Tea types and flavonoids
Tea is the second most consumed beverage in the world, after water. The four primary types of tea include white, green, Oolong, and black. All four teas are derived from the same plant, Camellia sinensis, but differ in how they are processed after harvesting.
Tea contains a wide array of components that have biological activity, including flavonoids, L-theanine, and caffeine. Many of the beneficial effects of tea are due to the high levels of flavonoids, such as catechins, which have antioxidative and anti-inflammatory properties.
The differences in the manufacturing process can influence the chemical composition and the beneficial effects of the different tea types. For instance, green tea is roasted before it can oxidize and hence, contains higher levelsTrusted Source of catechins. In contrast, black tea is allowed to oxidizeTrusted Source and has lower levels of catechins. Meanwhile, black tea contains larger amounts of other flavonoidsTrusted Source called thearubigins and theaflavins, which also possess antioxidant properties.
Cognitive function and cognitive decline:
Researchers think that the presence of theanine and caffeine may potentially produce a simultaneous feeling of calmness while improving attention. In addition, limited evidence suggests that the intake of theanine and caffeine together may result in a greater increaseTrusted Source in attention than either component alone.
Cardiovascular benefits:
A higher intake of dietary flavonoids is associated with a lower risk of cardiovascular diseases and metabolic conditions, including diabetes.
According to a meta-analysisTrusted Source synthesizing data from 39 studies, the daily intake of each additional cup of tea was associated with a 2% lower risk of a cardiovascular event, a 4% decline in the risk of stroke, and a 4% lower risk of mortality due to cardiovascular disease. These positive effects of flavonoids on cardiometabolic health are associated with lower inflammation and oxidative stress, improved regulation of blood glucoseTrusted Source and lipid levels, healthier gut microbiomeTrusted Source, and protective effects on blood vessels.
Thus, consumption of tea could be especially beneficial for individuals whose diets are deficient in other sources of flavonoidsTrusted Source, including whole grains, fruits, and vegetables.
Dr. Taylor Wallace, a professor in the Department of Nutrition and Food Studies at George Mason University, says, “Adding two cups of unsweet tea to the diet can be a simple and cost-effective [preventive] healthcare approach for cardiovascular diseases.”
Tea and immune function:
Tea consumption may also improve immune health, with studies suggesting a potential role of green tea in preventing bacterial and viral infections. For instance, a number of human studies, including randomized controlled trials, suggest that green tea consumption could reduce the risk of incidence of influenza infection.
Dr. Dayong Wu, a professor at Tufts University, Massachusetts, said the health benefits of consuming tea on the immune system fell into two categories.
“First is the protective effect against infection. Current research shows that tea/tea catechins may directly act on a variety of viruses and bacteria to prohibit them from attaching to and thus blocking their entry into the host tissues, inhibit their replication, and limit their spread. Tea/tea catechins may also enhance the anti-pathogen response of the host immune cells to help fight pathogens and clear the infection,” he explained.
Second, the antioxidant and anti-inflammatory properties of green tea may also help prevent tissue damage caused by excessive inflammation in response to an infection. Given its anti-inflammatory properties, green tea could also help alleviate symptoms of autoimmune diseases, such as inflammatory bowel disease and rheumatoid arthritis.
“Autoimmune disease represents a disrupted immune balance, and it is characterized by immune cells of a host attacking its own tissues. Tea/tea catechins have been shown to modulate complex immune cell function in a way to help correct this disorder, perhaps by suppressing overactive response and promoting tolerance,” Dr. Wu elaborated.
However, he also cautioned that most of these results are based on cell culture and animal studies, and more studies assessing the impact of green tea on immune function in humans are needed.
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