Lemon juice might not have enough vitamin C to make a difference. (Photo by ^Vanessa^)
The main health benefits of green tea come from its catechin content. Catechins belong to the flavonoid family and are the most abundant polyphenols of green tea. The main catechin of green tea is epigallocatechin gallate (EGCG), which also has more positive health data behind it than the other catechins, though other catechins seem to be useful as well.
While catechins are not well absorbed by the human body, drinking a few cups of green or black tea will nonetheless significantly increase antioxidant levels in plasma. Still, finding ways to improve the bioavailability of catechins is a worthwhile effort, especially if you want to get all the health benefits but don't want to drink ten cups per day or take supplements.
It will also give us an idea of whether adding green tea catechins to various products will actually do anything. For example, you've probably seen or heard that even Coca-Cola has now jumped on the bandwagon and added green tea extract to Coke to make it seem healthier. But if the catechins will just degrade, what's the use?
Ascorbic vs. citric acid in green tea
Chen et al. compared the effects of citric acid and ascorbid acid (another name for vitamin C) on the stability of green tea catechins. They dissolved catechins in a sodium phosphate buffer and then added either citric acid or ascorbid acid to see how they affected the degradation of the catechins.
After two hours, half of the catechins had degraded. Adding citric acid to the buffer didn't protect them from being degraded in any significant way. However, ascorbic acid improved the stability of the catechins for up to 20 hours.
The pH of the control sample (catechins in sodium phosphate buffer) was 7.42, while the samples containing citric acid or ascorbid acid were 7.28 and and 7.38, respectively. Thus, the acidity of the samples increased only slightly as a result of adding citric acid or ascorbic acid, and since the increase was smaller from ascorbic acid than citric acid, the stabilizing effect was not due to a change in pH.
The protection from degradation increased with dosage. When 0.005 mg/mL of vitamin C was used, there was no effect. With 0.05 mg/mL there was a slight effect. Increasing the dose to 0.1-0.2 mg/mL improved the stability further, with 0.2 mg/mL being significantly more effective than 0.1 mg/mL.
Green tea and vitamin C: Practical applications
So what does all this mean in practice? First, we know that the solution the authors used contained 0.5 mg of green tea catechins per mL. An average cup of green tea is 2.5 dL (or 250 mL). The amount of catechins depends on several things, but if we assume a conservative estimate of 100 mg catechins per cup, we get 100 mg / 250 mL = 0.4 mg/mL of catechins, which is pretty close to what the study used.
Second, the amount of vitamin C that showed the most benefit was 0.2 mg/mL. In a 2.5 dL cup of green tea, this would translate to 50 mg of ascorbic acid. One teaspoon of pure ascorbic acid powder contains about 5 grams of ascorbic acid, so only 1/100th of a teaspoon would be needed to reach the levels used in the study.
If you don't happen to have jars of ascorbic acid powder in your kitchen cabinet, the next question is: What about lemon juice? Lemon juice only contains about 110 mg of vitamin C per cup, so squeezing lemons into your green tea is not really a viable route to reach the levels used in the study. While a slice of lemon with green tea won't affect the catechins negatively, the vitamin C content is also not enough to prevent their degradation (of course, lemon juice may have other properties that have a stabilizing effect).
All of this may not matter, however, because earlier studies have shown that the stability of catechins depends in part on the pH of the solution, with more acidic solutions having less degradation than alkaline solutions. Since green tea has a lower pH than the buffer solution used in the study and fermented tea products have less catechin degradation than unfermented tea products, perhaps green tea doesn't really need ascorbic acid.
On the other hand, once the green tea comes into contact with the intestines, it may be susceptible to degradation, since intestinal pH is neutral or slightly alkaline. Therefore, while adding ascorbic acid to green tea might not do much while in the cup, it might have a benefit after consuming it.
If you're drinking products with added green tea extract (like the Coca-Cola one), it makes sense to see if the ingredient list also contains ascorbic acid. If it doesn't, you might be not be getting the health benefits you think you are. Whether the acidity of the drink is enough to prevent catechin degradation is unclear from this study.
Ascorbic acid protects green tea catechins from degradation in an alkaline solution for up to 20 hours. This stabilizing effect is dose-dependent and unrelated to changes in the pH status of the solution. Citric acid did not have a similar effect.
For more information on green tea, see these posts:
How Black Pepper Increases the Bioavailability of the Healthiest Green Tea Catechin
Green Tea Extract Enhances Abdominal Fat Loss from Exercise
Peak Increase in Antioxidant Activity Occurs 20-40 Minutes after Drinking Green Tea
Caffeine and Polyphenol Contents of Green Tea, Black Tea, Oolong Tea & Pu-erh Tea