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
[Leenen R, Roodenburg AJC, Tijburg LBM, Wiseman SA. A single
dose of tea with or without milk increases plasma antioxidant
activity in humans. European Journal of Clinical Nutrition
2000; 54: 87-92.
Serafini M, Ghiselli A, Ferro-Luzzi A. In vivo antioxidant effect
of green and black tea in man. European Journal of Clinical
Nutrition 1996; 50: 28-32.]