McKay's research was funded by the Agricultural Research Service (ARS), a scientific research agency of the U.S. Department of Agriculture (USDA), and by Boulder, Colo.-based Celestial Seasonings, a brand of The Hain Celestial Group, Inc. McKay works in the Antioxidants Research Laboratory of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston, Mass.

In a clinical trial, McKay tested 65 volunteers, aged 30 to 70 years, whose systolic blood pressure was 120 to 150 mm Hg and whose diastolic blood pressure was 95 mm Hg or less at the start of the study. Blood pressure readings of 120 over 80 or greater are considered a risk factor for heart disease, stroke and kidney disease.

For six weeks, about half the group was randomly selected to drink three cups of hibiscus tea daily. The others drank a placebo beverage containing artificial hibiscus flavoring and color. All participants were advised to follow their usual diet and maintain their normal level of activity. Before the start of the study, blood pressure was measured twice, one week apart, and at weekly intervals thereafter.

The findings show that the volunteers who drank hibiscus tea had a 7.2 point drop in their systolic blood pressure, compared to a 1.3 point drop in the volunteers who drank the placebo beverage.

In a subgroup analysis, 30 volunteers with the highest systolic blood pressure readings at the start of the study (129 or above) were found to have a greater response to hibiscus tea drinking compared to placebo drinkers. Their systolic blood pressure went down by 13.2 points, diastolic blood pressure went down by 6.4 points, and mean arterial pressure went down by 8.7 points.

This data supports the idea that drinking hibiscus tea in an amount readily incorporated into the diet may play a role in controlling blood pressure, although more research is required.