Green tea is a drink derived from the leaves from the Camellia sinensis plant, and has been traditionally drunk in East Asia for thousands of years. Green tea is thought to have healthy promoting effects based on the presence of a number of flavonoids of the flavan-3-ol class. The main flavan-3-ols present in green tea are epigallocatechin-3-gallate (EGCG ≈48 to 55%), epigallocatechin (EGC ≈9 to 12%), epicatechingallate (ECG: ≈9 to 12%) and epicatechin (EC ≈5 to 7 %). Studies using in vitro techniques have shown that green tea can reduce the oxidation of low density lipoprotein and superoxide production, as well as decrease vascular smooth muscle proliferation. Clinical trials have shown that green tea can induce weight loss, decrease the glycaemic effects of food, decrease lipid oxidation and decrease haemoglobin glycation. Green tea may inhibit formation of atheromas and protect against cardiovascular disease.
Green tea therefore may be of benefit to individuals at risk of metabolic syndrome, a risk factor for cardiovascular disease. Randomised clinical trials involving green tea on subjects with metabolic syndrome have been performed by researchers interested in the cardioprotective effects. For example, 35 subjects with obesity or metabolic syndrome were fed either 4 cups/d of green tea, a green tea extract with 4 cups/d of water, or a control consisting of just 4 cups/d of water1. The intake of flavan-3-ols from the treatments is shown in figure 1. After 8 weeks, the green tea drink and extract had caused a significant reduction in body weight (-2.5kg) and body mass index (-1.9kg/m2) when compared to controls. Although not significant, there was also a trend for the green tea to decrease low density lipoprotein (LDL) cholesterol, and the ratio of low to high density lipoprotein cholesterol.
Figure 1. The flavan-3-ol content of green tea drink and green tea extract used in the study1.
From these results just 4 cups of green tea per day may benefit individuals with metabolic syndrome or obesity by favourably modifying body weight. The lipid effects seen with green tea may result from the improvements seen in controlling glycaemia (reported elsewhere) or may be due to reductions in body weight. Alternatively, beneficial lipid effects may be attributed to direct effects by the flavan-3-ols present in the leaves. The antioxidant effects of green tea have been well reported, and this study showed that levels of malonaldehyde and hydroxynonenals, known markers for lipid peroxidation, both decreased with green tea supplementation, supporting previous findings. This study also supports the previous evidence that shows that green tea catechins are bioavailable in both tea and capsules, and have biological effects in humans. Of the flavon-3-ols, epigallocatechin was the most bioavailable, followed by epigallocatechin-3-gallate, epicatechingallate and epicatechin.
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