Tea is a drink that has traditionally been considered a health tonic. Modern science has corroborated this traditional belief, and many studies have provided evidence that regular consumption of tea confers a number of health benefits. The health benefits of tea are thought to result from its high concentration of polyphenols. While the polyphenols in black tea are a mixture of tannins and flavonoids, the polyphenols in green tea are almost all flavonoids. The flavonoids in green tea almost exclusively belong to a subgroup of flavonoids called the flavan-3-ols (catechins). Both green and black tea also contain flavonols, another subgroup of flavonoids, however the content of these is much smaller. Green tea is one of the best dietary sources of catechins, and regular consumption of tea can therefore provide a level of catechins not available through other foods at the same high concentration. Catechins may provide a number of health benefits, and have been researched for their weight loss effects.
The exact reasons for the weight loss effects of green tea are not fully understood. However a number of plausible mechanisms have been investigated and found to be with some merit. The most obvious and often overlooked weight loss effect of green tea is that the tea is often drunk in place of other beverages, and if these beverages contain sugar, then green tea can significantly reduce sugar intake. Fruit juice, soft drinks and other sugar sweetened beverages may be a significant factor in chronic weight gain, and so elimination of this sugar can be particularly beneficial. Another possible explanation for the benefits of green tea may relate to the beneficial glycaemic effects shown by the catechin polyphenols it contains. These polyphenols may interfere with the digestion and absorption of starch and sugar and in this way green tea may delay the absorption of glucose and limit postprandial glycaemia. This may explain the insulin sensitising effects of green tea, and contribute to its weight loss effects.
Green tea catechins are effective antioxidants and high intakes of green tea may decrease oxidative stress. Oxidative stress is now implicated as a causative factor in the deterioration of the β-cells of the pancreas and the insulin receptors on cells. High levels of oxidative stress may therefore cause a deterioration in the insulin system and this may lead to insulin resistance. Insulin resistance is now considered a significant factor in weight gain. By reducing oxidative stress green tea may improve insulin sensitivity and thus cause weight loss. Green tea may also have significant thermogenic effects and lead to increased metabolic rate. These thermogenic effects may relate to both the caffeine and the catechins within the tea. Caffeine may increase the circulating catecholamine concentrations, through antagonism of the adenosine receptor and through inhibition of caffeine degradation enzymes. The catechins in green tea are also thought to inhibit catecholamine degrading enzymes, increasing circulating adrenaline levels.
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