Despite their popularity, energy drinks are not the best option nutritionally, said Tammy Roberts, former University of Missouri Extension nutrition and health education specialist.

“Unfortunately, the energy in these drinks comes from sugar and stimulants like caffeine, guarana, ginseng and green tea,” Roberts said.

Caffeine is a stimulant that occurs naturally in coffee and is added to many soft drinks. Intake of up to 300 milligrams of caffeine is considered OK for most adults. Children should consume well under 100 milligrams per day, according to Roberts.

One cup of coffee has about 100 milligrams of caffeine. A 12-ounce cola drink has around 45 milligrams. Energy drinks have as much as 85 milligrams of caffeine per 8 ounces. That is less than a cup of coffee, but the can might be 24 ounces, which would provide 255 milligrams.

“It can be difficult to know exactly how much caffeine you are consuming from energy drinks because food manufacturers are required to list the presence of caffeine, but not required to list the exact amount,” said Roberts.

Negative side effects caused by caffeine include nervousness, irritability, increased heart rate, increased blood pressure and difficulty sleeping.

Guarana is a nervous system stimulant that comes from the seeds of the guarana plant. It is often found in herbal supplements that promote weight loss. The Food and Drug Administration does not do a premarket review for safety or effectiveness of dietary supplements.

Ginseng is thought to improve thinking or learning. In some studies, the benefits of using ginseng have been seen in healthy young people as well as in ill, older people. However, it is important to note that negative results have also been reported. According to the National Institutes of Health, more and better research is needed before a strong recommendation can be made about ginseng.

Green tea has about 50 milligrams of caffeine per cup.

“Some people believe that green tea helps with memory enhancement and improves mental performance, but, according to the National Institutes of Health, there is unclear scientific evidence for the use of green tea for that purpose,” Roberts said.

The amount of energy in the form of sugar in these energy drinks is about the same as that in other soft drinks, which is 40 to 50 grams of carbohydrates per 12-ounce can.

Although many people enjoy energy drinks, moderation is advised, said Roberts.