Energy drinks can contain anywhere from 80 to 400 milligrams or more of caffeine per can, along with sugar and other stimulants that may affect the heart and blood vessels.
Research cited by a UTHealth Houston cardiologist found that a large energy drink can impair blood vessels’ ability to expand within just 90 minutes.
Healthy adults should keep total caffeine consumption below 400 milligrams a day, while children, teens, and certain higher-risk adults should avoid energy drinks, according to a cardiologist.
That energy drink promising to get you through an afternoon slump may do more than make you feel awake.
Energy drinks typically combine caffeine with sugar and ingredients such as taurine, guarana, ginseng, and B vitamins. While that mixture can temporarily increase alertness, it can also raise heart rate and blood pressure and affect how blood vessels function, according to Dr. John Higgins, a professor of medicine and sports cardiology at McGovern Medical School at UTHealth Houston.
Caffeine works by blocking adenosine, a chemical in the brain involved in making us feel tired. Its effects can kick in within 30 to 120 minutes and persist for hours, depending on factors such as genetics and a person's tolerance to caffeine.
Some energy drinks also pack a significant amount of sugar — 50 grams or more in a single can, according to Higgins. That can contribute to a short-lived energy boost followed by a crash.
The ingredients aren't always straightforward
Consumers looking at the label may find more than caffeine.
Guarana and yerba mate, for example, can add additional caffeine. Other ingredients can include taurine and stimulants such as yohimbine, which Higgins says may increase anxiety, blood pressure, and the risk of heart rhythm problems.
Proprietary blends can make it more difficult to determine exactly how much of an individual ingredient a drink contains because manufacturers may disclose the combined weight of several ingredients rather than the amount of each one.
And those enormous doses of B vitamins advertised on some cans aren't necessarily providing extra energy. Higgins says taking far more than the recommended daily amount won't produce additional energy in someone who isn't deficient.
Your blood vessels may react quickly
The potential cardiovascular effects are one of the bigger concerns.
Research cited by Higgins found energy drinks increased systolic blood pressure by about 4.4 mmHg on average. Another trial found that an energy drink produced greater changes in the heart's electrical activity than caffeine alone, suggesting that the combination of ingredients may matter.
Higgins' own research has found effects on blood vessel function. In one study involving healthy young adults, consuming a large energy drink reduced the blood vessels' ability to relax and expand by nearly half within 90 minutes.
Another finding cited by Higgins involved a 24-ounce energy drink. A measure of artery function known as peak flow-mediated dilation dropped from 5.9% to 1.9% after 90 minutes — despite relatively little change in participants' heart rate and blood pressure.
That means a person might not necessarily feel that something is happening to their cardiovascular system.
Severe reactions aren't the norm, but cases involving irregular heart rhythms, chest pain, and cardiac arrest have been associated with heavy or rapid energy-drink consumption, particularly among people with underlying heart problems.
More common side effects include headaches, jitters, insomnia, nausea, and heart palpitations. One survey cited by Higgins found that more than half of young people who consumed energy drinks reported at least one adverse reaction.
How much is too much?
There's no universal answer because caffeine content varies enormously. One can may contain about 80 milligrams while another can pack 400 milligrams or more.
Higgins recommends healthy adults limit their total caffeine intake from all sources to less than 400 milligrams per day. That includes coffee, tea, soda, supplements, pre-workout products, and energy drinks. He also advises against combining several stimulant-containing products during the same day or mixing energy drinks with alcohol.
Children and teenagers should avoid energy drinks entirely, Higgins says. Women who are pregnant or breastfeeding, have high blood pressure or heart conditions, or are particularly sensitive to caffeine should also avoid them.
Consumers should also remember that energy drinks and sports drinks aren't the same thing. Sports drinks are intended to replace fluids and electrolytes, while energy drinks contain stimulants and shouldn't be used primarily for hydration.
Finally, some symptoms warrant immediate attention. Higgins says people should stop consuming caffeine and seek medical help if they experience chest pain, fainting, a racing or irregular heartbeat, severe shortness of breath, confusion, or seizures.