Many of green tea’s health benefits are due to the high amount of polyphenol catechins, which are antioxidants.
Green tea is an excellent source of epigallocatechin-3-gallate (EGCG), a potent antioxidant, according to the National Institutes of Health’s National Center for Complementary and Integrative Medicine. Green tea and its components, including EGCG, have been studied for their proposed ability to protect against inflammatory neurodegenerative diseases, such as Alzheimer’s disease.
“It’s recently been discovered that EGCG in green tea can break apart tangles of tau proteins in the brain, which are prominent in Alzheimer’s disease,” says Sarah Olszewski, a registered dietitian nutritionist and advisor for Cure Hydration, a plant-based electrolyte drink mix. “In Alzheimer’s, tau proteins abnormally stick together in fibrous tangles and lead to death of brain cells. Therefore, [it’s possible that] green tea consumption may be used as one measure to improve cognition and reduce risk of Alzheimer’s.”
Researchers are also exploring the effects of green tea on longevity, specifically regarding DNA sequences called telomeres. Shortened telomere lengths may be associated with shorter life spans and increased incidence of disease. A recent six-year study of more than 1,900 participants in Scientific Reports concluded that green tea consumption appears to reduce the chance of telomere shortening compared with coffee and soft drink consumption.
Regarding specific anticancer properties, green tea may reduce the risk of skin cancers and premature skin aging, says Smith. A 2018 review in Photodermatology, Photoimmunology and Photomedicine suggests topical application of tea polyphenols, ECGC in particular, might help prevent UVB rays from penetrating skin and causing oxidative stress, potentially reducing the risk of skin cancer. However, additional human clinical trials are needed to confirm these effects.
Drinking green tea may provide cognitive benefits, including reducing anxiety and improving memory and cognition, according to a 2017 review. A separate 2017 review concluded that the caffeine and l-theanine in green tea appear to improve attention and reduce distractibility.
“More research is needed to determine the full scope and mechanism of the neuroprotective effects of green tea compounds in humans,” cautions Smith.
Potential Side Effects of Green Tea
Drinking green tea may have certain side effects. According to Smith, green tea drinking might:
- Irritate the stomach lining when consumed on an empty stomach, possibly leading to stomach ache, constipation or nausea. It may also cause acid reflux or acidity in some individuals.
- Interfere with certain medications. For instance, green tea may increase the effect of stimulant drugs or affect the efficacy of medications for high blood pressure or chemotherapy medications.
- Increase the amount of calcium flushed out in the urine, potentially affecting bone health.
- Increase risk for iron-deficiency anemia. Tannins in green tea can bind to iron and form insoluble complexes in the digestive tract, making iron in foods and supplements less absorbable by the body.
- Exacerbate anxiety in susceptible individuals due to caffeine content.
- Be harmful for pregnant women when consumed in excess of six cups per day, as well as increase the risk of birth defects in babies related to folic acid deficiency.
- Be harmful for people with liver disease, particularly green tea extract in supplement form.
“It’s worth noting that most of these side effects are related to excessive consumption [of green tea] or the use of green tea supplements, which may contain much higher concentrations of bioactive compounds than brewed tea,” says Smith. “Drinking green tea in moderation is generally safe for most people. However, if an individual has specific health concerns or is taking medication, it’s always advisable to consult with a health care professional before making significant changes to one’s green tea consumption.”