Animal and human studies suggest that green tea has particular fat loss effects beyond its caffeine content. These effects have been attributed largely to its content of flavan-3-ols (catechins) and L-theanine. The former belongs to a group of phytochemicals called the flavonoids that are particularly concentrated in green tea. The latter is a non-proteinogenic amino acid found only in tea. Evidence suggests that green tea is an effective fat loss agent because the caffeine, L-theanine and catechins work synergistically to produce fat loss effects (here). In addition to a thermogenic or direct fat burning effect, green tea might have other mechanisms by which it can induce weight loss in mammals. One mechanism that has been explored is the ability of the components of green tea to slow the absorption of glucose to the blood and thus induce favourable effects on postprandial glycaemia. This might occur through the inhibition of starch digesting enzymes by the catechins in the tea or might involve inhibition of transporters in the enterocytes.
The possible beneficial glycaemic effects of green tea have been reported in a number of studies. In a recent meta-analysis1, researchers analysed the results of these previous studies in order to evaluate the effects of green tea on glucose control and insulin sensitivity. The authors of the study searched Medline and included studies that reported on the blood sugar effects of green tea, and then a detailed subgroup analysis was performed on the data. The data included 17 trials and 1133 subjects. The results of the analysis showed that green tea consumption significantly reduced fasting blood glucose levels. In addition, the lower fasting blood glucose were reflected in a lower level of glycated haemoglobin (A1c) in the blood, suggesting that green tea has long term beneficial effects at protecting haemoglobin from oxidation. In addition, green tea also significantly reduced fasting insulin concentrations, suggesting that insulin sensitivity had been improved in regular green tea consumers.
These results suggest that some of the beneficial weight loss effects of green tea come from its modulatory effects on blood glucose and insulin sensitivity. That green tea was able to improve fasting glucose and insulin levels suggests that this effect was not due to digestive effects of green tea. More likely, the antioxidant effects of green tea may improve insulin sensitivity and thus lower fasting blood glucose levels. Similar effects are known for cinammon, a herb with known insulin sensitising effects, that is thought to exert its beneficial effect through its antioxidant activity (here). One way this may work is through modulation of the redox state of the hepatocytes, which may favourably increase expression of certain enzymes (such as those involved in gluconeogenesis) that are necessary for blood glucose control. Such changes have been observed in animal studies. Animal studies also show that green tea can increase the capacity of adipocytes and skeletal muscle to uptake glucose by increasing synthesis of glucose transporter 4 (GLUT4).
Dr Robert Barrington’s Nutritional Recommendation: Green tea is a high quality food and should be incorporated into a healthy diet. The weight loss effects of green tea explain its beneficial effects on cardiovascular disease risk and suggest that it confers protection against most diseases associated with being overweight. Green tea has also been shown to keep the brain is a state of relaxation thanks to its content of L-theanine, which can modulate brain activity to increase the relaxing alpha brainwave state. Taking 5 to 6 cups of green tea each day is recommended.
RdB
