Effects of green tea on insulin sensitivity, lipid profile and expression of PPAR[alpha] and PPAR[gamma] and their target genes in obese dogs

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There is abundant evidence that, worldwide, obesity and type 2 diabetes have dramatically increased over recent decades⁽¹⁾. Obesity is closely associated with insulin resistance and other chronic conditions, including dyslipidaemia, hypertension and CVD, all components of the metabolic syndrome^(2,3). Tea, from Camellia sinensis, exists in three basic forms: fermented black tea; semi-fermented oolong tea; non-fermented green tea⁽⁴⁾. Due to inactivation of polyphenol oxidase, green tea is rich (up to 30 % of the dry leaf weight) in flavanols known as catechins⁽⁵⁾. Catechins have been reported to have many biological effects, including anti-mutation⁽⁶⁾, anti-carcinogenesis⁽⁷⁾, anti-oxidation⁽⁸⁾ and anti-angiogenesis⁽⁹⁾ properties.

Research into the relationship between green tea and obesity-related insulin resistance syndrome has shown that green tea enhances in vitro insulin activity⁽¹⁰⁾, enhances insulin sensitivity in human subjects⁽¹¹⁾ and rat⁽¹²⁾, and reduces hypertriacylglycerolaemia in mice⁽¹³⁾. The main active component in green tea mediating these effects seems to be epigallocatechin gallate (EGCG)^(14,15).

Mechanisms explaining the effects of green tea and its components on obesity-related metabolic disturbances are not fully understood and some findings are contradictory. For example, one study reported that green tea decreased the translocation of the insulin-dependent GLUT4 in adipose tissue⁽¹⁶⁾ while another⁽¹⁷⁾ found an increasing effect of green tea on GLUT4 content of adipocytes. There are also conflicting findings on the relationship between green tea polyphenols and the important transcription factors involved in lipid and glucose homeostasis (PPARα^(18,19) and PPARγ^(16,20)). Furthermore, many studies used high concentrations of catechins that may not reflect typical consumption.

Our laboratory has demonstrated that obesity in the dog produces a metabolic disorder nearly identical to that seen in the obese human subject^(21,22). Moreover, the dog genome has been sequenced⁽²³⁾ supporting use of the obese dog as a model to study human metabolic disorders and...