Antioxidant Activity of Tea Unaffected by Milk

The antioxidant activity of green and black tea (Camellia sinensis (L.) Kuntze, Theaceae) in the body (in vivo) is well established, but an important question remains: Does the addition of milk to tea inhibit the bioavailability of antioxidant tea polyphenols? Not according to the results of this Dutch study, which showed that a single dose of either black or green tea with or without milk caused a significant rise in plasma antioxidant activity (Leenan et al., 2000).

The crossover study compared the antioxidant effects of green tea, black tea, and non-carbonated mineral water with or without milk in 21 healthy volunteers. Each participant received a dose of one of the six test substances on six different days. A single dose of tea was defined as 2 g of tea solids in 300 ml of water (Lipton Research Blend, Lipton, Englewood Cliffs, NJ). The researchers utilized the ferric reducing ability of plasma (FRAP) assay to measure both plasma antioxidant and catechin levels. Blood samples were taken before consumption of the test substances and again 30, 60, 90, and 120 minutes after consumption. According to the results, both green and black tea caused a significant rise in plasma antioxidant and catechin levels, but the effect of green tea was significantly greater at all time points. The addition of milk to either type of tea did not significantly alter responses.

While a limited number of studies support these results, others have shown that milk had a negative impact on the antioxidant capacity of tea. An earlier study published in the European Journal of Clinical Nutrition concluded that while the addition of milk to tea had no effect on antioxidant activity in vitro, it did appear to interfere with absorption of tea polyphenols in vivo. The authors of the older study offered two possible explanations for this effect. First, because milk proteins can cause complexation (binding) of tea polyphenols, the researchers proposed that milk/polyphenol complexes resist gastric breakdown, rendering the polyphenols unavailable for absorption. They also theorized that milk might hinder polyphenol absorption by increasing gastric pH (Serafini et al., 1996).

On the other hand, the authors of the more recent study suggested that the antioxidant assay utilized by Serafini and colleagues (called the Total Radical trapping Ability of Plasma, or TRAP assay) might be less reliable than the FRAP method, as TRAP may be associated with a higher degree of variability. - Evelyn Leigh, HRF