Tea Leaves, Cocoa bean, Coffee bean

Tea

There are three types of tea: green, black and 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.

The main polyphenols in tea are catechins, which include 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). 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. In a Norwegian cohort the risk of dying from heart disease was 36% lower in tea drinkers compared with non-tea drinkers.

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

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. 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.

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.

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. 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).

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. 

Cancer: 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.

In summary: 

Evidence suggests that a substantial effect of tea drinking on serum lipids can now be ruled out and that a role in
limiting oxidation of LDL 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, inflammation and thrombosis. There is also evidence that tea, especially green tea, can prevent the spread of cancerous cells by of inhibiting angiogenesis (i.e. preventing new blood vessel formation).

For more information see: http://www.mayohealth.org/mayo/askdiet/htm/new/qd990310.htm and http://www.mayohealth.org/mayo/9901/htm/tea.htm

Coffee bean

Drinking 6 cups of unfiltered plunger coffee a day may increase the risk of heart disease (Grubben et al. AJCN 2000; 71: 480-4) because blood homocysteine and cholesterol levels increased by 10% and triglycerides by 36%. Sesquiterpenes in coffee is removed by filtering and is known to increase blood cholesterol levels. Caffeine may also lower blood levels of vitamin B6 which may explain the effect on homocysteine. However, coffee contains potentially cardio-protective flavonoids which have been shown to be antioxidants. Filtered coffee in moderation (3-4 cups per day) is ok.

Coffee has been shown to release fat from fat cells and hence have a possible benefit in a weight control diet. Marathon runners will drink a cup of coffee before a race to prevent hitting the wall. The theory behind this is that glycogen, or sugars, are the primary source of energy in the muscles, whereas fat is used as a ‘back-up’ fuel. Once glycogen supplies are exhausted (after 2 hours of vigorous activity) fat comes more into play, but this is not as easily accessible. Caffeine before a race may release fat into the blood stream quicker and help ‘save’ glycogen so this does not run out suddenly, leaving a feeling of ‘hitting a wall’ (Vinson & Dabbagh; Nut Research 1998; 18 (6): 1067-75).

Caffeine: Green tea has as much caffeine as black tea. A typical eight-ounce cup of tea prepared from one tea bag brewed for three to five minutes contains 40 milligrams of caffeine, compared with 100 milligrams in a cup of brewed coffee. The caffeine content of tea can range from 20 to 90 milligrams a cup, depending on the blend of tea leaves, method of preparation and length of brewing time, whereas a cup of coffee may contain from 60 to 180 milligrams of caffeine. Decaffeinated tea, like decaffeinated coffee, has about 4 milligrams of caffeine per cup. Instant teas and prepared iced teas, which can be purchased with or without caffeine, may be too highly processed to contain phytochemicals. 

If you would like to find out about 'caffeine' - its dietary sources and whether it is harmful of beneficial - read this good on-line summary by Choice Magazine May 2000

Cocoa Bean

Cocoa is a rich source of procyanidin flavonoids, including epicatechins and its derivatives. The antioxidant content is significantly higher than tea. Oligomeric polyphenols in cocoa are stronger antioxidants than monomeric polyphenols in tea. Oligomeric polyphenols inhibit reactive oxygen species which can damage cell membranes leading to diseases such as arteriosclerosis, cancer and the ageing process. After an acute chocolate meal, plasma concentrations of epicatechin and plasma antioxidant capacity increase compared to an acute white chocolate meal. The increase in antioxidant capacity has been observed by a decrease in lipid oxidation by-products. These results suggest that flavonoids can be absorbed in quantities to influence the body’s defence parameters. The lipid profile of cocoa butter, the major lipid in chocolate consists of oleic acid, stearic and palmitic acids. Several studies indicate that stearic acid has a neutral effect on blood lipids. However, stearic acid may activate clotting factor VII, may increase lipoprotein Lp(a) concentration and may impair fibrinolysis (AJCN 1999; 70: 951). However, cocoa butter appears to decrease platelet activity, potentially beneficial for cardiovascular health. 

Feeding studies in human showed that LDL cholesterol in the blood taken 2 hours post consumption of chocolate was less likely to oxidise. One study demonstrated that chocolate polyphenols affected platelet activation proteins. When platelets which were exposed to chocolate polyphenols were stimulated with epinephrine (known to cause platelet aggregation) aggregation decreased. More evidence is needed on how these findings translate to heart health. 

For more information (on-line) on chocolate go to: http://www.healthyeating.org/newsletters/march-00-newsletter.htm

 

Last Updated: March 28, 2001