’s linked to dangerous arrhythmias that can lead to sudden death, according to a recent study.
In the small clinical trial, published in June 2019 in the Journal of the American Heart Association, researchers randomly assigned 34 healthy young adults (average age 22) to drink 32 ounces of one of three drinks—a commercially available brand of caffeinated energy drink, another brand of caffeinated energy drink, or a non-caffeinated placebo beverage (containing carbonated water, lime juice, and cherry flavoring)—on three separate days, spaced at least six days apart.
Both energy drinks contained a combination of caffeine (304 to 320 milligrams per 32 ounces) and other ingredients, including the amino acid taurine, glucuronolactone (a naturally occurring chemical in the body and a common ingredient in energy drinks), and B vitamins, as well as various proprietary ingredients. The researchers measured the participants’ heart activity and blood pressure at baseline and periodically over the next four hours.
Compared with the placebo beverage, both energy drinks significantly prolonged the QT interval over the four hours following consumption. A prolonged QT interval is known to increase the risk of dangerous arrythmias and cardiac death, both adverse events that have been associated with energy drinks. Consistent with earlier research, the energy drinks also raised systolic and diastolic blood pressure significantly more than the placebo (by an average of five points and four points, respectively).
Interestingly, while the researchers attributed the blood pressure changes mainly to the beverages’ caffeine, they said that the QT interval changes couldn’t be explained by caffeine alone and likely stemmed from the drinks’ mix of caffeine and other ingredients, which may interact in ways not yet understood. Some evidence supports that idea: For instance, a small study by the same lead researcher in 2017 found that the QT interval was significantly longer two hours after participants drank an energy beverage versus when they ingested a caffeine-only control drink.
What you should do
The latest study was done in young and healthy people and measured only the short-term effects of energy drink consumption, so it’s not clear whether the findings would apply to other groups or to smaller amounts of the beverages—nor what the long-term effects of the observed changes might be. Though more investigation is needed, the authors concluded that based on the current evidence "individuals with acquired or congenital long QT syndrome and those with hypertension should be more vigilant and limit their energy drink intake."
Keep in mind that energy drinks have been linked in recent years with nearly three dozen deaths and hundreds of other adverse events, including seizures and cardiac arrests. There is no good reason to drink them. Pregnant women, children, teenagers, and people who are sensitive to caffeine in particular should avoid them altogether.
Also see Alcohol and Energy Drinks: A Bad Mix.