Health Professional Newsletter
Welcome to the sixth 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.
Research Update
The best way to get flavonoids from tea – let it brew
According to results from a new study, longer brewed tea has higher concentrations of flavonoids and antioxidant activity than tea that is only briefly brewed. These findings are unaffected by the addition of milk.
Flavonoids have recently received much attention in relation to their antioxidant activity. However there has been much debate as to whether these antioxidant properties are sustained once inside the body, as well as the effect of milk on this action. While some studies have suggested that adding milk can affect the bioavailability and antioxidant potential of flavonoids, others have found no such effect. It has been suggested that the reason for these differences is that brewing time does not get taken into account in these studies. The purpose of this investigation was to examine the effect of brewing time on flavonoid concentration as well as their antioxidant potential. The effect of adding milk on antioxidant capacity was further investigated in an intervention study with human volunteers.
Researchers at the Rowett Research Centre in Aberdeen brewed 3 grams (equivalent to 1 UK tea bag) of tea in 300mls of freshly boiled water. The tea was infused for 3, 5, 7 and 10 minutes. 6 different brands of tea were infused in this way. Each tea infusion was stirred once when the water was added and filtered at the end of the infusion time to remove any tea leaves. The antioxidant capacity was measured using the ferric reducing ability of plasma (FRAP) method.
The different brands of tea showed similar patterns in releasing flavonoids from tea leaves. Flavonoid content, as well as antioxidant concentrations, increased with infusion time, levelling off after 7 minutes. The 7 minute tea infusion had up to 60% more flavonoids than the tea that had been infused for 3 minutes. The increase in FRAP value (the measurement of antioxidant activity) correlated with the flavonoid content and was not affected by the addition of milk.
To examine the effect of tea once it had been drunk, 9 healthy male volunteers were randomly assigned to 3 groups and consumed 3 different beverages on 3 different occasions 1 week apart. After refraining from drinking tea for 24 hours and fasting overnight, volunteers drank 400mls (equivalent to 2 cups) of either black tea with water, black tea with semi-skimmed milk, or a control (water with semi-skimmed milk). The tea was made up using 3 grams of tea and infused for 7 minutes in 300mls of water (to allow the maximum release of flavonoids). This infusion was filtered before either 100mls of cold water or semi-skimmed milk was added. Blood samples were taken 10 minutes before the volunteers drank one of the test beverages, then 50, 80 and 180 minutes afterwards. Volunteers did not eat anything, nor drink more than 100mls of water, until after the last blood sample was taken.
Within 50 minutes of drinking black tea, the level of flavonoids in the volunteers’ blood rose by up to 45%. Within 80 minutes the antioxidant potential of the blood rose by 10%. The findings were unchanged when milk was added to the drink. No significant changes in plasma flavonoid concentrations were seen after drinking the control beverage.
As well as demonstrating that longer brewed tea has higher concentrations of flavonoids, this study provides further support that flavonoids from tea do have antioxidant properties in the blood once they have been consumed and that these effects are not altered when milk is added.
Kyle JA, at al (2007) Effects of Infusion Time and Addition of Milk on Content and Absorption of Polyphenols from Black Tea. J Agr Food Chem; May 10 (epub ahead of print)
Tea is the number one provider of flavonoids for the US population
A study that has sought to determine the average intake of flavonoids among the US population has found that, out of a range of fruit, vegetables and beverages, tea was the major flavonoid source in the US diet.
High consumption of fruit and vegetables has been associated with a reduced risk of diseases such as cancer, heart disease, and various other degenerative conditions. In addition to the vitamins and minerals present in fruit and vegetables, plant compounds, known as phytochemicals, may also contribute to these beneficial effects. One group of phytochemicals that have been the subject of much research are flavonoids. As well as fruit and vegetables, flavonoids are also present in certain beverages such as tea, wine and beer.
Data from the recently released United States Department of Agriculture (USDA) flavonoid database was expanded to include similar foods that had different processing techniques e.g. frozen, tinned, baked, steamed, etc. In this way a more complete flavonoid database was established. Twenty four hour dietary recalls from 8809 adults, which participated in the National Health and Nutrition Examination Survey (NHANES) during the period 1999-2002, were then analysed using this expanded database.
Results from this study established that the total daily flavonoid intake in subjects over 19 years old was approximately 190mg/ day. Flavon-3-ols were the major contributors (82.5%) to flavonoid intake, followed by flavanones (7.6%), flavonols (6.8%), anthocyanidins (1.6%), flavones (0.8%) and isoflavones (0.6%).
Tea was identified as the most important source of flavonoids contributing 157mg a day. Other important sources included fruit juices (8mg), wine (4mg) and citrus fruits (3mg).
This study is the first step towards gaining baseline data of flavonoid intake among US adults. From this information further investigations are required to determine the relationship between flavonoid intake and the prevention of chronic disease.
OK Chun, et al (2007) Estimated Dietary Flavonoid Intake and Major Food Sources of US Adults. J. Nutr 137:1244-1252
Other News
New flavonoid database published
The United States Department of Agriculture (USDA) has recently updated their flavonoid database. The new release provides analytical values for 26 selected flavonoid compounds in 393 foods, including tea.
For the update, scientists with the Agricultural Research Service analyzed the flavonoids in nearly 60 representative fruits, nuts and vegetables taken from a nationwide sampling. In addition, data from nearly 100 new scientific papers on the flavonoid content of various foods were evaluated for inclusion.
The new release includes the quantities of the 26 selected flavonoids found in 168 new foods that have been added to the original database. Data for many of the food items included in the first release were also updated.
Research studies have consistently provided evidence suggesting an association between consumption of diets high in flavonoids and reduced risk of cancer and heart disease. Documenting the flavonoid content of foods is essential to researchers who strive to evaluate associations between dietary flavonoid intake and risk factors for various chronic diseases.
This supplemental flavonoid database complements the National Nutrient Database for Standard Reference, SR19, which is the major authoritative source of food composition information in the United States.
Supplemental databases for other phytochemicals found on the Nutrient Data Laboratory website include those for proanthocyanidins and isoflavones.
The new database can be accessed by visiting www.ars.usda.gov/Services/docs.htm?docid=6231
An Update on Theanine and Tea
Drinking tea has traditionally been associated with stress relief with many people believing that having a ‘cuppa’ helps them relax after facing the stresses of everyday life. Despite limited scientific evidence for the relaxing properties of tea, one recent human study found that people who drank tea 4 times a day, for 6 weeks, were able to recover quicker after a stressful event and also had lower blood levels of the stress hormone cortisol, compared to a placebo drink.1
Although the exact mechanisms are not known, it has been proposed that the many ingredients in tea may somehow affect neurotransmitters in the brain.
One compound in tea that is receiving attention for its relaxing and calming properties is theanine.
Theanine is a unique amino acid present almost exclusively in the tea plant (Camellia sinensis). It appears to only occur in three other species; one mushroom species and two other species of the Camellia genus. Since tea is the second most consumed beverage in the world, a considerable amount of theanine is consumed daily throughout the world.
Theanine comprises 1-2 percent of the dry weight of tea leaves, makes up approximately 50% of the amino acids in tea, and is present as the free amino acid only – it does not occur in proteins. Theanine is synthesized in the root of the plant and concentrates in the leaves, where sunlight converts theanine to polyphenols. Because of this, some tea cultivators grow their plants out of direct sunlight to preserve the theanine content.
In rats theanine has been shown to cross the blood-brain barrier and on reaching the brain it has been shown to increase alpha-brain wave activity, a sign of induced relaxation.
A small Japanese study of university students showed oral theanine administration of 200 mg led to increased alpha-brain waves and a subjective sense of relaxation. This relaxed, yet alert, state of mind, began approximately 40 minutes after theanine was consumed.2
Another study subjected 12 participants to four separate trials: one in which they took theanine at the start of a stressful task, one in which they took theanine midway, and two control trials in which they either took a placebo or nothing. The results showed that theanine intake resulted in a reduction in the heart rate (HR) and salivary immunoglobulin A (s-IgA) after the stress induced task compared to the placebo control condition. Moreover, analyses of heart rate variability indicated that the reductions in HR and s-IgA were likely to be due to an effect on the sympathetic nervous activation. It was concluded that the oral intake of theanine could cause anti-stress effects via the inhibition of cortical neuron excitation.3
Although further human studies on the effect of theanine in tea are required before any firm conclusions can be drawn, this exciting area of research could help us to understand why tea is so different from other beverages.
References:
1. 1. Steptoe A, et al (2007) The effects of tea on psychophysiological stress responsivity and post-stress recovery: A randomised double-blind trial. Psychopharmacology (Berl). 2007 Jan;190(1):81-9.
2. 2. Ito K, Nagato Y, Aoi N, et al. (1998) Effects of L theanine on the release of alpha-brain waves inhuman volunteers. Nippon Nogeikagaku Kaishi 72:153-157.
3. 3. Kenta Kimura, et al (2007) L-Theanine reduces psychological and physiological stress responses. Biol. Psychol:74(1):39-45
