This piece was written by one of our contributors; registered nutritionist Ghazal Abrishamchi.
Chinese and Japanese people have been drinking green tea for centuries. It has also become more popular in Western culture. Green tea, brewed from the leaves of the Camellia sinensis plant, contains a number of polyphenolic compounds collectively known as catechins, with epigallocatechin-3-gallate (EGCG) being the most abundant catechin in green tea (1). The other active component of green tea is caffeine. The claimed health benefits of green tea, ranging from weight loss to cancer prevention, are often attributed to the catechins and the caffeine. Now, is there any evidence to support such “health benefits”?
Green tea & weight loss
Green tea supplements, also known as green tea extract, may seem like an attractive option based on their weight loss claims ; it is often assumed that these products are ‘natural’, and therefore, are safer than prescription drugs. Such supplements usually contain green tea catechins and caffeine at a concentration much higher than that of green tea beverage. Both catechin and caffeine, stimulate the sympathetic nervous system (SNS), this in turn leads to an increase in energy expenditure and lipolysis (i.e. fat breakdown)(2). Therefore, it would be expected that green tea, in the form of concentrated supplements, would result in weight loss. A single dose of green tea extract containing 270 mg EGCG and 150 mg caffeine, has been shown to increase energy expenditure by 4% in human studies (2). Now, the question is, does this increase in energy expenditure translate into weight loss? A Cochrane review (i.e. high quality study) that summed up the evidence from a number of well-designed studies, found green tea extract supplementation to have negligible effects on weight loss (3).
Green tea & cognition
Evidence from observational studies suggest that green tea may reduce the risk of developing dementia and Alzheimer’s disease, and may slow the rate of cognitive decline associated with ageing. For example, a study in a population of Japanese older adults found that those individuals who consumed green tea everyday had about 30% lower risk of developing cognitive decline compared to those who didn’t consume green tea at all (5). Although the mechanism is not well-understood, in-vitro (test tube/lab-based) and animal studies suggest that green tea may protect our brain cells through its powerful anti-oxidant and anti-inflammatory properties. Also, EGCG in green tea, has been shown to prevent amyloid-beta aggregation that is implicated in the pathology of Alzheimer’s disease (6).
Anti-diabetic effects of green tea
Some studies have found regular green tea consumption to reduce the risk of developing diabetes. Additionally, in a recent well-designed study (involving 17 healthy Asian men), a catechin- rich green tea beverage (containing 615 mg catechin in 350 ml water), was found to suppress the natural post-meal rise in blood sugar levels, especially when it was consumed before the evening meal (7). However, improvements in blood sugar levels have not been reproduced consistently across studies (8).
Green tea & Cancer
Green tea is thought to have anti-carcinogenic properties. Evidence in this field is conflicting; very limited evidence of variable quality indicates that it may be protective against prostate and breast cancer (1). However, studies that have reported reduced rates of cancer have often used high doses of green tea catechins (1). Such high doses require medical supervision.
Safety of green tea
Although green tea beverage, consumed in moderation, is generally considered to be safe, there have been reports of liver damage upon the use of green tea extract supplements, particularly in combination with other ingredients (4). Therefore, one should be wary of commercial green tea extract supplements.
Overall, evidence from several studies indicate that green tea does have a range of health benefits, owing to its catechins. However, most of the studies on green tea have been conducted in Asian population, and at high doses which require medical supervision. Therefore, applicability of such findings to the UK population, and at lower doses, is questionable. Also, it is the totality of the evidence that matters. While those studies that find health benefits get published, those that find non, often don’t get published!
References
1. Khan N, Mukhtar H. Tea polyphenols in promotion of human health. Nutrients. 2019;11(1).
2. Rains TM, Agarwal S, Maki KC. Antiobesity effects of green tea catechins: A mechanistic review. J Nutr Biochem [Internet]. 2011;22(1):1–7. Available from: http://dx.doi.org/10.1016/j.jnutbio.2010.06.006
3. Jurgens TM, Whelan AM, Killian L, Doucette S, Kirk S, Foy E. Green tea for weight loss and weight maintenance in overweight or obese adults. Cochrane Database Syst Rev. 2012;
4. Mazzanti G, Menniti-Ippolito F, Moro PA, Cassetti F, Raschetti R, Santuccio C, et al. Hepatotoxicity from green tea: A review of the literature and two unpublished cases. Eur J Clin Pharmacol. 2009;65(4):331–41.
5. Noguchi-Shinohara M, Yuki S, Dohmoto C, Ikeda Y, Samuraki M, Iwasa K, et al. Consumption of green tea, but not black tea or coffee, is associated with reduced risk of cognitive decline. PLoS One. 2014;9(5).
6. Kakutani S, Watanabe H, Murayama N. Green tea intake and risks for dementia, Alzheimer’s disease, mild cognitive impairment, and cognitive impairment: A systematic review. Nutrients. 2019;11(5).
7. Takahashi M, Ozaki M, Miyashita M, Fukazawa M, Nakaoka T, Wakisaka T, et al. Effects of timing of acute catechin-rich green tea ingestion on postprandial glucose metabolism in healthy men. J Nutr Biochem [Internet]. 2019;73:108221. Available from: https://doi.org/10.1016/j.jnutbio.2019.1082218. Yu J, Song P, Perry R, Penfold C, Cooper AR. The effectiveness of green tea or green tea extract on insulin resistance and glycemic control in type 2 diabetes mellitus: A meta-analysis. Diabetes Metab J. 2017;41(4):251–62.
