"Any association between the intake of green tea, a drink with little toxicity and no calorific value, and cognitive function could have considerable clinical and public health relevance," wrote lead author Shinichi Kuriyama from the Tohoku University Graduate School of Medicine, Japan.
Green tea is a rich source of catechins, compounds suggested to play a beneficial role in weight loss, cardiovascular and oral health, with some, namely epigallocatechin gallate (EGCG), now emerging as particularly powerful.
The new study, published in The American Journal of Clinical Nutrition (Vol. 83, pp. 355-361), analysed the consumption of six drinks (green, black and oolong tea, coffee, cola or juice, and 100 per cent vegetable juice) for 1003 Japanese subjects with an average age of 74.
The analysis was done using the Comprehensive Geriatric Assessment (CGA) questionnaire, which also asked about 55 other food items, as well as demographic, social, lifestyle, and physical habits of the participants.
Cognitive function was tested using the Mini-Mental State Examination (MMSE), which has a maximum score of 30 points. Three cut-offs were used to discriminate levels of cognitive impairment: less than 24 for severe, 24 to 26 for medium, and 26 to 28 for slight impairment.
Using the cut-offs, 85.3 per cent of people who drank less than three cups of green tea a week had some level of cognitive impairment.
Only 59.8 per cent of people who drank more than two cups a day had some level of cognitive impairment (39.2 per cent with slight impairment).
After statistical analysis, using the cut off at less than 26 points, the researchers found that people who drank more than two cups of green tea per day had a 50 per cent lower chance of having cognitive impairment, compared to those who drank less than three cups a week.
"In contrast, a weak or null relation between consumption of black or oolong tea or coffee and cognitive impairment was observed," reported Kuriyama.
"Green tea polyphenols, especially EGCG, might explain the observed association with improved cognitive function… Green tea contains 67.5 mg catechins per 100 mL, whereas black tea contains only 15.5 mg per 100 mL," said Kuriyama.
EGCG is said to be brain permeable, and its protection of the brain is proposed to be due to mechanisms other than its antioxidant and iron-chelating properties.
Possible mechanisms included "modulation of cell survival and cell cycle genes and promotion of neurite overgrowth activity."
The study does have limitations, with the authors noting: "Healthier and more active individuals might have more opportunities to consume green tea. Among the Japanese, green tea is often consumed as a social activity, and this in itself may contribute to maintaining higher cognitive function."
This study is good news for both the tea market and the tea extract market. European demand for tea extracts is currently surging, having reached 500 metric tonnes by 2003.
This has seen companies such as DSM, with its Teavigo boasting 95 per cent purity of EGCG, and Taiyo International, with its Sunphenon claiming more than 90 per cent purity, position themselves firmly in specific catechin markets.
The global tea market is worth about €790 (£540, $941) million. Green tea accounts for about 20 per cent of total global production, while black tea (green tea that has been oxidized by fermentation) accounts for about 78 per cent.