Effects of Tea on Blood Pressure

What is the problem and what is known about it so far?
High blood pressure (BP) increases the risks of heart disease. Elevated BP also increases the risks of stroke, heart failure and kidney dysfunction. Currently, high BP is a diagnosis made by three measurements of BP greater than 140/90 (mild: greater than 140/85; moderate: greater than 160/100). Dietary changes, including reduction in salt intake, can be initially tried to reduce mild BP problems. Moderately elevated BP usually requires treatment with BP lowering medications.

Why did the researchers do this particular study?
In Chinese medicine, tea is thought to have properties to reduce BP. Human and animal studies have not shown a clear relationship between BP and tea consumption. Researchers in this study wanted to clarify the long-term effects of tea drinking in a Chinese adult population in Taiwan who have just been diagnosed with high BP.

Who was studied?
2416 persons, 20 years or older, newly diagnosed with high BP were selected to participate in this study. Patients were randomly selected from every 5th household in seven designated districts in Tainan, Taiwan. Selected patients were contacted by letter and phone. 1507 Taiwanese patients (711 men, 796 women) responded. 600 patients were habitual tea drinkers, defined as persons who drank more than 120 ml per day for at least one year. It should be noted that the habitual tea drinkers were younger, more often male, of a higher educational and socio-economical level, centrally obese, consumed more alcohol, ate fewer vegetables, and ate more salt in their diet.

How was the study done?
Researchers accounted for differences in age, sex, socioeconomic status, family history of hypertension, body mass index, waist-hip ratio, lifestyle (including alcohol and tobacco use), and diet. Average amount of tea and type (green, black or oolong) of tea was considered, including whether milk or sugar was added.

What did the researchers find?
Habitual tea drinkers had a lower BP and fewer diagnoses of high BP compared to non-habitual tea drinkers. The risk of developing hypertension was decreased by 46% in patients studied who drank 4-20 oz (120-599 ml) /day and 65% for those who drank more than 20 oz (600 ml) /day. The risk of developing high blood pressure was lower in persons who drank tea for more than one year. However, drinking tea for more than 10 years did not have an added benefit.

What were the limitations of the study?
The study was carried out at one point in time which makes it hard to be certain that the findings represented cause and effect rather than just an association. Also, other factors, including socioeconomic and lifestyle factors and dietary habits associated with tea drinking in various cultures, could not be excluded as possible explanations for the findings in this study. The researchers were also unable to isolate an ingredient(s) that may create the beneficial blood pressure lowering effect. Finally, because only a selected Taiwanese population was studied, this study may not be applicable to the general population.

What were the implications of the study?
The results imply that there may be a beneficial association between green or oolong tea consumption and decreased BP. Although these benefits seemed to increase when tea was used in larger quantities for a longer duration of time, confirmation that tea is responsible for the effect will require studies that prospectively follow people over a period of time and where other factors such as socioeconomic status and dietary/lifestyle habits can be more closely accounted for. However if tea consumption does help lower BP, large groups of people could benefit

Summarized by Winifred Lee, College of Medicine, University of Vermont
Summarized from "The Protective Effect of Habitual Tea Consumption on Hypertension." Yang, Yi-Ching et al. Archives of Internal Medicine, July 26, 2004, Volume 164. page 1534-1540.