Monday, November 29, 2010

Current Use and What the Science Says

In addition to being an important component of commercially-marketed herbal tea blends (McKay et al., 2010), roselle is still used for its traditional medicinal properties, as well as for culinary purposes, in many countries around the world (Ojeda et al., 2010). Researchers are interested in exploring the anti-hypertensive properties of this plant, considering that a herbal tea remedy could be easily integrated into the diets of people with high blood pressure, especially in countries where Hibiscus tea is already consumed traditionally (McKay et al., 2010). 

A randomized controlled trial (RCT) was designed by McKay et al. (2010) to study the effects of Hibiscus tea on blood pressure in pre- and mildly-hypertensive adults. Participants in the experimental group received three servings of H. sabdariffa tea each day for six weeks, while members of the control group were provided with a beverage designed to mimic the real tea using food colouring and artificial flavours. The systolic and diastolic blood pressure of the participants was measured at baseline and once each week using an automated device. After six weeks, the decline in systolic blood pressure was significantly lower in the experimental group than in the control group (-7.2 ± 11.4 vs. -1.3 ± 10.0 mmHg; P=0.030). The decline in the experimental group’s diastolic blood pressure was also lower than that of the control group (-3.1 ± 7.0 vs. -0.5 ± 7.5 mmHg; P=0.160), but the difference was not significant.

Source: http://www.itechnews.net/wp-content/uploads/2008/12/usb-blood-pressure-monitor.jpg

Some of the strengths of this study were that it was double-blinded, and that it actually used a true placebo, rather than using black tea or a hypotensive drug for the ‘control’ group. The use of a standardized and automated method for measuring blood pressure, rather than having it measured manually by a physician, was also commendable. Weaknesses of the study included the sample size (n=66) and the fact that the active components of the tea were not isolated, suggesting that each tea bag could have contained a different quantity of anthocyanins, even if each one contained exactly 1.25g of dried calyces.

Herrera-Arellano et al. (2007) also conducted a double-blinded RCT to investigate the anti-hypertensive effects of H. sabdariffa on adults with existing stage 1 or 2 hypertension. The experimental group drank 250 mg of anthocyanins isolated from dried calyces dissolved in 250 mL of water every day for four weeks, while the control group was given 10 mg of the angiotensin converting enzyme (ACE) inhibitor lisinopril mixed with artificial flavour and colour. The blood pressure reduction in the H. sabdariffa group was 17.14/11.97 mmHg, which was less than that of the lisinopril group (23.31/15.39 mmHg; P<0.001). This study had a larger sample size (n=193), but 12% of participants were excluded from analysis due to non-adherence, which makes one question the validity of the results (Wahabi, Alansary, Al-Sabban, & Glasziuo, 2010). The main weakness of this trial was the lack of a true control.

Overall, it seems that there is reasonable evidence, based mostly on the results of the study by McKay et al. (2010), that the calyces of H. sabdariffa contain anti-hypertensive compounds, and that daily consumption of tea containing these compounds may lower blood pressure in adults with mild hypertension. However, further studies should be done with a larger sample size, H. sabdariffa extracts with standardized amounts of anthocyanins, and a true control group.

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