Wednesday October 13 2010
Tea is rich in potentially healthy anti-oxidant flavonoids
“Two cups of tea a day cuts heart disease,” the Daily Express has reported. The newspaper says that tea contains “health boosting flavonoids” that can reduce the risk of heart disease by 11%.
The news is based on a study examining a selection of research on the potential heart benefits of both green and black tea. It concludes that regular consumption of any type of tea is likely to reduce the risk of cardiovascular disease, probably due to levels of substances called flavonoids that are found in tea. The authors suggest that the reduction in risk is the result of actions such as preventing clogging of the arteries and improving body weight.
However, this review should be seen as a “round-up” of some of the existing research as it was not a systematic analysis of the research, and therefore cannot provide firm evidence. As the authors of the review point out, we now need good quality studies directly looking at whether tea can prevent heart disease in humans.
Overall, it is unlikely that drinking tea can prevent heart disease in isolation from other factors, or redress the effects of a poor diet or unhealthy lifestyle. Adopting a balanced diet, doing regular exercise, consuming moderate amounts of alcohol, and not smoking are all important ways to help reduce the risk of heart disease.
Where did the story come from?
The review was carried out by researchers from the School of Medicine and Pharmacology at the University of Western Australia. There is no information about any external sources of funding. The study was published in the peer-reviewed journal Molecular Aspects of Medicine.
The review was reported in both the Daily Mail and Daily Express, whose reporting generally did not reflect the uncertainty of the study’s conclusions. For example, the Mail reported that three cups of tea a day can prevent cardiac problems, while the Express said drinking tea two or three times a day could reduce risk of the disease by 11%. These claims appear to be based on a 2001 analysis, which the reviewers considered to be flawed. The review actually suggests that this earlier research had several problems that undermine the certainty of the results.
Both newspapers also claim that drinking two cups of tea will provide as many antioxidants as eating five portions of vegetables. Although tea does contain antioxidants, the suggestion that it can be a substitute for the numerous health benefits of fruit and vegetables is not supported by this research.
The Express did however feature the views of an external expert advising that more research is needed and that tea cannot protect from the effects of a poor diet or lifestyle. Both papers report views from the Tea Advisory panel, which support the researchers’ conclusions. The Mail points out that the Tea Advisory Panel is funded by the tea industry.
What kind of research was this?
This was a non-systematic, narrative review looking at the potential health benefits of both green and black tea. It examined evidence from various studies, including animal models, population studies and some randomised controlled trials (RCTs). It included research on the effect of tea on a number of cardiovascular endpoints related to health, including atherosclerosis (clogged arteries), blood pressure and cholesterol reduction.
The authors point out that tea is such a common drink worldwide that any health effects could have a significant impact on public health. Both green and black tea, they say, are rich in flavonoids – anti-oxidant substances that are thought to have health benefits. They say that as little as two to three cups a day will supply a “major contribution to total flavonoid intake in most individuals”.
What did the research involve?
The researchers viewed and summarised 63 studies on the possible effects of green and black tea. They looked at population studies on the possible link between tea and heart disease, but they also summarised studies of potential associations between tea and a number of clinical endpoints associated with cardiovascular disease. These were atherosclerosis, function of the endothelium (lining of the arteries), blood pressure, oxidative stress, cholesterol reduction, inflammation, function of the blood platelets, levels of homocysteine, body weight and body composition, and type 2 diabetes.
The review did not state how the researchers identified and selected the studies to be included. This means we cannot be sure if any relevant studies were omitted, nor whether studies not included came to similar conclusions. Although researchers describe the evidence they found, they do not systematically examine the quality of studies that were included.
What were the basic results?
The main findings reported by the authors were as follows:
- population studies suggest that tea may reduce cardiovascular disease risk
- mouse studies suggest that tea can inhibit development of atherosclerosis (although the authors recognise that human studies are needed)
- RCTs suggest that tea can enhance nitric oxide levels and improve endothelial function (which has a role in cardiovascular disease)
- RCTs provide limited evidence that green tea might reduce body fat
- there is inadequate evidence about the effects of tea on oxidative stress, inflammation, blood platelet activation, blood pressure and risk of type 2 diabetes
How did the researchers interpret the results?
The researchers suggest that tea is likely to provide protection against cardiovascular disease. Green and black tea appear to have similar health benefits.
This review summarises a selection of the existing evidence on the associations between tea and aspects of cardiovascular health. It is not clear whether researchers included all the available evidence, how they selected their studies, or whether any other evidence came to similar conclusions.
As the authors imply, large randomised controlled trials are needed to look at any association between tea and the prevention of cardiovascular disease. Where diet is concerned, RCTs are difficult to design because of the difficulties associated with trying to isolate dietary factors. Also, they are expensive to set up.
Moderate consumption of whichever colour tea you prefer is unlikely to damage your health, and it may have some benefits, as this research suggests. However, it is well established that the best way to reduce the risk of heart disease is to follow a healthy diet and lifestyle, which involves regular exercise, reducing fat and salt intake, eating plenty of fruit and vegetables, and not smoking.