Monday, May 23, 2011

Potential drawbacks of Tea

Effects of fluoride
All tea leaves contain fluoride; however, mature leaves contain as much as 10 to 20 times the fluoride levels of young leaves from the same plant. High fluoride intake (daily intakes over 2 mg for children, 4 mg adults) increases the risk of osteofluorosis and fractures. It is speculated that white tea should contain less fluoride than green tea and black tea, because it is made of buds and young leaves only.
The fluoride content of tea depends directly on the fluoride content of the soil in which it is grown; tea plants absorbs this element at a greater rate than other plants. Care in the choice of the location where the plant is grown may reduce the risk.
Effects associated with caffeine
Caffeine is an addictive drug and overuse of tea may result in harmful side effects, such as an increased likelihood of certain sleep disorders. Decaffeination reduces total catechins in both black and green dry teas by about 15 times and 3 times respectively.
One consideration to take into account when investigating the relationship between caffeine and diuresis is the dose size of caffeine ingested. Where the dose relationship has been systematically investigated it is only at a high dose of 360 mg that a diuretic action is found. A recent systematic review of the accumulated evidence has shown that acute diuretic effects are observed generally in cases where at least 300 mg of caffeine is ingested. This finding suggests that tea does not have a diuretic effect unless the amount of tea consumed at one sitting contains more than 250–300 mg of caffeine, equivalent to between 5 and 6 cups of tea.
Tea contains oxalate, overconsumption of which can cause kidney stones, as well as binding with free calcium in the body; other minerals may be bound as well. The bioavailability of oxalate from tea is low, thus negative effect requires a large intake of tea.
It has been suggested that chemicals known as tannins may increase one's risk of esophageal cancer, with some studies having found that tea drinking may in fact be negatively associated with risk of esophageal cancer.
Hot drinking temperature
Hot tea consumption has been linked to a higher risk for esophageal cancer: "In the case-control study, risk for esophageal cancer was increased for drinking hot tea vs lukewarm or warm tea. Risk was also significantly increased for drinking tea 2 to 3 minutes after pouring, or less than 2 minutes after pouring vs drinking tea at least 4 minutes after being poured." However, the act of boiling the water in the preparation of tea is known to kill harmful bacteria in the water and can be beneficial particularly in countries with lower quality drinking water.
Risk of liver damage
Consumption of some forms of tea has the potential to result in acute liver damage in some individuals. Several herbal & dietary supplements have been linked to liver damage, caused in part or completely by the presence of green tea extract in these supplements; the most notable cases include Hydroxycut (415 mg of a mixture of green, white, and oolong tea extracts, and several other herbal extracts, per dose); Exolise (360 mg of green tea extract per dose); and Tealine (250 mg of green tea extract per dose).

These concerns resulted in withdrawals of the first two products and a class action lawsuit against the manufacturer of Hydroxycut. The risk is thought to be quite small: in case of Hydroxycut, 9 million bottles were sold in the U.S. over the lifetime of the product, resulting in 23 known severe cases, however, these included at least one fatality and at least three cases of liver failure resulting in a liver transplantation. In case of Exolise, the risk of an adverse effect was estimated as less than 1 per 100,000. The mechanism is not completely understood, but it is thought that, in some genetically predisposed individuals, regular consumption of green tea, particularly when combined with fasting, can result in accumulation of a flavonoid EGCG (only present in green tea, but not in black tea) in the bloodstream to unsafe levels.
Effect of milk on tea
A study at the Charité Hospital of the Berlin Universities showed that adding milk to tea will block the normal, healthful effects that tea has in protecting against cardiovascular disease. This occurs because casein from the milk binds to the molecules in tea that cause the arteries to relax, especially EGCG. Milk may also block tea's effect on other things, such as cancer. Other studies have found little to no effect from milk on the observed increase in total plasma antioxidant activity. Teas with high EGCG content, such as green tea, are not typically consumed with milk. Previous studies have observed a beneficial effect from black tea which was not attributable to the catechin content. Plant-based "milks", such as soy milk, do not contain casein and are not known to have similar effects on tea.
Milk binds catechins, most notably EGCG. Milk also binds tannin, rendering it harmless, which helps to exemplify the effect on tea's constituent parts (i.e. EGCG binding).
Effect of citrus on tea
Drinking tea, particularly green tea, with citrus such as lemon juice is common. Studies, including a study from Purdue University in 2007, found that most of the antioxidant catechins are not absorbed into the bloodstream when tea is drunk by itself. The study, however, found that adding citrus to the tea lowers the pH in the small intestine and causes more of the catechins to be absorbed.



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