There is increasing evidence that green tea is beneficial to the health. A large body of scientific studies show possible benefits including protection from cancer, cardiovascular disease and obesity. The health benefits of green tea are thought to originate from its high concentration of flavonoids, which include a particularly high amount of flavan-3-ols, a sub-group of flavonoids often called the catechins. Flavan-3-ols may have a range of physiological effects that include antioxidant and gene regulatory roles. The effects of green tea may include improvements to insulin sensitivity, and this may explain some of the weight loss effects of consuming green tea. Black tea is compositionally different from green tea because it is fermented during manufacture and this oxidises many of the flavan-3-ols to tannins, producing a high tannin content and a low flavan-3-ol content. However, despite these compositional changes, black tea may also have weight loss effects and this may be due to its ability to modulate the insulin system.
For example, in one study1, researchers investigated the effects of instant black tea on postprandial insulin and glucose concentrations in healthy humans using a randomised crossover trial. Subjects consumed either 250 mL water with instant black tea, 250 mL water or 250 mL water plus caffeine with a test meal of 75 grams of glucose. The results of the study showed that consumption of black tea caused a reduction in the plasma glucose concentrations at 120 min. when compared to either water or caffeine controls. In addition, consumption of instant black tea caused an increase in postprandial insulin concentrations up to 90 min compared to the water or caffeine control. A chemical analysis of the black tea showed that it contained 350 mg per gram total polyphenols, 39 mg per gram flavan-3-ols, 21 mg per gram theaflavins and 52 mg per gram caffeine. Interestingly a certain amount of flavan-3-ols have survived fermentation and were still present in the instant black tea.
The results of this study are interesting because of the increase in the insulin concentrations up to 90 min and the reduction in the glucose at 120 min. This may indicate that the black tea stimulated the release of insulin from the pancreas and this then was the reason for the reduction in plasma glucose concentrations at 120 min postprandially. Evidence suggests that flavonoids and other antioxidants increase insulin sensitivity in the long term through an ability to prevent oxidative stress. However, in this study the effects of the black tea resulted from a single consumption with a glucose load, suggesting that other shorter term mechanism of modifying the insulin system exist. While the phenolic compounds in tea have been suggested to be responsible for this effect, tea does contain other chemicals, and these may play a part in the beneficial glycaemic effects. L-theanine for example is biologically active and is known to have central nervous system effects, and therefore may play a part in regulating postprandial glycaemia.
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