Health Professionals Newsletter

Welcome to the latest newsletter from the Tea Avisory Panel (TAP). This service will keep you up to date with current 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.

Research Update

A dietary pattern that includes a low tea intake is associated with unfavourable left ventricular function.

Some studies suggest that dietary factors may influence heart failure development but these relationships are not well characterised or understood. Significant associations of individual foods or nutrients with heart failure have been demonstrated such as deficiencies in vitamins B6 and B12. But individual foods or vitamins are rarely consumed in isolation thus their combined effects probably differ from their isolated effects. Asymptomatic left ventricular systolic dysfunction (LVSD) is used to identify people at high risk of developing heart failure. Furthermore diabetes has been established as a major risk factor for heart failure. It has been suggested that the association between diabetes and heart failure is partly due to LVSD.

In a recent study, Liu and colleagues examined the influence of certain dietary patterns on Left Ventricular (LV) function and the potential role of metabolic dysfunction.

Data was taken from the cross-sectional Multi-Ethnic Study of Atherosclerosis (MESA). 6,814 men and women aged 45-84 years who identified themselves as white, black, Hispanic or Chinese were recruited from 6 US communities. Those with self-reported physician-diagnosed cardiovascular disease or other long-term conditions were excluded, as were pregnant women.

Of the 6,814 participants: 5,004 (73%) had a cardiac MRI scan and a measure of LV function. Of those 4,601 (92%) went on to complete a food frequency questionnaire. The current study used data from these 4,601 participants, of whom 2185 were men and 2416 were women.

Dietary assessment was undertaken at baseline using a 120-item validated food frequency questionnaire. Foods and beverages we categorised into 47 groups. Other data collected at baseline included age, gender, race-ethnicity, educational level and lifestyle traits including smoking, alcohol consumption and physical activity. Measures of blood pressure, BMI, waist circumference, total, HDL and LDL cholesterol, triglycerides, glucose and insulin were also taken at base line.

Presence of the metabolic syndrome was confirmed according to the American Heart Association diagnostic criteria which are defined as the presence of 3 or more of the following:

• Increased waist circumference (≥102cm for men and ≥88cm for women);
• Raised triglycerides (≥150mg/dL);
• Reduced HDL (<40mg/dL in men and <50mg/dL in women);
• Increased systolic or diastolic blood pressure (≥130 / 85mm Hg) and
• Raised fasting glucose (≥100mg/dL).

analysis was then undertaken to evaluate associations between RRR dietary patterns and LV function.

The RRR dietary pattern score was significantly and positively correlated with an intake of foods with a high glycaemic index, high fat meats, cheeses, and processed foods and negatively correlated with intake of vegetables, soy, fruit, green and black tea, low fat dairy desserts, seeds, nuts and fish.

This RRR dietary pattern score was significantly associated with major components of the metabolic syndrome (waist circumference, triglycerides, glucose, HDL and systolic blood pressure). The participants with a higher RRR score also had worse serum profiles (lower HDL, higher triglycerides and glucose concentrations and greater insulin resistance) (P<0.001) and poorer LV function (greater LV mass, lower stroke volume and lower ejection fraction) (P<0.001).

The authors concluded that their findings need to be confirmed by longitudinal studies but nevertheless their study highlighted that a diet rich in foods with a high glycaemic index, high fat meats, cheeses, processed foods and low in vegetables, soy, fruit, green and black tea, low fat dairy desserts, seeds, nuts and fish, is significantly and unfavourably associated with LV mass and LV function. This association may be mediated by metabolic dysfunction.

This study does confirm that no single food can be responsible for chronic and complex conditions like the metabolic syndrome and heart failure. But tea consumption featured as part of a healthy diet can have positive outcomes for health.

Liu, J., Nettleton, J. A., Bertoni, A. G., Bluemke, D. A., Lima, J. A. and Szklo, M. (2009) Dietary Pattern, the metabolic syndrome, and left ventricular mass and systolic function: the Multi-Ethnic Study of Athersclerosis Am J. Clin. Nutr. 90, 1-7

Long-term tea consumption is associated with a reduced risk of ischemic stroke

A recent study found that older people who consumed at least one cup of tea per week enjoyed a significant reduction in stroke risk compared with infrequent or non-consumers. People who consumed tea for more than 30 years also had decreased risk of ischemic stroke.

Experimental studies have suggested tea may prevent ischemic stroke but underlying mechanisms are not completely understood. The literature suggests that tea (or its components) could reduce hypertension, atherosclerosis and thrombogenesis, which are important risk factors for stroke.

However epidemiological evidence investigating a link between tea drinking, particularly long-term intake, and stroke is still emerging and quite limited. Thus the present study, a hospital-based, case-control study conducted in the Guandong province of Southern China, aimed to ascertain the relationship between tea consumption and ischemic stroke.

Subjects were recruited from 3 teaching hospitals. Cases (n = 374) were patients with ischemic stroke referred by neurology wards. Only patients with first ever stroke were included. Controls (n = 464) were recruited from various outpatient departments. Eligibility criteria included no past history or clinical evidence of stroke, cardiovascular disease, cancer or diabetes. Those with Alzheimer's or requiring long-term dietary modification were also excluded. The average age of both cases and controls was 69 years.

Cases were interviewed at their bedside within 10 days of admission or before discharge. If cases were unable to provide answers as a result of morbidity the next of kin were interviewed instead. Controls were interviewed in an out-patient setting. During the interview data collected included age, gender, weight, height, educational level, smoking status, alcohol consumption, life-long physical activity and the presence of conditions such as hypertension, hyperlipidemia and diabetes. A 125-item semi-quantitative food frequency questionnaire developed and validated for a Southern Chinese population, recorded habitual food intake for the previous year.

Regarding tea intake, subjects were classified as 'ever' or 'never' (<1 cup per month) tea consumers. The 'ever' tea drinkers were then asked to report frequency of consumption, the duration of regular tea drinking, types of tea drunk and the average quantity of tea leaves used for brewing tea per year.

For the analysis tea drinking frequency was reclassified into 4 levels; <1 cup per week, 1-6 cups per week, 1-2 cups per day and >2 cups per day. The lowest intake level served as the reference group. The amount of tea leaves used per year was divided into 3 levels of exposure and <50g per year was taken as the reference group. Controls consumed significantly more tea and had been consuming tea for a longer period than cases (P<0.05). Men tended to consume more tea leaves and had a longer history of tea drinking compared to women across both cases and controls (P=0.001). Overall a decrease in ischemic stroke risk was observed for those who drank at least one cup per week (P=0.015) when compared to infrequent or non-tea drinkers. The greatest reduction in risk was seen for those drinking one to 2 cups per day. When the risk of ischemic stroke was analysed according to the type of tea drunk, the consumption of oolong and green tea were associated with a significant reduction (P<0.001) but not black tea (P=0.73).

There was a significant inverse association with ischemic stroke and drinking duration especially for those drinking tea for >30 years (adjusted OR, 0.40; 95% CI, 0.25-0.64). Total tea leaf consumption was also inversely associated with ischemic stroke, for those consuming ≥4kg per year (adjusted OR, 0.27; 95% CI, 0.16-0.46), although black tea appeared less beneficial than green and oolong teas.

This is the first study to document detailed tea consumption and stroke risk in a Chinese population. The results indicated that the risk of ischemic stroke was significantly lower among subjects who regularly drank one to 2 cups of tea per day. Although black tea does not appear to be as beneficial as green and oolong teas, this is in contrast to 2 prospective cohort studies in Finland and Netherlands that found a clear association between black tea consumption and the risk of ischemic stroke. The authors suggested that this difference maybe as a result of the types of black tea consumed by the current study population including a concentrated postfermented tea called 'Pu-erh'. This is not typically drunk in Western countries and the lack of a significant effect observed for black tea in this study represents a combination of 'Pu-erh' mixed with other subtypes of black tea.

This study provides good evidence that tea drinking may be beneficial for ischemic stroke prevention in a Chinese population. These results are consistent with similar studies carried out in Finnish male smokers, Dutch men and Japanese men and women, although the latter investigated the effects of green tea rather than black tea. Further studies are required to determine whether similar effects would be observed in other populations and indeed whether it would infer protection from other kinds of stroke.

Liang, W., Lee, A. H., Binns, C. W., Huang, R., Hu, D. and Zhou, Q. (2009). Tea consumption and ischemic stroke risk. A case-control study in Southern China. Stroke 40: 0. Published online May 28, 2009

Drinking tea may protect against Parkinson's Disease

A review of the current literature has concluded that tea consumption may provide protection from Parkinson's Disease (PD), the results being most striking in Chinese populations.

The lower rates of cardiovascular disease and cancer in Asian populations have been attributed, by some, to tea consumption. The prevalence of PD in Chinese populations is much lower than in Caucasian populations and some have also linked this to a high tea consumption.

The present study involved a comprehensive review of published data investigating the association between PD and tea consumption.

Published observational studies on PD and tea consumption were identified through a comprehensive search including MEDLINE and PsycLIT. Twelve studies were identified based on the inclusion criteria: 11 case-control and 1 cohort study published between 1981 and 2003. The studies represented different populations from 3 continents; North America, Europe and Asia.

Data were extracted from each study in the form of a risk estimate (odds ratio, OR) and its 95% confidence interval. Pooled risk estimates were calculated and the validity of this pooling was then tested (the test of homogeneity). If this test was significant, denoting heterogeneity across the studies (i.e. studies being grouped together differed from one another), a random effects analysis was performed instead. The random effects analysis accounts for the heterogeneity between different studies. The pooled OR for all 12 studies investigating the risk of PD in tea consumers versus non-consumers suggested a protective effect of tea (pooled OR, 0.83; 95% CI, 0.74-0.92) and included 2,215 cases and 145,578 controls. However the test for homogeneity was significant and therefore the random effects analysis was applied (OR, 0.81; 95% CI, 0.67-0.98). As these results were close to no effect, in reality there may be little effect of tea drinking on PD. However, pooling the results of the 7 case-control studies using only population based controls showed a clear, protective effect (OR, 0.76; 95% CI, 0.66-0.88) with homogeneity between the populations. Therefore it could be that the reason for heterogeneity when all the studies were analysed is due to the study design, i.e. population based versus hospital based. The risk of PD according to numbers of cups of tea consumed was also assessed. Three case-control studies had sufficient data to calculate the risk for consumers of ≤1 cup of tea per day compared to non-consumers. The pooled OR indicated a probable protective effect of drinking up to one cup per day (pooled OR, 0.75; 95% CI, 0.60-0.93). Four case-control studies had sufficient data to calculate the risk for consumers of >1 cup of tea per day compared to non-consumers. However the 95% confidence interval crosses the line of no effect (pooled OR, 0.81; 95% CI, 0.64-1.03) and thus is not significant.

Pooled data for the 3 studies carried out in Chinese populations was homogeneous with a suggestion that tea drinking is protective against PD in this population (pooled OR, 0.73; 95% CI, 0.60-0.90).

The limited number of identified studies on tea consumption and PD reveal it is an under researched area. Nevertheless the findings of a protective effect of tea consumption are encouraging and warrant further research to confirm the association and to quantify the level of consumption needed in order to confer protection.

Quintana, J.L.B., Allam, M. F., Del Castillo, A. S. and Navajas, R. F. (2009). Parkinson's Disease and Tea: A Quantitative review. J. Am. Coll. Nutr. 28:1, 1-6.