Re: Control of Obesity and Diabetes among Health Benefits of Green Tea
Chacko SM, Thambi PT, Kuttan R, Nishigaki I. Beneficial effects of green tea: a literature review. Chin Med. 2010;5:13. DOI:10.1186/1749-8546-5-13.
Tea (Camellia sinensis), which is usually consumed
as green (unfermented), black (fully fermented), or Oolong (partially
fermented) tea, is associated with many health benefits. The most significant
effects have been reported with the consumption of green tea.1 Among
those health benefits are the prevention of cancer and cardiovascular diseases,
as well as anti-inflammatory, antiarthritic, antibacterial, antiangiogenic,
antioxidative, antiviral, neuroprotective, and cholesterol-lowering effects.
These authors researched the available literature to highlight the efficacy,
mechanisms of action, and adverse side effects of green tea and its constituent
catechins.
Green tea is composed of proteins (15%-20% dry weight);
amino acids (1%-4% dry weight); carbohydrates (5%-7% dry weight); minerals and
trace elements (5% dry weight); and trace amounts of lipids, sterols, vitamins,
xanthic bases, pigments, and volatile compounds. The health benefits of green
tea are attributed to its polyphenol content. Most of the polyphenols are
flavonols, commonly known as catechins (epicatechin, epigallocatechin,
epicatechin-3-gallate, and (-)-epigallocatechin-3-gallate [EGCG]).
For this review, the authors searched PubMed, EMBASE, AMED,
and China Academic Journals Full Text Database. They included articles about
green tea's health benefits in humans and animals, absorption of metal ions and
drug-metabolizing enzymes, antioxidation and inhibition of oxidative stress,
carbohydrate metabolism and diabetes mellitus, and adverse effects. The authors
selected 105 peer-reviewed articles in English for this review.
Cited in this review are animal studies reporting that green
tea catechins provide some protection against degenerative diseases; that green
tea has an antiproliferative activity on hepatoma cells and hyperlipidemic
activity in hepatoma-treated rats; that green tea catechins could act as
antitumorigenic agents and as immune modulators in immunodysfunction caused by
transplanted tumors or by carcinogen treatment; and that green tea is effective
in preventing oxidative stress and neurological problems.
Green tea has been linked to the prevention of certain
types of cancer. The antioxidant, antimutagenic, and anticarcinogenic effects
of green tea could offer protection against cancer caused by environmental
agents. Tea's anticarcinogenic effects against breast cancer have been reported
in experimental studies; however, say the authors, epidemiologic evidence is
inconsistent. Several epidemiological studies and clinical trials have shown
that green tea may reduce the risk of chronic diseases, particularly
hypertension and coronary heart disease. Other cited animal and human studies
report an inhibitory effect of green tea on Helicobacter
pylori infection and on the influenza virus as well as an antifungal
activity against Candida albicans.
Green tea consumption has also been associated with
increased bone mineral density and protection against hip fractures.
The authors report that tea catechins can affect iron
absorption, particularly in those at risk for iron deficiency. Noting that the
catechins' effects on other ions are not well understood, the authors suggest
that they may affect absorption and metabolism of ions because flavonoids
interact with various metal ions.
Green tea catechins, along with the antioxidant vitamins C
and E and the tea's enzymes, are hypothesized to contribute to the total
antioxidant defense system. In vivo studies have reported that green tea
catechins increase total plasma antioxidant activity. The content of the
oxidative stress marker malondialdehyde has been shown to decrease after green
tea intake. One cited study of 25 patients with different gastrointestinal
complaints measured the tolerance of green tea tablets and their effect on
antioxidant status indices. A decreased level of all antioxidant status indices
was seen in the treatment group.
Tea catechins, especially EGCG, appear to have antiobesity
and antidiabetic effects. Recent data from human studies indicate that the
consumption of green tea or green tea extracts may help reduce body weight,
mainly body fat, by increasing postprandial thermogenesis and fat oxidation.
One cited study of six overweight men given 300 mg EGCG daily for two days
suggests that EGCG alone has the potential to increase fat oxidation in men and
may thereby contribute to the antiobesity effects of green tea. (See HC
041031-399 for Consumption of Green Tea May Help Prevent Obesity)
Cited studies in animal models of diabetes reported
reductions in serum glucose levels with the administration of green tea
polyphenols. In normal rats, green tea catechins reduced plasma triglyceride
levels in an oral glucose-tolerance test. Green tea and green tea extracts have
been demonstrated to modify glucose metabolism beneficially in experimental
models of type 2 diabetes mellitus. A human study reported that green tea
promoted glucose metabolism in healthy human volunteers as shown in oral
glucose-tolerance tests.
The authors point out that the effects of green tea and
its constituents may be beneficial up to a certain dose and may cause some as
yet unrecognized adverse effects at higher doses. Also, the effects of green
tea catechins may vary from person to person. The harmful effects of too much
tea are mainly due to its caffeine content, the presence of aluminum, and the
effects of tea polyphenols on iron bioavailability.
The authors conclude that long-term consumption of tea
catechins could be beneficial to high-fat diet-induced obesity and type 2
diabetes and could reduce the risk for coronary disease. Further research
should focus on the pharmacological and clinical effects of green tea and its
mechanisms of action.
—Shari
Henson
Reference
1Cabrera C, Artacho R, Gimenez R. Beneficial effects
of green tea: a review. J Am Coll Nutr. 2006;25:79-99.