Thursday March 20 2008
Three cups of tea or more a day were linked to lower plaque levels
Women who drink three cups of tea a day “are less likely to have heart attacks and strokes,” the Daily Express reports. The newspaper adds, however, that “strangely, no added benefit of tea drinking was found among women who only had one or two cups a day or for men”.
The story is based on a French study that looked at how the thickness of ’plaques’ in the carotid artery were related to the amount of tea that men and women drank. Plaques are build-ups of fat and cholesterol in the blood vessels (known as atherosclerosis). They have been linked to an increased risk of heart attack and stroke. The women who drank three cups of tea or more a day were seen to have fewer plaques in their arteries. However, because of its design, this study cannot prove that tea consumption was the cause of less plaque build-up in the arteries; a randomised controlled trial would be the best way to show this.
On the strength of this study alone, it would be better to suggest to women (and men) that they should drink tea because they enjoy it, rather than in any hope that it may reduce their risk of heart disease or stroke.
Where did the story come from?
Stéphanie Debette and colleagues of the University Hospital of Lille and Broussais Hospital, Paris, France carried out this research. The Fondation pour la Recherche Médicale funded the preparation and initiation of the study. Various other institutions and foundations also provided support. It was published in Arteriosclerosis, Thrombosis and Vascular Biology, a peer-reviewed medical journal.
What kind of scientific study was this?
This was a cross-sectional analysis of data collected at the start of the Three-City (3C) Study, a large cohort study of the French population. The 3C study attempted to clarify the relationship between tea consumption, thickness of the carotid artery and plaque build up.
In the 3C study, 9,693 people aged 65 years or older were recruited from the cities of Dijon, Bordeaux and Montpellier between March 1999 and March 2001. Of these people, 6,635 (excluding those aged over 85) had an ultrasound examination to determine their carotid artery thickness. The participants were also interviewed by a nurse and data was collected on past medical history (including cardiovascular disease and medications), lifestyle factors, education, average daily consumption of tea and coffee (divided into none, one to two cups, or three or more cups), and average weekly consumption of various foods. The study also recorded blood pressure, body mass index (BMI), blood sugar levels and blood cholesterol levels.
Of the people with ultrasound scans, 6,597 also provided data on their tea consumption and these were included in the final analysis. The researchers carried out statistical tests to see whether there was any link between the three tea-drinking categories and thickness of the carotid arteries. The researchers adjusted for possible confounding factors, including age, smoking, BMI, diabetes, cholesterol, alcohol consumption and diet.
EVA, a matching study, was carried out following these results to investigate whether similar associations were found in a younger population group aged between 51 and 79.
What were the results of the study?
When the researchers looked at the association between level of tea consumption and the other variables tested, they found that tea consumption was inversely associated (i.e. when one increases the other decreases) with age, blood pressure, BMI, diabetes and smoking in women. It was inversely associated with alcohol consumption in men, and with coffee and meat consumption in both. In both men and women, tea consumption was positively associated (an increase in both variables at the same time) with consumption of fish, fruit and vegetables and a higher education level. It was also positively associated with hormone replacement therapy (HRT) in women.
The researchers found that drinking three or more cups of tea reduced the risk of finding plaques on the carotid arteries in women, even after adjustment for potential confounding factors. However, there was no link between tea consumption and thickness of the artery wall. In men, there was no link between tea consumption and carotid plaques or thickness of the artery wall.
In the EVA study of younger men and women, there was no link between tea consumption and carotid plaques or thickness of the artery wall in either sex.
What interpretations did the researchers draw from these results?
The researchers conclude that, in their large French sample, carotid plaques occurred less frequently in women who drank three or more cups of tea per day. However, this association was not seen in men.
What does the NHS Knowledge Service make of this study?
This is a large population study and, reportedly, the first to examine the link between tea consumption and atherosclerosis. However, interpretations from the results should be made with care.
- The cross-sectional design of this study means it cannot prove that tea consumption reduced fatty build-up in the arteries. To get a better idea of whether this is the case, researchers should, over time, follow two groups of people: those who drink tea and those who do not. Ideally, the groups should be randomised to balance other potential confounding factors. Both groups should be free from atherosclerosis at the start of the study and should be monitored to see whether there is any difference in the development of the condition between the groups.
- The study considered many confounding factors, but there are others that may have influenced the results, e.g. exercise level and family history of heart disease.
- The data on tea consumption is self-reported on only one occasion. A single estimate of average daily tea consumption may not be a reliable estimate of long-term tea-drinking patterns. This would also be the case for other one-off measures, such as blood pressure, BMI, smoking, alcohol or other dietary factors. Additionally, there is no information on the type of tea that the participants drank, e.g. black tea, white tea, green tea or others.
- This study only looked at the degree of atherosclerosis in the carotid arteries; it did not looked at other blood vessels in the body (heart attacks and stroke can be caused by an embolus, which is a section of plaque that has travelled to other parts of the body, as well as thrombosis at the site).
- This research did not look at long-term outcomes in the study population. Those with signs of greater thickening on the arteries at ultrasound scan may not necessarily be the ones who will go on to have a heart attack or a stroke. There are many risk factors for these conditions.
- Finally, these results are not applicable outside the French population. Dietary patterns and health in general may be different elsewhere.