Nearly 500,000 tea drinkers were scrutinized. This is the largest cohort (tracking of people) ever conducted. Surprisingly, it is not the green tea drinkers who are in the spotlight but the black tea drinkers. This study, published this summer 2022, has made the English tabloids. From a scientific point of view, what is behind the "reduction of all causes of death by black tea"?
The follow-up of 500,000 black tea drinkers aged 40 to 69 years was carried out between 2006 and 2010. Each person had to answer a questionnaire about their tea consumption ranging from 0 to more than 10 cups of tea per day but also about the temperature of the tea (lukewarm / hot / boiling), the type of tea (here mostly black) as well as the addition of additives (sugar, milk, the study is British, it is common to sweeten and / or put milk).
Why obtain this information? To understand if beneficial effects appear in a dose-dependent way (the more tea you drink, the better), if the addition of sugar/milk interferes or not with the tea and if the heat would favour the appearance of pathologies (some studies show that boiling tea could favour the appearance of gastric and oesophageal cancers, the results are still to be discussed).
Other information was listed such as gender, ethnicity, medical history (including overweight/obesity, alcohol and smoking).
Why all this information? The aim is to obtain a maximum of information allowing to understand if: tea has an impact on the prevention of the appearance of diseases (cancers, cardiovascular diseases, respiratory diseases) and if there are differences or even a gain for populations at risk (obesity, sex-dependent diseases, etc).
The results of the study have been widely reported in the media, so I will not dwell on them too much, all causes of mortality were reduced by 4% to 50% in this cohort. However, there are extremely important elements to discuss in order to understand all the nuances of the study which make it not just another "tea is good for you" study!
First of all, the study was interested in the "dose-response" curves, i.e. from how many cups of tea people consumed, they reacted the best. The first information to remember is that neither the addition of additives nor the temperature of infusion had any impact on these curves. So whatever the number of cups of tea drunk with sugar/milk/heat did not affect the quality of response of the people.
It seems that the best dose-response data were obtained from 2 cups. While the results were still slightly better than non-tea drinkers for 8-9 cups, they were less so than for drinkers of 2 cups of tea per day.
Why is this so? We must look at the profiles of the drinkers. Those who drank the most tea daily were people with a less favorable health record, including a greater presence of obesity. It is therefore not possible to conclude that tea from 8 to 9 cups of tea has a more limited effect on the occurrence of diseases.
Surprisingly, the researchers did not find any differences between men and women or between ethnic groups. However, we know that certain diseases occur more or less frequently in certain populations.
Why is this? This question could not be answered by tracking the individuals alone. They took samples to obtain the metabolic potential of the people, the capacity of each person to "integrate" and metabolize caffeine. Indeed, between different people, there are differences when it comes to the management of caffeine in the body (we often say "I am sensitive to caffeine" or not), and this is dictated by our genetics, you can not do anything about it. Interestingly enough, people's metabolic profile has no impact on the results. Whether you have good caffeine management or not, the results would be the same.
What makes this study an interesting publication is also the fact that the authors pointed out the big limitations. Indeed, to facilitate the follow-up, people were asked to evaluate their consumption in "cups". However, a cup can represent very diverse volumes. The classic cup measures between 15 and 20cl while the modern mugs >30cl, which completely changes the volume of water absorbed and/or the quantity of tea used.
Moreover, the main drawback of the study is the lack of traceability of the quantities of tea brewed by people. It is therefore not possible to understand whether it is the frequency of consumption or the total quantity of tea that would have a beneficial effect on health.
On a personal note, another important criterion is the quality of the leaves, since each part of the plant (bud, leaf position, stems) produces different quantities of molecules (https://sciencestea.wixsite.com/monsite/post/th%C3%A9ine-vs-caf%C3%A9ine-qu-en-est-il-exactement )
Another limitation, which should have been explored, is the simultaneous consumption with other drinks such as coffee. Indeed, the authors have pointed out that some people drank both black tea and coffee, or even more coffee than tea, without tracing the quantity. It is therefore difficult to know whether it is caffeine that is the cause of these effects or other cocktails of molecules with similar effects present in both drinks.
DOI article https://www.acpjournals.org/doi/10.7326/M22-0041