Tea is the most consumed beverage in the world after water, drunk in the UK for 350 years and in Asia for more than 4000
years. Data on over 7000 adults from the UK National Diet and Nutrition Survey (NDNS) indicate that 77% of people drink tea, with a mean consumption of 2.3 mugs (540 ml) per day. Men and women drink similar amounts, while 46% drink unsweetened tea (52%
women, 39% men). Those aged 50–64 years consume more black tea than 19- to 24-year olds (mean consumption 644 vs 298 ml).
Antioxidants play an important role in the prevention of chronic diseases. Fruits and vegetables are frequently cited as good sources but mean European intakes remain below the recommended 5-a-day. Black tea, as typically brewed in the UK, contains about
200mg flavonoids per cup. Therefore, tea makes a significant contribution to dietary intakes of flavonoids. Drinking three cups of tea per day for 2 weeks (at a strength of 2 g dry tea per cup) increased the concentration of flavonoids in the blood by 25%.
Cardiovascular diseases
Consumption of black tea has been associated with a lower incidence of heart disease/cardiac death and a reduction in risk factors. One study reported that the incidence rate of MI was estimated to decrease by 11% with an increase in black tea consumption of three cups per day. Flavonoids are shown to prevent the oxidation of LDLcholesterol, reduce clotting and improve coronary vasodilation. While plant polyphenols, such as those in tea and cocoa, increase plasma antioxidant levels. It has been hypothesized that manganese in black tea could impact positively on heart disease risk, via the role of manganese superoxide dismutase in supporting cardiac muscle function and attenuating lipid peroxidation.
Cancer
It has been suggested that plant antioxidants, such as those found in tea, red wine and cocoa, can help prevent and
control cancer development. Flavonoids may exert other effects unrelated to their antioxidant capacity, for example, anti-inflammatory effects and inhibition of tumorigenesis. However, there is no consistent evidence in the literature, that black tea is beneficial in cancer prevention. For the colorectal cancer, there is moderate evidence for a slightly positive or no effect of black tea consumption.
Dental health
The pathogenesis of dental caries involves the fermentation of carbohydrates by plaque bacteria, a byproduct of which is
acid. This causes demineralization of tooth enamel over time. The tea plant naturally accumulates fluoride from the soil
and can contain 196 mg per 2 g dry tea (around one teabag), although the fluoride content of a cup of tea can exceed this if fluoridated water is used during brewing. The FSA Total Diet Study estimated that 1l of tea (four to five cups) prepared with fluoridated water would make a significant contribution to fluoride intake. The most positive evidence suggested that brews of black tea suppress salivary amylase activity. This, in turn, can reduce the cariogenic potential of starch which acts as a slow-release source of fermentable carbohydrate. Other studies showed that black tea decreased tooth surface pH and suppressed the growth and virulence of periodontal pathogens.
Bone health
There have been suggestions that bone mineral density (BMD) may be influenced by chemical compounds in tea such as caffeine, fluoride and phytoestrogens. The available evidence suggested that black tea consumption had a moderately positive effect on BMD, particularly in older women. There was a significant increase in BMD with higher levels of tea consumption (four or more cups per day). Black tea was also identified as an independent protective factor for the risk of hip fractures in men, this effect was independent of the addition of milk to tea. In the UK, black tea consumption increases the overall calcium intake of middleaged women by around 3% of the Reference Nutrient Intake due to the routine addition of milk.
Caffeine
Data from UK studies suggest that the caffeine content of an average cup of tea is 17 mg/100 ml (40mg per 235 ml cup with a range of 1–90 mg). In comparison, coffee supplies 75–100mg per cup. There is controversy about the effects of caffeine on health. Some authors claim that excessive intakes of caffeine are related to hypertension, dehydration, anxiety, insomnia and birth defects. Others suggest positive effects on cognitive performance, physical endurance, fatigue and alertness at intakes of 60–400mg caffeine per day.
Mood
Black tea ingestion seemed to produce a rapid increase in alertness and self-reported improvements in mood. The capacity to process information was shown to be increased, while adverse effects on sleep duration or quality were not evident.
Iron status
It has been suggested that phenolic compounds in black tea could have an adverse effect on iron uptake in the diet, particularly in vulnerable groups such as children, elderly, pregnant women and those with low iron stores. It is recommended that healthy people with a minimal risk of iron deficiency had no cause to restrict tea consumption while, for groups at risk of iron deficiency, tea drinking should be avoided at mealtimes.
Gardner EJ, Ruxton CH, Leeds AR. (2007) Black tea--helpful or harmful? A review of the evidence. Eur J Clin Nutr.: 61(1): 3-18.
http://www.ncbi.nlm.nih.gov/pubmed/16855537
