Green Tea and Disease Prevention

Green tea was discovered in China in approximately 3000 BC. Buddhist priests carried the tea drinking tradition to Japan. Even in ancient times, tea was believed to have many medicinal qualities. In 1211 AD, the Japanese Zen priest Yeisai published the book ‘Kitcha-Yojoki’ which translated to ‘Tea and Health Promotion’. At the time, civil war had broken out and tea drinking was all but forgotten for two centuries. With peace restored, Yeisai reintroduced the tea plant to Japan. He wrote about methods of harvesting, production, and also it’s many healthful attributes. He proclaimed that tea was a “divine remedy and a supreme gift of heaven”. Where before tea drinking was restricted to nobility and priests, tea drinking now spread to the general population. When the Shogun Sanetomo became ill from over-feasting, he summoned Yeisai to offer prayers. The Buddhist abbott supplemented his prayers with tea, and after recovery, the shogun became a great tea devotee. Modern medicine is just now catching up to this ancient wisdom regarding tea’s health benefits.

Obesity and the closely related type 2 diabetes significantly increase the risk of death and heart disease. Green tea contains many chemical compounds that may play a role in reducing the burden of disease. Some studies have shown benefit in adding green tea compounds to the diet as opposed to oolong tea that contains lower levels of catechins, the chemicals thought to be effective for weight loss. In one experiment, patients were assigned to two different beverages — oolong tea compared to an enriched green tea. Oolong tea is made from the same tea leaves, but the fermentation process changes many of the catechins to theaflavins instead. Oolong tea is the most widely sold tea in Japan. The green tea contains larger amounts of catechins, but this amount was enriched by concentrating some of the catechins.

After a period of 12 weeks, there was a noticeable difference in body weight, with the green tea group losing an extra 1.1 kg compared to the oolong group. There were other benefits, too. Waist size decreased 3.4 cm in the green tea group compared to only 1.6 cm in the oolong group. This abdominal obesity is far more injurious to human health than overall body weight, and therefore the criteria for metabolic syndrome are based on waist circumference, not body mass index.

The effect of reducing body fat and waist circumference would also be expected to have a beneficial effect on reducing type 2 diabetes. Indeed, the same researchers as the first study undertook a 2009 study to determine the effects of a catechin enriched green tea in a randomized placebo controlled trial in type 2 diabetic patients who were not taking insulin.

The results were simply spectacular. Body weight was largely unchanged between the two groups, but the waist circumference, indicative of the more dangerous abdominal fat was reduced by 3.3 cm in the catechin group but not in the placebo. Systolic blood pressure was reduced by 5.9 mmHG and diastolic by 3.0 mmHG in the catechin group. Triglycerides improved by over 10%. This improved metabolic profile was confirmed by the blood testing which showed reduced blood glucose and hemoglobin A1C, a sort of 3 month average of blood glucose measurements. In the catechin group, A1C was lowered by 0.37, almost as powerful as some of the medications used today for treatment of diabetes.

This beneficial effect was also found in several other studies. A 2006 study published in the Annals of Internal Medicine known as the JACC (Japan Collaborative Cohort Study for Evaluation of Cancer Risk) followed over 16,000 subjects with a mean age of 53 years and body mass index of 22.7 (normal weight). Consumption of green tea and coffee were both protective against the development of type 2 diabetes, whereas oolong and black tea did not show these benefits. Both green tea and coffee drinking afforded about a 30–40% decreased risk. Other epidemiological studies have shown similar benefits for coffee drinking.

The MEDIS study studied 1,190 elderly patients in the Mediterranean Islands of Greece, Cyprus and Crete. Once again, moderate (1–2 cups) long term consumption of green tea significantly reduce levels of blood glucose. In these elderly people with a high risk of disease, the consumption of 1 additional cup of tea per day was associated with a 70% lower likelihood of developing diabetes.

This improvement in metabolic syndrome would be expected to have dividends for heart disease, which we’ve discussed previously. As well, we see these benefits in prevention of stroke, as well.

Despite wide variation in tea consumption and across 6 countries, there was a consistent beneficial association between tea drinking and risk of stroke. Subjects who drank 3 or more cups of tea per day had a 21% reduced risk of stroke. There are three main mechanisms of protection theorized. First, catechins help to lower blood pressure, a key risk factor for stroke. Second, tea drinking may be beneficial for endothelial function (discussed previously), the interface between blood vessels and blood flow that is so important for atherosclerosis. The third possible benefit involves theanine, an amino acid found in high concentrations in tea leaves coming almost exclusively from that source. Theanine is easily absorbed into the body, crosses the blood brain barrier, and may help protect against damage from stroke. Tea drinking may help turn minor strokes into undetectable ones.


Most of the data regarding green tea and cancer comes from animal studies, which is interesting but not definitive. EGCG, the main catechin in green tea is able to prevent metastasis by blocking the adhesion of cancer cells to the endothelial layer. Cancer cells often depend upon a matrix metalloproteinases (MMP) in order to metastasize, and EGCG has been shown to inhibit MMP-2 and MMP-9. In isolated cell studies, this helps block metastasis of cancer.

The other major pathway where green tea catechins may benefit is in the induction of apoptosis(programmed cell death). EGCG binds to the death ligand to activate the mitochondrial pathway. Once activated, the cell dies and never has a chance to become cancerous.

Small epidemiological studies seem to suggest a benefit for stomach cancer in Japan, but overall studies have been equivocal.

These benefits are likely quite small, but the risks are equally small. Given the relatively low cost of drinking tea, this makes tea drinking, along with fasting, one of the cheapest and most powerful biohacks.



Nephrologist. New York Times best selling author. Interest in type 2 diabetes reversal and intermittent fasting. Founder

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store
Dr. Jason Fung

Nephrologist. New York Times best selling author. Interest in type 2 diabetes reversal and intermittent fasting. Founder