However the manufacturer of one of the supplements - the ingredient Advantra Z - says the communication of the study results by the media has been misleading.
The researchers from the University of California at San Fransisco reported on Friday that both Advantra Z and Xenadrine EFX, products promoted as safe alternatives to the herbal weight loss supplement ephedra, caused a rise in heart rate of around 18 percent in a placebo controlled trial on 10 healthy subjects.
Heart rate and blood pressure were recorded in the study for an hour before dosing and then at several intervals afterwards. To rate physical symptoms, moods and emotions, questionnaires were administered one, two and six hours six hours after taking the supplements or placebo.
Xenadrine EFX was found to also raise blood pressure significantly, write the authors in the 9 September issue of The American Journal of Medicine.
"These findings indicate that ephedra-free dietary supplements could have some of the same adverse health effects associated with previously available ephedra products, such as Metabolife 356 and Ripped Fuel," said Christine Haller, UCSF assistant professor of medicine and lead author of the paper.
But Ellen Miller, public relations counsel for Advantra Z maker Nutratech, said: "When the study was presented at a conference some months ago it concluded that Advantra Z did not have negative cardiovascular effects, even at eight-fold higher doses."
"We were thrilled with these results and very surprised by the conclusions reported in this recent press release," she told NutraIngredients.USA.com.
Single doses of both products increased heart rate by an average of 11 to 16 beats per minute over baseline, the scientists said, an equivalent of an 18 percent increase if baseline rate is 80 beats per minute.
Nutratech says it has been advised by independent experts that such an increase is 'negligible', and could occur simply by eating a meal or doing some gardening.
The company has recently reported on a study demonstrating the safety and efficacy of the supplement ingredient.
Xenadrine EFX, however, also contains other ingredients, including caffeine equivalent to three cups of coffee. In the new study, it increased blood pressure by 7 to 12 percent (9-10 mm Hg).
The increased blood pressure is likely not due to caffeine alone, say the researchers, but potentially related to the actions or interaction of other constituents in the multi-ingredient supplement.
They conclude that Xenadrine EFX appears to have similar acute cardiovascular stimulant actions as the weight loss herbal ephedra, banned in 2004.
The Xenadrine manufacturer was not available for comment at the time of going to press.
The scientists call for longer term dosing studies to determine whether the blood pressure effects of Xenadrine EFX persist with repeated use.
They also want further research into the safety and effectiveness of bitter orange-containing supplements in general - particularly among those most likely to take them: overweight people who may have other health conditions.
Until such data are available, they conclude, doctors should caution patients about using ephedra-free weight-loss dietary supplements and should monitor blood pressure in those who choose to use the products.