After examining the effect of a combination of green tea with different amounts of dietary protein, the results show that no additional effect occurs when green tea and milk protein are simultaneously administered. GT + 15 MP had a similar significant effect on DIT compared to PL + 15 MP. Yet, since the DIT of GT + water was significantly elevated, we may conclude that 15 MP inhibited a possible additional effect of GT. Furthermore, GT + water increased DIT compared with PL + water, GT + 3.5 MP or PL + 3.5 MP. Apparently there is no synergistic effect from catechins and milk protein when given simultaneously, regardless of the amount. The diet-induced thermogenesis from milk protein, for 3.5 MP and 15 MP alone, was not elevated by the catechins to a higher extent, as compared to placebo (GT + water vs.
PL + water) [2
]. With GT + 15 MP the milk protein prevented an additional effect, thereby this condition showed a similar effect compared with PL + 15 MP. Even small amounts of milk protein (3.5 g) were sufficient to suppress the effect of green tea on diet-induced thermogenesis. Milk protein reduced the average post-meal AUC over 3.5 h from 42 kJ to 14.5 kJ, without an appreciable increase in DIT attributable to the milk itself. These results were still present when corrected for multiple testing. With respect to fat oxidation, previous studies did find an effect of green tea on fat oxidation [2
]. However, there was no additive effect of green tea on fat oxidation seen in the current study, since the differences between baseline and post-treatment RQ values did not differ. It could be that with GT, mainly dietary fat oxidation is stimulated, and that this effect was not shown in the present study because the breakfast contained hardly any or very little fat.
Previously, in concurrence with the theory of Brown and Wright [7
] we reported that the formation of complexes between catechins and proteins, in particular the proline-rich β-casein that is preferred by polyphenols, prevented a synergistic effect of catechins and green tea after 3 months of weight maintenance. Controversy remains about whether combining proteins with tea inhibits the beneficial effect of tea drinking, and different mechanistic explanations exist for this phenomenon [13
]. For instance, the addition of protein increases the pH of the stomach resulting in a reduced absorption of catechins and affects the weak acid compounds of the polyphenols. Polyphenols are easily absorbed in their non-ionized form, but a rise in stomach pH increases the ionization thereby impeding the passage of the polyphenols through the gastric mucosa [13
]. This suggestion leads to another possible explanation for the lack of a synergistic effect in the current study, namely, that the change of pH in the stomach by the proteins would make it impossible for the casing of the capsules to dissolve. Nevertheless, this seems rather unlikely because the raw material of which the casing consisted was gelatin. This is a common material for producing coating/casing of capsules.
Some other studies did not find an inhibition of the anti-oxidant capacity of tea after the addition of proteins. Therefore, Van het Hof et al.
suggest that not the protein content itself is the cause of the inhibition but possibly the fat content. Catechins have a complex ring structure and are fat-soluble which causes the formation of complexes between fat and polyphenols [18
]. This was contradicted by Krul et al.
who examined three types of milks (whole milk, semi-skimmed milk and skimmed milk) and found that the fat content of milk did not play a role. After the addition of whole milk that has the largest amount of fat, the anti-mutagenic effect of tea was reduced the least compared to the semi-skimmed and skimmed milk [15
]. In this study, where a non-fat dairy product was used for the 3.5 MP conditions, the effect of the green tea that was present with water but without 3.5 MP was completely inhibited.
The quantity of milk protein for the 15 MP conditions is sufficient to increase DIT itself, yet also inhibits an additional effect of the green tea. The quantity of milk protein was 15 energy% of the breakfast. On average this was 16.1 g for each subject, which usually is considered to be a normal protein intake. The explanation that casein forms complexes with the polyphenols and thereby slows the emptying of the stomach because casein coagulates under the acidic condition of the stomach, is the most acceptable one [30
]. Finally, it might be possible that protein-polyphenol complexes form metabolites that are taken up in the small intestine similar to the GT metabolites but do not have the same effect. This, however, has never been studied and should be investigated.
Limitations of the study were the substantial dropout of 17 subjects after the 15 MP conditions and, although randomization took place within, subjects first received the 15 MP conditions and thereafter the 3.5 MP conditions.