Health Professional Newsletter
Welcome to the eighth newsletter from the United Kingdom Tea Council. This service will keep you up to date with current tea4health activities and events and inform you of the latest published research. We'll keep you updated on a quarterly basis with news and views from across the tea industry.
Black tea has the same effect on endothelial function as green tea
The beneficial effects of green tea on endothelial function are well established. It has been proposed that the catechins present in green tea are responsible for this effect. The present study published ahead of print in the British Journal of Nutrition has shown that black tea improves endothelial function to the same degree as green tea despite having a lower catechin concentration.
This study was conducted in 3 parts; parts one and two were in vitro studies using animal tissue and in part three they recruited human volunteers.
Part one – Bovine aortic endothelial cells were subject to increasing doses of green tea, black tea or control (water). Endothelial nitric oxide synthase (eNOS) activity was used as a measure of endothelial function.
Part two – Pre-constricted, thoracic rat aorta was exposed to increasing doses of green tea, black tea or control to measure vasorelaxation.
Part three – 21 healthy women were recruited to the cross-over study and asked to abstain from tea drinking for 4 weeks prior to commencing the study. The effects of green tea, black tea or control (boiled water) were assessed separately at least three days apart. Changes in brachial artery diameter, used as a measure of endothelial function, were assessed by high resolution vascular ultrasound at base line and two hours after beverage consumption.
In each of – the studies freshly brewed tea was used rather than extracts to mimic real-life conditions. Black tea contained 45% less catechins than green tea but did contain more caffeine.
Increasing concentrations of both green and black tea increased eNOS activity in the bovine preparation – the magnitude of effect of green and black tea were similar.
Both green and black tea induced pronounced, dose-dependant relaxation in rat aorta. The magnitude of the effect of green and black tea were almost identical despite black tea containing half the concentration of catechins.
In the human study drinking green and black tea increased brachial artery diameter in women by a maximum of 10.2% and 9.1%, respectively. Drinking water had no effect on brachial artery diameter.
This study has demonstrated significant effects of both green and black tea on markers of endothelial function both in vitro and in humans. Despite having a lower catechin concentration black tea produced the same magnitude of effect as green tea. The authors suggest that other polyphenolic compounds, such as theaflavins and thearubigins (generated when catechins are oxidised in the manufacturing process of black tea) may compensate for the reduced catechin content in black tea.
Since black tea is the major beverage of choice in Western Countries the extent of the potential cardiovascular health benefits of tea drinking cannot be under estimated.
Jochmann, N., Lorenz, M., von Krosigk, A., Martus, P., Bohm, V., Baumann, G., Stangl, K. and Stangl, V. (2007). The efficacy of black tea in ameliorating endothelial function is equivalent to that of green tea. Br. J. Nutr. Oct 5; 1-6. Epub ahead of print
Tea has protective effect on carotid atherosclerosis in elderly women
Drinking 3 or more cups of tea a day is associated with a reduced frequency of carotid plaques in elderly women according to new research from France.
Researchers from a French multi-centre cohort study (Three-City Study) evaluated the relationship between tea drinking and markers of atherosclerosis (Common Carotid Artery Intima-Media Thickness – CCA-IMT & carotid plaques); they report this as the first study of its kind. Results were then tested for replication in another younger French population sample (EVA-Study, 1123 subjects).
The Three-City Study is a three centre prospective cohort with a study population of 9693 non-institutionalised persons aged 65 and over living in three cities in France.
Six thousand five hundred and ninety seven participants under 85 years underwent an ultra sound examination of the left and right carotid arteries to identify plaques and measure the CCA-IMT. Information regarding medical history, demographics, diet, tea and coffee consumption were also collected.
Results were reported for 2613 men and 3984 women with a mean age of 73 years and a mean Body Mass Index (BMI) of 26. In women there was an inverse association of tea drinking and hypertension, diabetes and smoking habits. These associations were not seen in men. Increasing daily tea consumption was associated with a lower prevalence of carotid plaques in women: 44%, 42.5% and 33.7% in women drinking no tea, 1 to 2 cups daily and 3 or more cups daily (P=0.0001). This association was independent of major vascular risk factors, dietary habits and educational level. The effect was not seen in men. There was no association of tea drinking and CCA-IMT in either men or women.
The EVA study is another French prospective cohort study including a total of 1389 subjects aged between 59 to 71 years. For the purpose of this current study an ultrasound examination of the carotid arteries was examined among 1123 participants (462 men and 661 women). There was a trend towards a lower frequency of carotid plaques with increasing tea consumption in women, but no association in men. Again there was no association of tea drinking and CCA-IMT in either men or women.
The results of these studies are consistent with others looking at tea consumption and either vascular events or aortic and coronary atherosclerosis. The reason why tea consumption was associated with carotid plaques in women, but not men, is unclear. A number of explanations have been proposed including estrogen-like activity of tea constituents and better bioavailability of quercetin (a tea flavonoid with antioxidant properties) in women.
These studies were in a French population where tea drinking is much less common than the UK; in the Three-City study less than 10% of participants drank 3 or more cups of tea per day. The authors concede that in a French population tea consumption may be an overall indicator of a healthy diet and lifestyle that may not have been fully accounted for in adjusting for covariates. They report that in the UK tea drinking is associated with lower social status and a less healthy diet and lifestyle. It is unclear how the results of this study would apply to a UK population.
However due to the large size of the population, the consideration of a number of potential confounders and the fact that this trend was replicated in another independent study, all suggest a true inverse association between tea consumption and carotid plaques in elderly French women. The authors conclude by suggesting that further studies are required to confirm the findings and determine underlying mechanisms.
Debette, S., Courbon, D., Leone, N., Gariepy, J., Tzourio, C., Dartigues, J., Barberger-Gateau, P., Ritchie, K., Alperovitch, A., Amouyel, P., Ducimetiere, P., and Zureik, M. (2007). Tea consumption is inversely associated with carotid plaques in women. Arterioscler. Thromb. Vasc. Biol: Dec 6; Epub ahead of print
Black tea can benefit blood sugars
Drinking one gram of instant black tea increases the insulin response and reduces blood glucose levels in healthy volunteers according to a study published in October’s issue of the Journal of the American College of Nutrition.
Researchers from King's College London and the University of Central Lancashire devised, a four-way randomised, crossover trial and assigned 16 healthy subjects (12 females and 4 males, mean age of 35.5 years and mean BMI of 23.8) to consume 75 grams of glucose dissolved in:
- 250ml of water (control)
- 250ml of water plus 0.052g of caffeine (positive control)
- 250 ml of water plus 1g instant tea
- 250 ml of water plus 3g of instant tea
Following an overnight fast (12 hours) each subject received each of the four drinks on a different day in a randomised order. Each study drink was consumed within a 15 minute time period. All participants were asked to maintain their usual dietary habits and physical activity patterns for the duration of the study.
Some subjects who were allocated the strongest tea drink, 3g, experienced vomiting and palpitations after ingestion, so this part of the study was abandoned.
The researchers report that plasma glucose concentrations during the first hour in response to the drinks were not significantly different. However, after two hours plasma glucose concentrations were significantly lower in the group who consumed 1g of instant black tea, relative to the control and caffeine drinks.
Also, drinking the instant black tea was associated with increased insulin levels compared with the control and caffeine drinks at 90 minutes. This is consistent with other studies that have reported the polyphenols found in tea enhanced insulin secretion.
Chemical analysis showed that the instant black tea was rich in polyphenolic compounds (total, 350mg/g) and was similar to published values for black leaf tea products.
Bryans and colleagues state that the polyphenol content of the tea was most probably behind the effects. They believe that these compounds could have an insulin-stimulating effect on pancreatic B-cells – the cells responsible for insulin production.
However another possible role of these polyphenol compounds could be on gut hormones that stimulate insulin secretion (Glucose-dependant Insulinotropic Polypeptide and Glucagon-like peptide-1).
This study has demonstrated that instant black tea, consumed at an amount equivalent to 1.5 cups a day, can reduce the late phase of postprandial glycaemia. It is unclear if or how these effects on plasma glucose and insulin will be altered in everyday patterns of food intake and tea consumption. However it is an area that warrants further investigation.
Bryans, J. A., Judd, P. A., and Ellis, P.R. (2007). The Effect of Consuming Instant Black Tea on Postprandial Plasma Glucose and Insulin Concentrations in Healthy Humans. J. Am. Coll. Nutr. 26(5) 471-7.