Beneficial Effects of Green Tea

Dr. Jason Fung
Mar 14, 2018 · 7 min read

Tea is one of the world’s most most popular beverages. Tea drinking very likely originated in China and there are various legends surrounding the origins of tea drinking. One legend claims that Shen Nong in 2700 BC set out to find out about the effects of eating various plants, so he tasted over 100 plants in a single day. Luckily he had a transparent stomach so he could directly observe their effects. Tea leaves apparently were able to clear out poisons. Ummm…. yeah. A slightly more believable legend claims that Shen Nong was boiling some water in a pot, when some leaves fell in. After tasting over 100 plants that day, he discovered that tea, while bitter, could make him think quicker and see clearer.

Tea drinking went ‘viral’ and would have broken the internet had the internet only existed in 2700 BC. It spread throughout the world (a little more slowly) by the various ancient trade routes. Because unprocessed tea is quite bitter, the origins of the word come from ‘tu’ meaning bitter. In the mid 7th century, a pen stroke was removed from the word and the word became ‘cha’. Virtually all the words for tea in all the different languages of the world are variations are either ‘tea’ or ‘cha’. The ancient Chinese Min Nan dialect of the Fujian province used the word ‘te’ so spread via sea trade translated into the English ‘tea’ to as far away as the Maori ‘tii’. The dialects in landlocked regions of China used the word ‘cha’ and spread via the ancient Silk Road led to the Swahili ‘chai’ and as far as the Russian ‘chay’.

Interestingly, much of the early writings about tea touted its medicinal effects, particularly on digestion, rather than the (bitter, kind of metallic) taste. Tea is the leaf of the plant Camellia sinensis, and is consumed as green, black and oolong. Most of the purported health benefits have focused on green tea because of the high concentration of polyphenols and the beneficial effects of a chemical compound called catechins, the most abundant of which is epigallocatechin-3-gallate (EGCG). According to traditional Chinese beliefs, tea may be beneficial for weight control. This fascinates me, because our current research may only now be catching up with what those ancient Chinese people knew. I’ve long recommended tea drinking in my Intensive Dietary Management program for weight loss.

An estimated 2.5 million tons of tea leaves are produced annually, with some 20% becoming green tea. The oldest tree in existence is an estimated 3200 years old (pictured here) and lives in the Yunnan province of China where Pu-er tea is believed to have originated. China’s culture, along with tea drinking spread to Korea and Japan by 200AD.

In the 1500s, Portuguese traders brought tea to Europe and by the 1600s it spread to England, who spread their cultural tastes (and their famous stiff upper lip) to much of the rest of the world. The English bought so much tea from China that they developed a huge trade deficit as the Chinese didn’t really want any English stuff, other than their silver.

So, the English introduced opium for the express purpose of creating a nation of addicts, which would happily (for the English) balance their trade deficit. The Chinese government were not nearly as happy about their burgeoning opioid crisis and moved to ban the trade. In true gangland, drug-pusher style, the English sent in their big gunships to make sure the opium flowed freely. Thus began the two Opium Wars that eventually won England the ports of Hong Kong, Shanghai and others. As if that were not enough, the English then proceeded to smuggle some trees out of China and set up plantations in India to break China’s 4000 year old monopoly. That’s the kind of ruthlessness that wins you a global empire and probably why the English are the bad guys in the movie ‘Pirates of the Caribbean’.

Different teas come from the same tree species but undergo difference processing. Freshly harvested leaves are immediately steamed, rolled and dried. This inactivates the enzymes responsible for breakdown of color so it becomes the stable green tea leaves you can buy anywhere. This also helps preserve the natural polyphenols in the leaves. If the leaves are fermented, they become Oolong tea, and further fermentation produces black tea. The polyphenols and catechins become changed to theaflavins, which may have their own beneficial effects.

Green tea contains much higher concentrations of catechins than black teas, accounting for up to 30% of the dry weight. However, standard brewing methods are insufficient to fully extract them so it may take drinking 10 cups of tea per day to reach the doses of catechins used in studies. Many studies instead use industrially extracted catechin enriched green tea extracts to get the high doses necessary. A more natural, cold brewed green tea is another potential solution to that problem (available here). These catechins are potent anti-oxidants, which may help the body protect against the ravages of inflammatory insults. Anti-oxidants are compounds that protect cells against reactive oxygen species, which may lead to cellular damage. In addition, there may be some protective effect against cancer, Alzheimers and Parkison’s disease, albeit mostly in animal studies.


Two human studies suggested that catechins in green tea may help with weight loss. The first study gave healthy volunteers a beverage containing green tea catechins, caffeine and calcium 3 times per day and compared the 24 hour energy expenditure.

The green tea concoction increased the amount of calories burned by 106 cal/day or 4.6%. It is not possible to know whether the effect was due to the catechins, caffeine or calcium. Given that calcium has no known effect on metabolic rate, most believe the effect is due to a combination of catechins and caffeine. Luckily for us, green tea naturally contains both catechins and caffeine. In comparison to other studies of caffeine and metabolic rate, however, this green tea drink performed 50–100% better than expected compared to caffeine alone. Heart rate and blood pressure did not differ between the groups, so there was no evidence of excessive sympathetic stimulation by the caffeine that could explain the increased energy expenditure.

This echoes the results of an earlier study (Dulloo AG 1999) that found almost identical results, even though this study used half the amount of caffeine, they also showed a similar 4% increase in metabolic rate.

In this study, caffeine given alone did not increase energy expenditure. Most studies support the combination of caffeine and catechins, which is found naturally in green tea, although the concentrations of both are much higher in the extracts compared to brewed tea. The 2011 meta-analysis, comparing all studies on green tea, came to similar conclusions that catechin-caffeine mixtures increased energy expenditure by 4.7% and further, found evidence of a dose-response relationship. Only the catechin-caffeine mixtures increased fat oxidation. It was noted during the Cochrane review that no study supported the use of brewed green tea, compared to the small but consistent benefits seen with green tea preparations that contain higher levels of catechins. Naturally enriched green tea was discussed in our last post.

Catechins inhibit the enzyme COMT that degrades noradrenalin and cAMP. Blocking COMT increases noradrenalin, explaining the increased energy expenditure. Interestingly, there are genetic differences between COMT activity between races. Asians have higher rates of the high activity COMT(H) allele compared to some Caucasians, which have higher rates of the low-activity COMT(L) allele. Since Asians have the higher activity of COMT, blocking it with green tea catechins would be predicted to show greater effects, explaining the racial difference in the effect of the green tea catechins seen in some studies (1.51 kg versus 0.8 kg weight loss in Asians versus Caucasians).

Green tea promote long term weight loss by maintaining basal metabolic rate. Yes, the effect is not huge, but battles are always won on the margin. The difference of 100 calories per day multiplied over years can be very significant. This is reminiscent of the discussion of high blood pressure. We know from many clinical trials that if you lower the blood pressure by a little bit, it has huge benefits when multiplied over millions of people and over decades of time. The same applies to obesity and control of metabolic rate.

All of medicine comes down to risk versus reward — you can’t measure one alone to get a sense of whether or not a treatment is useful. That is, cutting off you arm is a very good treatment for a mosquito bite, but the risk is not worth the reward. So, yes, the rewards are relatively small, but perhaps important over the long term. But the risks are virtually non-existent. Even the most strident critic would be hard pressed to come up with a problem with green tea consumption. Thus the benefit/risk ratio is very heavily in favor of green tea.

So why wound’t you use it? I can’t think of a single reason.

Dr. Jason Fung

Written by

Nephrologist. Special interest in type 2 diabetes reversal and intermittent fasting. Founder of Intensive Dietary Management Program.