Among those with SBP over the median of 129 mm Hg, the reduction was double that, almost 14 mm Hg after six weeks, and produced significant reductions in diastolic and mean arterial pressures.

The finding "suggests that regularly incorporating hibiscus tea into the diet may actually help control blood pressure in people who are at risk for developing hypertension," said lead author Dr Diane L McKay (Jean Mayer USDA Nutrition Research Center on Aging, Tufts University, Boston, MA) at a press conference here at the American Heart Association 2008 Scientific Sessions.

On a population basis, even small changes such as those seen in this study would be expected to reduce stroke, coronary artery disease, and all-cause mortality, McKay added.

Bioactive Phytochemicals

Hibiscus is among the most common ingredients found in herbal tea blends sold in the US, McKay said. The principal components of hibiscus include anthocyanins and other flavanoids, as well as polyphenolic compounds and phenolic acids.

A variety of bioactivities have been attributed to these compounds, she noted, including the ability to act as an ACE inhibitor. Earlier short-term trials in humans used black tea as a control, which also has an effect on vascular reactivity, making it not a proper control to look at the effects of hibiscus tea, she pointed out. "It is also interesting to note that a study comparing hibiscus tea with captopril, an ACE inhibitor, found no difference in blood-pressure-lowering effects."

The aim of their study, she said, was to determine whether hibiscus tea, "in an amount that can be readily incorporated into the diet," will lower blood pressure in prehypertensive and mildly hypertensive adults compared with a placebo beverage.

The study was a randomized, double-blind, placebo-controlled trial of 65 generally healthy men and women aged 30 to 70 years who had SBP readings of 120 to 150 mm Hg and a diastolic blood pressure (DBP) of <95 mm Hg.

Subjects were not taking antihypertensive medications or other supplements or medications that could affect their blood-pressure level. They were not excluded on the basis of body-mass index (BMI), and BMIs in the study ranged from 18.5 to 34.9.

Participants were randomized to receive either three 8-oz servings daily of hibiscus tea for six weeks or a placebo beverage. The hibiscus tea was prepared by brewing one tea bag containing 1.25 g of dried hibiscus calyces in 8 oz of boiled water for six minutes, after which the tea bag was removed. The placebo beverage was prepared by adding a small amount of hibiscus-flavored concentrate to 8 oz of water.

The beverage had to be consumed within 12 hours of preparation and could be served hot or cold and with or without milk and a sweetener of the subjects' choice, she noted. The placebo beverage had no anthocyanins, which they believe is the active component, McKay pointed out.

Outcomes of interest were the change from baseline in SBP, DBP, and mean arterial pressure (MAP).

"After six weeks, we found that subjects who consumed hibiscus tea had a significantly lower systolic blood-pressure level compared with people in the placebo group," said McKay. A drop in DBP and MAP was also seen, but these changes were not significantly different from placebo.

However, in a subgroup analysis of subjects with systolic pressures higher than 129 mm Hg, the magnitude of the change found with hibiscus tea was almost double that seen in the overall group, and differences with placebo with regard to systolic, diastolic and mean arterial pressure were all statistically significant, McKay said.

McKay put their findings into the context of previously published work suggesting that, on a population basis, a reduction of 3 mm Hg in systolic blood pressure would be expected to translate into a reduction in stroke mortality of 8%, of 5% in coronary heart disease mortality, and of 4% in all-cause mortality [1].

During her presentation, she was asked by an audience member about any potential adverse events from this amount of hibiscus tea.

No adverse effects were reported, she responded. "There are some data from Nigeria, where we know that the average per capita consumption of a hibiscus-containing beverage is about the equivalent to 25 cups of our hibiscus tea every day, and no adverse effects have been demonstrated."