Tea is the world’s second favorite beverage, following closely on the heels of water, and actually outdoing coffee, in spite of America’s obsession with the popular beverage. In fact, hot and iced teas are a $90 billion business worldwide. Last fall Starbucks opened its first “tea bar,” a rival to its coffee shops, inside one of its Teavana stores that sells loose leaf teas, teapots and tea-related gifts.
But there’s a long-simmering scientific tempest in that teapot; namely, whether or not it’s OK to put milk in your tea. Black tea with milk is popular in England, and many Americans have followed suit. However, early epidemiological studies have indicated that those who drank their tea with milk did not get all the potential health benefits of the tea. This led to a scientific quest to determine the impact milk might have on the healthful phytochemicals in black tea. The latest paper in this dispute recently appeared online, and will hit the streets in the February issue of the journal Food Chemistry. In addition to milk, it also sheds light on another accompaniment to tea, that is, the natural non-caloric sweetener stevia.
Before exploring the Food Chemistry article, though, let’s look at some of the earlier studies on the milk question. Previous work had indicated that the problem with milk in tea might be the affect of milk on the antioxidant phytochemicals in tea, the catechins. A 2005 paper in the Annals of Nutrition and Metabolism examined the effect of black tea with and without milk on the absorption of catechins in men. The study did indeed find that drinking tea with milk resulted in lower levels of catechins in the blood of the experimental subjects. However, milk didn’t affect measures like total antioxidant capacity of the blood or measures of free radicals in the blood after drinking tea. Those measures, though, might be influenced by so many other variables in the diet that drinking a small amount of tea might not be able to affect them.
The next study we reviewed was published in the 2007 European Heart Journal. Adult women drank either 500 ml of black tea, black tea with skim milk, or water. Researchers then measured flow-mediated dilation in the women, which assesses how much abnormal blood vessel constriction they have (a condition associated with high blood pressure and high cholesterol). Plain black tea improved flow-mediated dilation, but water and tea with milk had no effect on it. To check these results, researchers also applied tea with and without milk to isolated rat blood vessels, and found that milk inhibited the relaxation of these blood vessels caused by black tea. They also ascertained that it was the protein casein in the milk that was responsible for this effect, probably because it can chemically attach to the catechins in tea.
Another 2007 study, this one from the Journal of Agricultural and Food Chemistry, found that milk did not affect the absorption of catechins from black tea or the antioxidant capacity of plasma. What was significant, however, was how long the tea was allowed to steep before being drunk. Stronger tea led to higher catechin levels. The study authors noted that increases in catechin levels were detected 80 minutes after drinking tea. In 2010, though, a study in Nutrition Research found that adding milk to tea decreased its antioxidant capacity. This study just looked at the characteristics of tea, rather than giving it to human subjects. Skim milk decreased the antioxidant capacity more than whole milk, possibly because skim milk has more protein and thus more casein, whereas the protein in whole milk is diluted with milk fat.
The new 2014 study is from researchers in Kenya, a major tea-growing country. These researchers studied both green and black tea, and measured the antioxidant capacity with and without milk, sugar, honey and stevia. They also gave tea to mice, and then repeatedly measured the blood levels of glutathione, an important antioxidant compound in the body. They found that milk, sugar and honey all decreased the antioxidant capacity of the tea, and the larger the amounts of these additives, the greater the decrease in antioxidant ability. However, stevia did not have any effect on the antioxidant capacity, and the researchers thus recommend it as a preferred sweetener for tea. In monitoring the effect of tea on glutathione in mice, they found that tea was able to increase glutathione levels, and that the glutathione levels were highest 2 hours after tea consumption, suggesting that it’s best to take tea about every two hours to maximize its health benefits. This may sound like the Kenyan researchers are trying to get us all to drink more tea, but it should be noted that this has been demonstrated in other studies as well. It’s also worth mentioning that in Asia, where several studies document healthful effects of green tea, many people drink green tea throughout the day.
So what do these studies tell us about what to add to our tea? For starters, it looks like stevia comes out as a winner. Stevia is listed as a 3-star sweetener in Life Over Cancer, and its status as a non-caloric natural sweetener means it’s better for weight control, as well as for the antioxidant capacity of your tea (we don’t know what the impact of artificial sweeteners like aspartame are on tea’s antioxidant capacity). Whether milk decreases the antioxidant or other health effects of tea is still controversial. However, it’s worth noting that it’s the milk protein casein that seems to be responsible for disabling tea catechins. And while we don’t know whether the proteins in soy, rice or almond milk affect tea catechins, we always recommend them as an alternative to dairy, due to studies that show casein protein may fuel the growth of cancer. And if you are a tea drinker? We suggest spreading your intake of tea throughout the course of the day.
Finally, in case you don’t like tea and coffee is your beverage of choice, you should know that there’s also disagreement about whether milk decreases the antioxidant effect of coffee, with similar contradictory results among studies. So the tempest is not confined to the teapot, but continues to rage in the latte machine as well!