Increases metabolic rate
Clinical trials conducted by the University of Geneva and the University of Birmingham indicate that green tea raises metabolic rates, speeds up fat oxidation and improves insulin sensitivity and glucose tolerance. In addition to caffeine, green tea contains catechin phenolics that raise thermogenesis (the production of heat by the body), and hence increases energy expenditure. There is also a suggestion that it can increase endurance in exercise by improving fat metabolism.
Possible anti-diabetes effect
There is also epidemiological evidence that drinking green tea and black tea may help prevent diabetes, although it is worth noting that this is evidence of an association, and that future studies are needed to confirm the effect.
Boosts mental alertness
The amino acid L-theanine, found almost exclusively in the tea plant, and in the highest doses in green tea, actively alters the attention networks of the brain, according to results of human trials announced in September 2007. It has been proposed that theanine is absorbed by the small intestine and crosses the blood-brain barrier, where it affects the brain’s neurotransmitters and increases alpha brain-wave activity. The result is a calmer, yet more alert, state of mind.
Boosts immune system
On 21 April 2003 the Brigham and Women’s Hospital released details of a research project which indicated that theanine may help the body’s immune system response when fighting infection, by boosting the disease-fighting capacity of gamma delta T cells. The study included a four-week trial with 11 coffee drinkers and 10 tea drinkers, who consumed 600ml of coffee or black tea daily. Blood sample analysis found that the production of anti-bacterial proteins was up to five times higher in the tea-drinkers, an indicator of a stronger immune response.
Lowers chances of cognitive impairment
A 2006 study showed that elderly Japanese people who consumed more than 2 cups of green tea a day had a 50 percent lower chance of having cognitive impairment, in comparison to those who drank fewer than 2 cups a day, or who consumed other tested beverages. This is probably due to the effect of EGCG, which passes through the blood-brain barrier.
In 2010 researchers found that people who consumed tea had significantly less cognitive decline than non-tea drinkers. Study participants who drank tea 5-10 times/year, 1-3 times/month, 1-4 times/week, and 5+ times/week had average annual rates of decline 17%, 32%, 37%, and 26% lower, respectively, than non-tea drinkers.
Coffee consumption did not show any effect except at the very highest level of consumption, where it was associated with significantly decreased decline of 20 percent. The study used data on more than 4,800 men and women aged 65 and older from the Cardiovascular Health Study to examine change in cognitive function over time. Study participants were followed for up to 14 years for naturally-occurring cognitive decline. (AAICAD 2010; Lenore Arab, PhD; UCLA )
Lowers stress hormone levels
According to a study by researchers at University College London, drinking black tea can lead to lower levels of the stress hormone cortisol after a stressful event. Fifty minutes after being subjected to challenging tasks, subjects who had been drinking 4 cups of black tea daily for 6 weeks, had a 20% greater drop in cortisol than the placebo group. Blood platelet activation, which is linked to blood clotting and the risk of heart attacks, was also lower for tea drinkers.
Effects on HIV
A recent study appearing in the Journal of Allergy and Clinical Immunology was the subject of an article on BBC News. It stated that epigallocatechin gallate (EGCG) found in green tea can lead to the inhibition of HIV binding and may be used as a complementary therapy for HIV patients, but qualified it by noting that “It is not a cure, and nor is it a safe way to avoid infection, however, we suggest that it should be used in combination with conventional medicines to improve quality of life for those infected.”
It was an in vitro (test tube) study, not an in vivo study, which only tested effects of a chemical in green tea. “Many substances shown to prevent HIV infection in the test tube turn out to have little or no effect in real life, so I think there’s a long way to go before anyone should rely on green tea to protect against HIV infection.”
Effects on inflammatory bowel disease (IBD)
The phenolics in green tea have been shown to reduce intestinal inflammation in mouse models of IBD. This effect seems to be related to tea’s ability to interrupt the cascade of inflammatory reactions that are the cause of IBD.
Effects on bad breath
Researchers at the University of Illinois at Chicago stated that tea phenolics help inhibit the growth of bacteria that cause bad breath.
Iron overload disorders
Researchers in Germany have found that a daily cup of black tea can help stop excess iron damaging the bodies of people who suffer from hemochromatosis due to its high content of flavonoids (commonly mistaken for tannins), which limit iron absorption.
References available from source website.