Green tea is prepared from dried leaves of Camellia sinensis, a perennial evergreen shrub. It contains several compounds that are possibly beneficial to brain health, including caffeine, catechins (polyphenols like EGCG), and L-theanine (an amino acid derivative). Several observational studies and clinical trials suggest that green tea consumption might promote cognitive function, but no studies have tested whether it can prevent dementia.
Observational studies suggest that greater green tea consumption is associated with lower dementia risk, but no clinical trials have tested whether green tea can prevent age-related cognitive decline or dementia. Our search identified:
• 2 double-blind randomized controlled trials, 1 on cognitive function and the other on acute effects of green tea extract
• 2 observational studies on the incidence of dementia and cognitive decline
• 2 observational studies on cognitive function in older adults
• 1 review on tea and cognitive health in late life
• Multiple preclinical studies
Greater green tea consumption was associated with lower risk of dementia in two studies conducted in Japan, with the larger study reporting 27% lower risk in people who drank at least 5 cups a day . Tea drinking was also associated with higher verbal fluency in elderly Chinese people (i.e., 80–115 years old) .
Two double-blind randomized controlled trials have evaluated the effects of green tea extract on cognitive functions. One trial in 91 patients with mild cognitive impairment reported that the combination of green tea extract and L-theanine for 16 weeks resulted in significant improvements in memory and attention, particularly in patients who had relatively severe baseline impairment . The second trial examined the acute effects of a drink containing 27.5 g of green tea extract and reported that the drink increased brain connectivity associated with working memory and the degree of connectivity correlated with the magnitude of improvement in working memory . Green tea contains several compounds that may have beneficial properties, including caffeine, L-theanine, and green tea catechins (e.g., EGCG) .
No studies have reported whether green tea can improve cognition or slow decline in people with dementia. In a clinical trial, markers of oxidative stress were decreased in Alzheimer's patients who consumed a beverage that included green tea extracts for eight months . However, it is unknown whether this beverage helps patients. A different antioxidant therapy (not containing green tea extract) was reported to lower oxidative stress in Alzheimer's patients but accelerated cognitive decline .
Multiple meta-analyses have reported that green tea consumption is safe at moderate and regular amounts (3 to 5 cups per day, up to 1200 ml/day) and side effects are mild [10-12]. Three drugs are known to interact with green tea: warfarin (also known as Coumadin™ and Jantoven™), anisindione (or Miradon™), and dicumarol . Green tea consumption may reduce the levels of folic acid in the body and interfere with iron absorption . Because green tea contains caffeine, pregnant women and people with cardiovascular problems or other health risks should consult their physician or healthcare provider about consuming green tea. Caffeine in green tea can also interact with some medications .
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
Green tea is available loose or in bags and a typical cup (200 mL) contains 40–60 mg of caffeine, 8–25 mg of L-theanine, and 25–200 mg of catechins . Additionally, green tea supplements (e.g., green tea extract, catechins, L-theanine) are available in the forms of pills, capsules, liquid, and powder.
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Information on green tea from the National Center for Complementary and Integrative Health
An analysis of commercially available green tea supplements at Labdoor
A review of test results on green tea supplements and tea at ConsumerLab.com
Additional information on caffeine doses in common drinks can be found at the Mayo Clinic
Check for drug-drug and drug-supplement interactions on Drugs.com