Epidemiological evidence suggest that tea is protective of cancer and cardiovascular disease. However, in general, studies show a more consistent health effect for green tea compared to black tea, suggesting that nuances exist between their physiological effects. The chemical composition of green and black tea has been analysed extensively in the literature and related to the processing methods of the tea. During the manufacturer of green tea, leaves of the Camellia sinensis plant are lightly steamed, and this results in a tea that retains much of the original catechin content of the whole leaves. In contrast, black tea preparation involves fermentation of the leaves, and this fermentation step alters the structure of the original catechins to produce a range of polymers called tannins. These chemical differences give green and black tea their distinctive flavours. However, while these structural differences may explain the different protective effects of green and black tea, confounding variables do exist that deserve further examination.
Green tea is traditionally consumed without milk, whereas black tea is consumed with the addition of milk. Based on this observation it is tempting to speculate that milk could alter the health effects of black tea, perhaps by modifying the bioavailability of the tannins and catechins contained within it. However, research published in the European Journal of Clinical Nutrition in 1998<sup?1 suggests that this may not be the case. The study authors fed 12 health volunteers either green tea, black tea or black tea with milk (each containing 3 grams of tea solid) is a randomised cross-over design. Consumption of both green and black resulted in an increase in plasma catechin levels of 13 % with peak levels occurring 2.3 hours after consumption. Blood levels then declined rapidly as the catechins were excreted, with plasma concentration falling by half in 4.8 hours and 6.9 hours for green and black tea. respectively. When the authors analysed the data for black tea with milk they found no difference in bioavailability or metabolism compared to milk-free black tea.
Therefore milk does not appear to alter the bioavailability of black tea. The differences in the health effects of black and green tea are therefore likely due to other factors. In particular, this adds weight to the viewpoint that the superior health protective effects of green tea are due to differences in the polyphenol profiles when compared to black tea. That isolated catechin from green tea have shown to confer protection from cancer and cardiovascular disease in animal experiments suggests that the higher catechin content of green tea can explain the epidemiological evidence of improved protection over tea. However, Another confounding variable that is often ignored when comparisons are made between black and green tea is the fact that sugar is often added to black tea. The detrimental effects of sucrose are well reported and include an increased risk of cardiovascular disease and cancer. Therefore while tea is protective of diseases, the addition of sugar to black tea may negate some of the benefits, possibly due to the fructose that it contains.