Black tea compounds may protect against diabetes

Compounds in black tea called theaflavins and thearubigins were found to be able to mimic the effects of insulin, according to the results of the study published in the journal Aging Cell .

The study looks set to add to an ever-growing body of science linking consumption of the beverage to a wide range of health benefits, including lower risk of certain cancers, weight loss, and protection against Alzheimer's.

"In some studies, black tea consumption has been associated with a lower incidence of heart disease and cancer, but the mechanisms are far from clear," wrote lead author Amy Cameron from the University of Dundee.

"We have now identified a family of tea compounds that mimic the effect of insulin signalling to FOXO1a and PEPCK, key downstream effectors of cellular insulin/longevity signalling."

The researchers explain that forkhead transcription factor family O (FOXO) plays a role in regulating the rate of ageing in response to dietary factors, and obesity and age-induced deficits in the same pathway are linked to genes such as phosphoenolpyruvate carboxykinase (PEPCK).

"With the exception of water, tea is the most popular drink globally, but despite this, little is known about the biological availability of black tea polyphenols in vivo or the molecular target(s) mediating the effects presented here," they added.

The health benefits of tea have been linked to the polyphenol content of the tea.

Green tea contains between 30 and 40 per cent of water-extractable polyphenols, while black tea (green tea that has been oxidized by fermentation) contains between 3 and 10 per cent.

The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate, and epicatechin.

In the new study Cameron and co-workers screened dietary compounds for their ability to effect insulin-like signalling to FOXO1a and PEPCK in the hepatoma HL1c cell line, a cells previously shown to faithfully model many aspects of insulin's repression of glucose in the liver.

Three black tea theaflavins, theaflavin 3-O-gallate, theaflavin 3'-O-gallate, theaflavin 3,3'di-O-gallate and thearubigins were identified to possess novel insulin mimicking effects.

And putting the compounds in the context of green tea, and especially EGCG, Cameron noted that this compounds effects on the FOXO1a and PEPCK pathway of depend on the conversion of EGCG to these larger structures.

"We have now identified a family of tea compounds that mimic the effect of insulin signalling to FOXO1a and PEPCK, key downstream effectors of cellular insulin/longevity signaling," wrote the researchers.

"Further investigation in this area to identify not only the molecular target mediating the effects that we have observed, but also agents capable of acting in this manner in vivo, might lead to the generation of drugs or specific dietary interventions to treat or defer the onset of these age-related diseases," they concluded.

An estimated 19 million people are affected by diabetes in the EU 25, equal to four per cent of the total population.

This figure is projected to increase to 26 million by 2030.

In the US, there are over 20 million people with diabetes, equal to seven per cent of the population.

The total costs are thought to be as much as $132 billion, with $92 billion being direct costs from medication, according to 2002 American Diabetes Association figures.

Source: Aging Cell (Blackwell) February 2008, Volume 7, Issue 1, Pages 69-77, doi:10.1111/j.1474-9726.2007.00353.x "Black tea polyphenols mimic insulin/insulin-like growth factor-1 signalling to the longevity factor FOXO1a" Authors: Amy R. Cameron, S. Anton, L. Melville, N.P. Houston, S. Dayal, G.J. McDougall, D. Stewart, G. Rena