Green tea (Camelia sinensis) has been shown to have weight loss effects in both animal and human studies. The exact reason for the anti-obesity effects of green tea are not known, but have been traditionally attributed to the caffeine content of tea. Caffeine belongs to the methylxanthine group of chemicals which have been shown to be potent stimulators of the central nervous system. In this regard they can stimulate the release of adrenaline, increase metabolic rate and increase lipolysis and fatty acid oxidation. However, studies investigating the weight loss effects of green tea have shown that caffeine alone cannot account for all of the weight loss effects of the herb. Other factors within tea may therefore contribute to its anti-obesity effects. A number of other components within tea have been investigated and the amino acid L-theanine has been shown to possess some benefits to weight loss. However, most of the attention of researchers has been focused on the anti-obesogenic effects of the green tea catechins.
One mechanism that has been postulated to give catechins their weight loss effects is through improved glycaemic responses. Flavonoids may be absorbed in the small intestine using sugar transporters (sugar glucose transporter 1; SGLT1) and as such may interfere with the absorption of glucose. Delaying the absorption of glucose may improve glycaemic control and ultimately have weight loss effects. Searching the literature reveals a number of studies that have tested the ability of catechins to cause weight loss and measured their effects on the blood glucose response to meals. In a meta-analysis of such studies, researchers investigated the ability of green tea catechin to cause weight loss and modulate fasting blood glucose levels1. The results showed that green tea catechins were able to cause a significant improvement in blood glucose levels which equated to around 1.5 mg/dL. This effects was independent of the present of caffeine, providing evidence that green tea catechins may be partly responsible for the weight loss effects of green tea.
These results therefore suggest that the catechin content of green tea favourably modulates blood glucose levels. As high blood glucose levels are associated with weight gain, abdominal obesity and type 2 diabetes, these results may explain the weight loss effects of catechins. Previously published studies have compared the weight loss effects of green tea on mice and concluded that the caffeine and L-theanine content of the tea may be jointly responsible for the weight loss effects. However, in the same study the authors noted that the weight loss effects of caffeine were enhanced by addition of catechins. The inconsistent results from clinical trials involving catechins and weight loss suggest that the benefits of catechins are not as pronounced as for caffeine in terms of weight loss. This would be expected from the observation that while caffeine is a potent stimulator of fatty acid oxidation, catechins moderately improve fasting blood glucose levels. Catechins may therefore only work over longer time periods and to a lesser effect than caffeine.
The ability of caffeine and catechins to cause weight loss effects by different mechanisms also explains their synergistic action. Evidence from the above cited meta-analysis shows that the beneficial effects of green tea catechins is only evident in studies of over 12 weeks. If green tea catechins are able to inhibit the absorption of glucose and this has long term benefits of fasting blood glucose, then green tea consumption may have beneficial effects at protecting against the deleterious effects of sugar and refined carbohydrates in the Western diet. Another possible explanation for the glycaemic effects of green tea catechins that has particular relevance to sugar and refined carbohydrates is the ability of the catechins to inhibit carbohydrate digesting enzymes. Legumes too possess the ability to inhibit carbohydrate digesting enzymes such as amylase and this is known to account for at least some of their beneficial effects on blood sugar levels. If this is the case for green tea, then other foods containing catechins such as apples, may have similar effects.
The green tea catechins may also improve blood sugar levels through inhibition of the expression of enzymes required for gluconeogenesis in the liver. Gluconeogenesis is the formation of new glucose from other non carbohydrate sources such as amino acids and glycerol and in this regard is an important mechanism to increase blood glucose levels. By inhibiting the expression of enzymes in this pathway, green tea catechins may have a long term benefit to blood glucose levels. Of course a more obvious explanation for the beneficial glycaemic effects of green tea may relate to their potent antioxidant effects. Oxidation of cellular membrane components is increasingly being implicated in the development of insulin resistance, and plant antioxidants that can inhibit this effect may improve insulin sensitivity. Cinnamon has also been shown to have beneficial glycaemic effects because of its potent antioxidant effects. The likelihood that green tea catechins have a similar effect is therefore a possibility.
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