Drinking five cups of green tea per day may reduce the incidence of psychological distress by 20 per cent, says a new study from Japan.
In a study with 42,093 Japanese individuals 2,774 people, or 6.6 per cent of the study population, suffered from psychological stress, and green tea consumption was said to improve psychological well-being.
According to WHO estimates, more than 450 million people suffer from stress worldwide, with 17 per cent of Europeans stating that stress is the most important risk factor to health. The related costs of stress estimated at €20bn in Europe (WHO) and $200bn in the US (International Labor Office).
Researchers led by Atsushi Hozawa from the Tohoku University Graduate School of Medicine report their findings online in the American Journal of Clinical Nutrition.
Green tea and its extracts already have a positive reputation, with studies reporting they may offer protective effects against Alzheimer's and certain cancers, improve cardiovascular and oral health, and play a positive role in weight management.
Despite reports already stating that green tea or its constituents might reduce psychological stress, no large-scale study has evaluated the relationship between green tea consumption and psychological distress, said the researchers.
After adjusting their results for potential confounding factors, including age, sex, history of disease, BMI, alcohol consumption, cigarette smoking, diet, and other factors, a significant inverse association between green tea consumption and psychological distress was observed for people who drank at least five cups of green tea per day, compared to those who drank less than one cup per day.
Being an epidemiological study, the authors could not offer any evidence as to what the active constituents behind the apparent benefits could be. Further study is needed to elucidate the bioactives and mechanism of action.
The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC).
A previous Japanese study reported that green tea extracts may offset the signs of physical and mental fatigue associated with modern stressful lives. Five days of supplementation with EGCG was found to reduce levels of oxidised species related to fatigue in an animal model, according to findings published in the journal Nutrition.
Global tea market
The global tea market is worth about €790 (£540, $941) million, with green tea accounting for about 20 per cent of total global production, while black tea accounts for about 78 per cent.
Green tea is said to contain over four times the concentration of antioxidant catechins than black tea (green tea leaves that have been oxidized by fermentation), about 70 mg catechins per 100 mL compared to 15 mg per 100 mL for black tea.
Consumer awareness of the benefits of green tea and green tea extracts continues to rise with growing numbers of studies, from 430 papers in 2000 to almost 1500 in 2003, reporting benefits of the main compounds, catechins.
This has seen European demand surge, having reached 500 metric tonnes in 2003. Companies such as DSM, with its Teavigo boasting 95 per cent purity of epigallocatechin gallate (EGCG), and Taiyo International, with its Sunphenon claiming more than 90 per cent purity, position themselves firmly in specific catechin markets.
Source: American Journal of Clinical Nutrition
Published online ahead of print, doi:10.3945/ajcn.2009.28214
“Green tea consumption is associated with lower psychological distress in a general population: the Ohsaki Cohort 2006 Study”
Authors: A. Hozawa, S. Kuriyama, N. Nakaya, K. Ohmori-Matsuda, M. Kakizaki, T. Sone, M. Nagai, Y. Sugawara, A. Nitta, Y. Tomata, K. Niu, I. Tsuji