Polyphenols, or flavonoids, are likely a key component to what makes tea a healthful drink. These chemical compounds act as antioxidants, which control the damaging effects of free radicals in the body. Free radicals can alter DNA by stealing its electrons, and this mutated DNA can increase LDL cholesterol or alter cell membrane traffic—both harmful to our health.
Though green tea is often believed to be richer in polyphenols than black or oolong (red) teas, studies show that—with the exception of decaffeinated tea—all plain teas have about the same levels of these chemicals, albeit in different proportions. Green tea is richest in epigallocatechin-3 gallate whereas black tea is richest in theaflavins; research has shown that both can exert health benefits. Herbal teas contain polyphenols as well but will vary highly depending on its plant origin.
Indeed, one reason for conflicting results in observational studies may be the wide variations in tea types with varying flavonoid content. Where the tea leaves are grown, the specific blend of tea leaves, type of processing, and addition of ingredients such as milk, honey, and lemon can alter specific flavonoid content. How accurately people report their tea intake (e.g., type, amount, brew strength) and their overall diet (e.g., do they eat other foods rich in flavonoids?) are other factors that need to be clarified as they can affect study results. For example:
Some research suggests that the protein and possibly the fat in milk may reduce the antioxidant capacity of tea. Flavonoids are known to “deactivate” when binding to proteins so this theory makes scientific sense.
One study that analyzed the effects of adding skimmed, semi-skimmed, and whole milk to tea concluded that skimmed milk significantly reduced the antioxidant capacity of tea. Higher-fat milk also reduced the antioxidant capacity of tea but to a lesser degree. All said, in practice, it’s important to keep in mind that tea—even tea with a splash of milk—can be a healthful drink.
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Source: Excerpt from Tea, The Nutrition Source. Harvard T. H. Chan School of Public Health. Published April 2023. Teas and herbs referenced in the article have been linked to Tea Market products.