“Drinking more than three cups of tea a day cuts the risk of a heart attack”, The Daily Telegraph reported today. The newspaper said that according to research, tea - with or without milk - may also strengthen bones. It adds that the “beneficial effects on many biochemical effects in the body” are due to polyphenols, which are natural plant antioxidants in tea.
This study is a review which has pulled together the results from several reviews and trials of tea-drinking and its affects. Although the research has been thoroughly carried out, the high variability in trials conducted in this field makes it difficult to draw the results of studies together to provide conclusive results.
For now, it may be best for people to continue to enjoy a cup of tea because they like it, rather than hoping for any protection from heart attacks. There are many medical and lifestyle risk factors which contribute to the development of heart disease.
Where did the story come from?
Dr Carrie H.S. Ruxton of Nutrition Communications, Cupar, UK, carried out the research. The study was funded by the Tea Advisory Panel and the Tea Council. The study was published in the (peer-reviewed) medical journal British Nutrition Foundation .
What kind of scientific study was this?
This review of research into black tea and health was written in a narrative style. It adds supportive evidence to a previously conducted systematic review on the benefits of tea, balanced against concerns over the effects on body hydration and iron status. The author explores reasons for conflict among studies found, including lifestyle factors.
The review begins with a brief introduction to the tea-making process including market data showing how much tea is consumed in the UK. The author then discusses the health properties of black tea, supporting each point with a referenced piece of research. The search methods that the researcher used to identify these results are not provided.
The discussion includes the polyphenols in tea - part of the flavinoid group – and their availability in the body once they are consumed. Black tea is also compared with green tea and other drinks such as red wine and orange juice. The effects of adding milk to tea, and the effects on chronic diseases in the body (including cancer and heart and bone disease) are also examined.
What were the results of the study?
- A review conducted in 2007 found conflicting evidence that milk binds to the flavanols in tea, therefore affecting their availability for absorption in the body. An additional clinical study, also conducted in 2007, found that adding milk did not affect the concentration of flavonoids in the blood following consumption. However, with the small amount of data available, much more research will be needed to verify this.
- The studies investigating tea consumption on cardiovascular health varied in design between observational studies linking tea drinking to CHD risk factors or mortality, trials designed to investigate the effect of drinking tea on CHD risk, and experimental studies where human tissues were exposed to polyphenols in the lab. A review conducted in 2001 of 10 follow-up studies, found that the risk of heart attack was reduced by 11% when three cups of tea per day were drunk (237ml). Other studies that have since been conducted showed similar results, with four studies demonstrating links between tea consumption and reduced risk of CHD death and risk factors, while one study did not.
- Five studies investigating bone health suggested that tea had a ‘modest beneficial effect’ on bone mineral density, with most benefit seen in elderly women who drank four or more cups per day.
- Evidence from cancer studies was too conflicting to draw any conclusions.
What interpretations did the researchers draw from these results?
The author concludes that ‘the clearest consistent evidence points to an association between tea consumption, in excess of three cups per day, and a reduced risk of myocardial infarction’. She says that there is no consistent evidence to indicate that tea drinking has detrimental effects on hydration, bone health, or iron status. However, she acknowledges the non-uniformity of both observational and interventional studies, and says that tea is ‘worthy of further research.’
What does the NHS Knowledge Service make of this study?
The author has carried out a very thorough analysis of the status of research into black tea consumption and its effects. However, due to the nature of the topic, there are several limitations to the trials conducted in this field. The author says, ‘Taken together the evidence indicates a positive role for tea in human health, although the final proof from intervention studies remains elusive.'
- The methods used in each of the individual studies were not reported, and therefore the reliability of their findings are unclear from this report. In particular, in many of the follow up studies tea consumption was assessed by phone or questionnaire, which introduces recall bias and differences in reporting between those investigated. For example, if the participants were questioned on their tea intake on that day, this may not reflect a consistent pattern, and the size of one person’s cup of tea may be different to another’s.
- Intervention trials were very variable in the outcomes that were examined. For instance, some looked at measures in the blood while others looked at blood pressure. They also varied in the people they included, tea preparation and amount of tea consumed. Additionally, many were only one-day interventions, which cannot reliably represent effects in the long term.
- Other risk factors for heart disease or for other outcomes examined such as bone disease are not clear. One previous review acknowledged that inadequate control of confounders (such as lifestyle and genetic predisposition to disease) was a problem across intervention studies.
- The high variability in the trials makes it difficult to draw the results of the studies together to provide conclusive results.
- The methods used to conduct the search for reviews or individual tea studies are not clear from the report.
For now, it may be best for people to continue to enjoy a cup of tea because they like it, rather than hoping for protection from heart attacks. There are many medical and lifestyle risk factors that contribute to the development of heart disease.
Sir Muir Gray adds...
Tea? I love it.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Daily Mail, 11 June 2008
Metro, 11 June 2008
Daily Express, 11 June 2008
The Daily Telegraph, 11 June 2008
Links to the science
Nutrition Bulletin 2008; 33 91-101