Green tea is made solely from the leaves of Camellia sinensis that have undergone minimal oxidation during processing. Green tea originates from China and has become associated with many cultures throughout Asia. It has recently become more widespread in the West, where black tea is traditionally consumed. Many varieties of green tea have been created in countries where they are grown. These varieties can differ substantially due to variable growing conditions, horticulture, production processing, and harvesting time.
Over the last few decades green tea has been subjected to many scientific and medical studies to determine the extent of its long-purported health benefits, with some evidence suggesting that regular green tea drinkers may have a lower risk of developing heart disease and certain types of cancer. A green tea extract containing polyphenols and caffeine has also been shown to induce thermogenesis and stimulate fat oxidation, boosting the metabolic rate by 4% without increasing the heart rate.
Green tea contains salubrious polyphenols, in particular catechins, the most abundant of which is epigallocatechin gallate. Green tea also contains carotenoids, tocopherols, ascorbic acid (vitamin C), minerals such as chromium, manganese, selenium or zinc, and certain phytochemical compounds. It is a more potent antioxidant than black tea, although black tea has substances that green tea does not such as theaflavin.
Green Tea & brain health
One of the first studies to investigate the link between green tea and brain health in humans has found that green tea consumption is linked to a reduced prevalence of cognitive impairment. Past cell culture and animal studies have found that green tea may protect the brain against degenerative processes leading to Alzheimer’s and Parkinson’s and that it can inhibit the build up of amyloid proteins associated with Alzheimer’s. However, until now, human studies have not been done.
In the current study, researchers analyzed data from 1,003 Japanese subjects who had participated in a community-based "Comprehensive Geriatric Assessment" conducted in 2002. The subjects, aged 70 or older, completed a self-administered questionnaire that included questions about their diets, overall physical health, lifestyle habits and the frequency and regularity of green tea consumption. Researchers evaluated the detail of cognitive function by using the "Mini-Mental State Examination", which measures memory, attention and language use.
The results indicated that higher green tea consumption was associated with a lower prevalence of cognitive impairment. Subjects who consumed two or more cups of green tea per day had about half the chance of developing cognitive impairment compared to those who drank three cups or less each week. Participants who drank about one cup per day also reduced their risk of mental decline, although not by as much as those consuming two or more cups per day. Green tea’s protective effect on cognitive function remained even after the researchers adjusted for overall diet, smoking and exercise habits.
The study authors suggest that green tea’s potential ability to support brain health may help explain the lower rate of Alzheimer’s disease in Japan compared with Europe and North America. Reference: Kuriyama S, Hozawa A, Ohmori K, Shimazu T, Matsui T, Ebihara S, Awata S, Nagatomi R, Arai H, Tsuji I. Green tea consumption and cognitive function: a cross-sectional study from the Tsurugaya Project 1. Am J Clin Nutr. 2006 Feb;83(2):355-61.
In a study performed at the Israel Institute of Technology, it was shown that the main antioxidant polyphenol of green tea extract, EGCG, when fed to mice induced with Parkinson's and Alzheimer's disease, helped to protect brain cells from dying, as well as 'rescuing' already damaged neurons in the brain, a phenomenon called neurorescue or neurorestoration. The findings of the study, led by Dr. Silvia Mandell, were presented at the Fourth International Scientific Symposium on Tea and Human Health in Washington D.C., in 2007. Resulting tests underway in China, under the auspices of the Michael J. Fox Foundation, are being held on early Parkinson's patients.
Green Tea & Heart Disease
In May 2006, researchers at Yale University School of Medicine weighed in on the issue with a review article that looked at more than 100 studies on the health benefits of green tea. They pointed to what they called an "Asian paradox," which refers to lower rates of heart disease and cancer in Asia despite high rates of cigarette smoking. They theorized that the 1.2 liters of green tea that is consumed by many Asians each day provides high levels of polyphenols and other antioxidants. These compounds may work in several ways to improve cardiovascular health, including preventing blood platelets from sticking together (this anticoagulant effect is the reason doctors warn surgical patients to avoid green tea prior to procedures that rely on a patient's clotting ability) and improving cholesterol levels, said the researchers, whose study appeared in the May issue of the Journal of the American College of Surgeons. To be specific, green tea may prevent the oxidation of LDL cholesterol (the "bad" type), which, in turn, can reduce the buildup of plaque in arteries, the researchers wrote.
A study published in the August 22, 2006 edition of Biological Psychology looked at the modification of the stress response via L-Theanine, a chemical found in green tea. It "suggested that the oral intake of L-Theanine could cause anti-stress effects via the inhibition of cortical neuron excitation."
In a double-blind, randomized, placebo-controlled trial conducted by Division of Cardiovascular Medicine, Vanderbilt University Medical Center, Nashville, Tennessee; 240 adults were given either theaflavin-enriched green tea extract in form of 375 mg capsule daily or a placebo. After 12 weeks, patients in the tea extract group had significantly less low-density lipoprotein cholesterol (LDL-C) and total cholesterol (16.4% and 11.3% lower than baseline, p<0.01) than the placebo group. The author concluded that theaflavin-enriched green tea extract can be used together with other dietary approaches to reduce LDL-C.
A study published in the January, 2005 edition of the American Journal of Clinical Nutrition concluded "Daily consumption of tea containing 690 mg catechins for 12 wk reduced body fat, which suggests that the ingestion of catechins might be useful in the prevention and improvement of lifestyle-related diseases, mainly obesity."