January 9, 2020

expert reaction to study looking at tea drinking, and health and longevity

A study, published in the European Journal of Preventive Cardiology, reports that drinking green tea regularly can improve health and longevity.

Dr Jenna Macciochi, Lecturer in Immunology, University of Sussex, said:

“The press release reflects the findings but it’s important to note this study hasn’t found a causal link, and there are confounders and limitations with this type of study – so ‘tea drinkers live longer’ cannot be assumed to mean ‘drinking tea leads to living longer’.

“This is a prospective cohort study, which is perhaps the only practical research design to investigate long term relationships between food and disease such as the one being studied in this paper.  It reflects a ‘real’ exposure to a component (in the case tea) in a more real-life context, improving the generalisability to the wider population.

“The dietary assessment method (face-to-face questionnaires), despite the misconceptions that they do not accurately reflect diet intake, do have solid reliability and validation.

“Nevertheless, these studies are inherently subject to confounding e.g. it doesn’t take into account other lifestyle factors.  A body of evidence in nutrition suggests that whole diet patterns are more informative of diet-disease relationships than any isolated food or nutrient.

“It does build on a growing body of evidence that tea contains many healthful properties (such as flavonoids) and builds on the several previous studies which collectively did not find a unanimous conclusion – this study strengthens the body of evidence that habitual tea drinking is associated with lower rates of atherosclerotic cardiovascular disease, though it cannot prove that it’s definitely the tea that’s responsible.  It also highlighted notable sex differences, although these may be accounted for by proportion of men drinking tea.”

Jodie Relf, Freelance Registered Dietitian and spokesperson for the British Dietetic Association, said:

“It’s been thought for years that drinking tea can have a positive impact on our health, however, there is currently not enough evidence to support exactly what compounds in tea elicit these health benefits and exactly what these benefits are.  What we need to be mindful of is that drinking tea alone is not going to improve our health, it is merely another tool to add to our belts.  We cannot continue to eat high fat foods, live a sedentary lifestyle and expect tea to solve all our problems.

“In this study the authors clearly state that black tea does not have the same benefits as green tea, so if you’re looking for the ‘supposed’ health benefits this study suggests you may need to swap your breakfast tea for green, but you might want to wait until there are more conclusive studies before you make this change.  The researchers do not mention how the tea is consumed or in what form – in China green tea is often consumed as a loose tea and not in tea bag form as it is in more westernised cultures; this may change the health enhancing properties of the tea.

“In China drinking tea is not only about quenching thirst or staying hydrated – the act of drinking tea involves taking time out of your day to brew the tea and drink it whilst taking time to slow down and bring calmness and serenity to your day.  This time of calm alongside the health promoting properties of the tea may be what improves their health by reducing stress levels.

“Other things to consider that are not mentioned in the study are: firstly, what those who weren’t drinking tea were drinking – was tea replaced by sugary drinks or caffeinated beverages in these individuals, and was that what increased their risk of atherosclerotic cardiovascular disease and all cause mortality?  Secondly, we’d need to consider other lifestyle factors alongside drinking tea – was it that those that still practiced the weekly ritual of drinking tea also practiced other healthful behaviors like consuming a healthy diet and exercising?”

Prof Gunter Kuhnle, Professor of Nutrition and Food Science, University of Reading, said:

“There is a lot of interest in the potential health benefits of tea, especially in a country such as the UK where people drink large amounts of tea.  Various studies have suggested a beneficial effect, for example by improving blood lipids, and some of the compounds found in tea are believed to reduce the risk for heart disease – but so far this has not been shown in large, controlled studies.

“It is currently not known how tea – or compounds found in tea – affect health.  The antioxidant effect of polyphenols found in tea has long been assumed to be responsible, but this has been resoundingly disproved in the last decade.  Some of the compounds found in tea might have a beneficial effect, but this is currently still under investigation.

“This study is an observational study and can therefore only establish an association – not a causal relationship.  The authors found that ‘habitual’ tea consumption was associated reduced mortality and risk of heart disease.  However, most people in the UK would qualify as ‘habitual tea drinkers’ in this study as they used two cups per week as cut-off point – which is very little when compared to the average consumption of 3 to 4 cups per day in the UK.  It is not clear from the study whether there is any benefit from higher tea intake – and therefore there is no likely benefit from increasing tea intake by the majority of the British public.

“Among the habitual tea drinkers, only about 10% consumed black tea whereas almost 50% consumed green tea – which is different from the UK population.  A beneficial effect was only observed for green and other types of tea, but not black tea: however, this might be due to the very small number of black tea drinkers.  While black and green tea contain similar amounts of caffeine, they differ in their content and composition of polyphenols.  However, the data available do not allow us to attribute any observed effect to these differences.”

Dr Duane Mellor, Registered Dietitian and Senior Teaching Fellow, Aston Medical School, Aston University, said:

“Tea is the second most consumed drink globally after water, and this study suggests that Chinese tea drinkers who continue to drink tea have a reduced risk of stroke and death related to stroke or heart disease.  It is important to note that the proportion of women who drank tea in this population was a lot lower than seen in men, and as men have a higher risk of these diseases it could have influenced the results.  What was interesting was that those who continued to drink tea had a lower risk of dying during the study, however what is not known is why people stopped drinking tea and that could be key.  As with all studies of populations like this there is no evidence of causality, only that tea drinking is associated with a longer life.  It is possible that what people did instead of drinking tea, including those who stopped regularly consuming tea, could be the factors that increased their risk.

“The study also looked at the differences between black tea, which is more commonly consumed in the UK, and green tea.  Green tea has been linked with a number of health benefits and this was the case in this study and the journal’s editorial links the associated benefits of green tea to compounds called catechins.  If this is correct, the way tea was consumed in this Chinese population could be a factor, and in other countries tea such as the UK and much of Europe it is drunk in very different ways, e.g. with milk.  Also, there is unclear evidence whether the catechins in tea are readily available for us to use and absorb.  It is possible that they may have beneficial effects in the colon, but that too is an area of debate and study.

“However, what should be remembered is that although green tea is safe and may have benefits, the use of green tea supplements should be considered carefully as there has been a number of cases of liver damaged reported in individuals who have consumed these in large doses.”

‘Tea consumption and the risk of atherosclerotic cardiovascular disease and all-cause mortality: The China-PAR project’ by Xinyan Wang et al. was published in the European Journal of Preventive Cardiology at 05:15 UK time on Thursday 9 January 2019.

DOI: 10.1177/2047487319894685

Declared interests

Dr Jenna Macciochi: “No conflicts of interest.”

Jodie Relf: “No conflicts of interest to declare.”

Prof Gunter Kuhnle: “I conduct research into the association between flavanols and health. Flavanols are a group of compounds found in tea.”

Dr Duane Mellor: “None.”