ENERGY drinks that promise to boost performance and rev up metabolism can contain three to four times as much caffeine as a typical soda, a new study shows.

The concern, researchers say, is that consumers who are vulnerable to the ill effects of caffeine - including children, pregnant women and people with cardiovascular disease - may unknowingly ingest large amounts.

Energy drinks like Red Bull, Red Devil and Sobe "No Fear" typically contain a mix of carbohydrates, B vitamins, amino acids and caffeine. Manufacturers claim the beverages boost physical performance, concentration, metabolism.

So it's not surprising that energy drinks would contain large amounts of caffeine, said study co-author Dr. Bruce A. Goldberger of the University of Florida College of Medicine in Gainesville.

But the actual caffeine dose might surprise some consumers, he said.

Given the danger of high caffeine intake to some people, energy drinks should be required to state their caffeine content clearly, Goldberger and his colleagues argue in their report, published in the Journal of Analytical Toxicology.

"We believe that these products should be labeled," Goldberger said, noting that in Europe, drinks with more than 150 milligrams (mg) of caffeine per liter must be marked as "high-caffeine."

Nearly all of the 10 energy drinks he and his colleagues tested would fall under that rule.

In the United States, Goldberger said, the Food and Drug Administration has had a long-standing "proposed" rule that soft drinks limit their caffeine content to no more than 65mg per 370g (12 ounce) serving.

But neither sodas nor energy drinks are required to put their caffeine content on the label.

In their tests, Goldberger and his colleagues found that all 19 soft drinks they sampled contained less than often far less than - the recommended 65mg of caffeine per 370g.

Most of the energy drinks, however, boasted at least that much caffeine in an 248g serving.

A serving of one of the products had 141mg of caffeine, or about twice the amount in a double-shot espresso drink. Others typically contained 65 to 75mg of caffeine per 248g, whereas a regular Coke or Pepsi had about 30mg per 370g.

Labels on that product, and two of the other energy drinks, did state that they are not recommended for children and pregnant women.

High caffeine intake may cause headaches and sleep problems in children, and some studies of pregnant women have linked caffeine intake higher than 150mg per day to an increased risk of miscarriage and low birth weight.

But other groups - such as people with high blood pressure, heart rhythm abnormalities or anxiety disorders - should limit their caffeine intake as well, and the majority of the energy drinks in this study had no warning label of any kind, Goldberger noted.

"I think that labeling is now due," he said.

Editor: Wing