TEA

Tea has been implicated in protecting against heart disease, osteoporosis, cancer and even possibly skin cancer if the tea is drunk with lemon! The most recent studies published in 2002 showed that tea drinking can reduce the risk of death after a heart attack by 44% (read abstract on-line) and may also help to maintain total bone mineral density, thus reducing the risk of developing osteoporosis Read abstract on-line

Tea Types

There are three types of tea:
green
black
oolong.

Green tea is the least processed of all the teas, made by quickly steaming or heating the leaves of Camellia sinensis. Black tea is prepared by exposing tea leaves to air which causes oxidation, which turns them a deep brown and intensifies their flavour. The leaves are then crushed. Black tea contains as much of the protective phytochemicals as green tea, though the form may differ. Oolong is between the two: more processed than green tea but less so than black tea. It is exposed to heat, light and crushing for less time than black tea. Herbal teas, on the other hand, come from a wide variety of plants other than the tea plant and may include roots and flowers as well as leaves. Most herbal teas do not possess the antioxidant properties of real tea, although they may contain certain other biologically active compounds. A few also contain caffeine.

Antioxidants

The main antioxidants in tea are polyphenols, such flavonoids and catechins. The catechins includes the compound epigallocatechin gallate (EGCG). EGCG seems to inhibit cell growth and play a role in apoptosis (programmed cell death) - both of which appear to be important in the prevention and control of cancer. Polyphenols are also potent antioxidants that help prevent free radical damage to cells and the oxidation of LDL cholesterol. These two functions seem to inhibit the formation of atherosclerotic plaques, which narrow blood vessels and can lead to heart disease. The bulk of evidence for tea's health benefits comes from studies in animals that were treated with amounts of tea polyphenols equivalent to what might be consumed by a regular tea drinker. 

Cardiovascular disease

Several prospective studies have provided inconsistent results regarding tea intake and decreased risk of cardiovascular disease (CVD).

1. In a long-term study of a Dutch cohort, the risk of dying from heart disease and stroke was significantly lower in men with a high intake of tea which was independent of the major established risk factors.

2. In a Norwegian cohort the risk of dying from heart disease was 36% lower in tea drinkers compared with non-tea drinkers.

3. A Californian study found no effect of tea on cardiovascular risk.

4. Tea drinking was positively associated with increased coronary risk in a Welsh population. The investigators suggested that the addition of milk to tea may bind the tea phytochemicals thereby preventing their absorption. However, subsequent experiments by other investigators have not been able to show that milk decreases the absorption of tea flavonoids. Also, in the Welsh study, men with the highest intake of tea tended to smoke more, eat more fat, and consume less alcohol. This result may be residual confounding by unmeasured or imperfectly measured coronary risk factors which clustered with tea intake. 
5. A Harvard study by Dr. Howard Sesso indicated that people who drank one or more cups of black tea a day were half as likely to suffer a heart attack as those who did not drink tea, regardless of other risk factors for heart disease.

6. Several cross-sectional studies in Japan have found an inverse relationship between the consumption of green tea and serum cholesterol levels. An inverse association between black tea and blood cholesterol levels has also been observed.

7. A study of 1,330 Chinese men found a significantly lower level of serum cholesterol and triglycerides among those who drank more than 10 cups of green tea a day. However, short-term controlled trials in humans have not been able to show any effect of green or black tea on blood lipids.

8. The most recent US study published in 2002 showed that drinking tea can cut the risk of death after a heart attack by up to 44%. A study of 1900 patients found that those who drank the most tea were least likely to die during the three or four years after a heart attack. Death rates for moderate tea drinkers were 28% lower than those who drank no tea. Death rates for heavy tea drinkers were 44% lower (read abstract on-line).

How does tea protect against heart disease?

Evidence suggests that a substantial effect of tea drinking on blood fats can now be ruled out and that a role in limiting oxidation of blood LDL cholesterol is possible. More likely mechanisms by which green or black tea reduces the risk of heart disease and stroke is probably through its effect on blood vessel health, dilation, inflammation and thrombosis.

a) Prevents oxidation of LDL cholesterol?

The antioxidant catechins in green tea and the aflavins in black tea have been shown in bench top studies to inhibit the oxidation of LDL cholesterol and to chelate free metal ions which can act as pro-oxidants. Oxidised LDL cholesterol is more atherogenic (i.e forms a fatty plaque in blood vessels) more readily than unoxidised LDL cholesterol.

However, studies in humans have not found that there is resistance to LDL oxidation when consuming 6 cups of green or black tea (Hof et al. Am J Clin 1997; 66: 1125-32). It is believed that if tea polyphenols protect LDL from oxidation the mechanism is more likely to be at play in the blood vessel wall than in the LDL particle itself. This avenue of research is currently being explored (Tijburg et al. Food Sci Nutr 1997; 37 (8): 771-85).

b) Inhibit inflammation?

Coronary heart disease is considered to be a disease with a strong inflammatory component and tea polyphenols may inhibit inflammation. Tea polyphenols have a pronounced antagonism to bradykinin, which is released during some inflammatory responses, they can also inhibit arachidonic acid and histamine metabolism. However, most of these studies have been conducted in isolated cells or in animals. There are indications that both green and black tea taken orally may inhibit platelet activity in experimental animals and possibly humans, but more evidence from human trials is needed. 

c) Improved blood vessel dilation?

Nitric oxide is a chemical produced by blood vessel cells (endothelium). It prevents blood clots and inflammation occuring in blood vessels. One of the most important functions of nitic oxide is as a potent dilator of blood vessels, to permit the passage of a greater volume of blood. Healthy people with few cardiovascular risk factors have an appropriate production of nitric oxide, normal vessel dilation and normal formation and breakdown of blood clots. A healthy artery can dilate 13%. In individuals with heart disease risk factors such as obesity, diabetes etc blood vessel dilation is markedly reduced because normal production of nitric oxide is lost.

High levels of free radicals (created by smoking, pollution, UV, diabetes) inactivate nitric oxide. Antioxidant vitamins (such as vitamin C) and phytochemicals (such as flavonoids) appear to be involved in blood vessel dilation.

A study by Duffy et al (Circulation 2001) showed that consuming 450ml black tea significantly improved blood vessel dilation (flow mediated dilation) 2 hours after drinking tea. The observed effect was shown to be due to the effect of tea on blood vessel cells (endothelium) and not due to caffeine. There were no effects of drinking tea on blood pressure or blood fats.

d) Prevents blood clots?

A study published in the British Journal of Nutrition in 2002, conducted by Hodgson et al on 20 healthy people in Australia, were unable to show that black tea had any effect on platelet aggregation (clumping) and thus the prevention of blood clot formation.

How does tea protect against cancer?

There is evidence that tea, especially green tea, can prevent the spread of cancerous cells by of inhibiting angiogenesis (i.e. preventing new blood vessel formation).

Several dozen animal studies indicate that the polyphenols and related compounds in tea are protective, especially against cancers of the oral cavity and digestive tract. Tea chemicals are believed to act by preventing damage to DNA that could result in a loss of control over cell growth. Tea is one of the few agents that can inhibit carcinogenesis at the initiation, promotion and progression stages. However, it is not yet known how effective tea can be in preventing human cancer, what dose is most effective or what is the best way to administer the active compounds.

A study of more than 35,000 postmenopausal women in Iowa suggested that women who drank two or more cups of tea daily were less likely to develop cancers of the digestive tract and urinary tract. However, no protection was found against other cancers. However, a study in the Netherlands among 58,000 men and 62,000 women found no link between tea drinking and a reduced risk of cancers of the lung, breast or colon.

Skin cancer?

Researchers at the University of Arizona College of Public Health, in Tucson, compared the abilities of hot and cold black tea - and each with citrus peel - to protect against squamous cell carcinoma (SCC), a common form of skin cancer. About 450 people, half with a history of SCC, were interviewed. Subjects who drank hot black tea had only 60% of the SCC risk and participants who consumed citrus peel only had 30% of the risk compared to those who consumed neither black tea nor citrus peel. Those who consumed both hot black tea and peel had almost 80% lower SCC risk. Cold tea had no effect, possibly because it is often diluted.

Tea and bone health

High caffeine intake is reportedly a risk factor for reduced bone mineral density (BMD). Most studies, however, are from populations in which coffee drinking predominates and is the major caffeine source. Tea contains caffeine but also has other nutrients, such as antioxidants (e.g flavonoids), that may influence bone mass in different ways.

A study published in the Archives of Internal Medicine in 2002 showed that habitual tea drinkers in Taiwan (number of people studied was 1037) had a higher bone mineral density. People who consumed tea for at least -610 years had a higher total bone mineral density as well as a higher bone density at the lumbar spine and hip bone, than non-habitual drinkers. This was true for green, black and oolong tea.

This study is supported by another study reported in the American Journal of Clinical Nutrition in 2000 which investigated the tea intake and BMD of 1256 free-living women aged 65-76 years in the United Kingdom. This study also found that tea drinkers had higher BMD measurements than non-tea drinkers. These findings were independent of smoking status, use of hormone replacement therapy, coffee drinking, and whether milk was added to tea.

The mechanism for bone protection is not certain, but it probably involes fluoride and antioxidants. Tea is a good source of fluoride and it is known that fluoride is protective against bone loss. The antioxidants in tea (e.g polyphenols) are probably also important for bone, as are fruits and vegetables in general, by mechanisms which include antioxidation.

Conclusion

Consider consuming some tea daily (green, black or oolong) - maybe with a twist of lemon.

References

Mukamal K, Maclure M, Muller JE, Sherwood JB, Mittleman MA. Tea Consumption and Mortality After Acute Myocardial Infarction. Circulation May 6, 2002

Hodgson JM, Puddey IB, Burke V, Beilin LJ, Mori TA, Chan SY. Acute effects of ingestion of black tea on postprandial platelet aggregation in human subjects.
Br J Nutr 2002 Feb;87(2):141-5

Chih-Hsing Wu, Yi-Ching Yang, Wei-Jen Yao, Feng-Hwa Lu, Jin-Shang Wu,
Chih-Jen Chang. Epidemiological Evidence of Increased Bone Mineral Density in Habitual Tea Drinkers. Archives of Internal Medicine, 2002: 162 (9): 1001-6

Hegarty VM, May HM, Khaw KT.Tea drinking and bone mineral density in older women. Am J Clin Nutr 2000 Apr;71(4):1003-7

Last Updated: March 28, 2003