While the authors did not identify which of the 19 ingredients in the supplement caused this effect, they warn that there could be a lack of data on herbal preparations that is putting many consumers in danger.
"Dietary weight-loss supplements often combine ephedra and caffeine with various other natural ingredients," write researchers in today's issue of The Journal of the American Medical Association (JAMA 2004;291:216-221).
In the United States, more than 3 billion doses of these herbal preparations are sold annually, resulting in $7 billion in sales.
In the UK, ephedra is classed as a schedule 3 herb and can only be prescribed by herbal practioners. "If indicated, ephedra will usually be prescribed alongside other botanicals," Trudy Norris, practising herbalist and member of the National Institute of Medical Herbalists, told NutraIngredients.com.
"But it would not be used with caffeine. And it is not generally used for weight loss. It is more likely to be indicated to influence overall health, such as in treatment for chronic chest complaints, resulting from asthma for example," she added.
"Consumers are drawn to herbal preparations because of their nonprescription status, direct-to-consumer advertising, and the perception that natural products are innately safe. Unfortunately, the perception of safety may be the result of a lack of data," add Brian F. McBride, from the University of Connecticut Schools of Pharmacy, and colleagues.
The researchers evaluated the impact of Metabolife 356, the best-selling dietary supplement containing ephedra and caffeine, in addition to several other ingredients, on corrected QT (QTc) interval duration and systolic blood pressure. The QTc interval is a measure of the time intervals that occur during the electrical impulses that stimulate the heart to contract. A longer QTc interval can increase the risk of developing abnormal heart rhythms. Systolic blood pressure is the higher number of a blood pressure reading, representing the blood pressure when the heart is contracting.
Fifteen healthy volunteers with an average age of almost 27 years were recruited for the double-blind study conducted earlier last year. The individuals were randomised to receive either the dietary supplement containing 19 ingredients, including ephedra (12 milligrams) and caffeine (40 milligrams), or a placebo capsule (sugar pill) as their first treatment.
With a week off between the treatment sessions, the two groups returned for the second phase of the study and received the other treatment. The participants' heart rate and blood pressure were monitored immediately before taking the supplement or placebo and one, three and five hours after.
"Individuals receiving the dietary supplement containing ephedra and caffeine had a longer maximal QTc interval (mean [average], 419.4 versus 396.1 milliseconds) and higher systolic blood pressure (mean, 123.5 versus 118.93 mm Hg [millimeters/mercury] compared with placebo," the authors report.
"Overall, 53 per cent of participants had QTc intervals increases of at least 30 seconds while taking the dietary supplement containing ephedra and caffeine."
The authors note that the European Center for Proprietary Medicinal Products recognizes a drug-induced increase in the QTc interval of at least 30 milliseconds as a possible cause of concern in developing a potentially fatal irregular heart beat, known as torsade de pointes.
"The ephedra- and caffeine-containing dietary supplement Metabolife 356 increased the mean maximal QTc interval and systolic blood pressure. Since the actual ingredient or ingredients in Metabolife 356 responsible for these findings are not known, patients should be instructed to avoid this and similar dietary supplements until more information is known about their safety," conclude the authors.
Ephedra was until recently included in a large number of best selling weight loss products in the US. The US government looks set however to ban the herb after a number of reports linked it to serious side effects and some deaths.
Metabolife International, a major manufacturer of ephedra products, has however always maintained its safety when used as directed. David Cohen, national science counsel for the company, told a BBC Online report on the study that the new data "is inconsistent with prior studies which have measured this exact same interval".