Green tea has been cultivated for centuries and was used in traditional Chinese medicine to treat a range of conditions from from headaches to depression.
SETTING THE SCENE
Green tea leaves are deemed to be richer in antioxidants than other types of tea because of the way they are processed. Antioxidants are substances that fight free radicals and have been shown to bring about a myriad of potential health benefits. Free radicals are harmful as they have the potential to cause damage to cells and alter genetic make-up (DNA); these manifestations have been implicated in inducing cancer, coronary heart disease and ageing. The mechanism by which antioxidants act is by preventing the formation of free radicals or by neutralising them, thus protecting the integrity of cells.
Tea Processing Methods
All varieties of tea (green, black and oolong) are produced from the Camellia sinensis plant using different processing methods. Green tea processing, as opposed to the other types of tea, does not involve fermentation; the fresh leaves from the plant are steamed to produce the green tea as we know it today. As green tea leaves do not undergo the fermentation process, they are reputed to contain a high concentration of polyphenols, also termed antioxidants.
Green tea contains B vitamins, folate (naturally occurring folic acid), manganese, potassium, magnesium, caffeine and other antioxidants, notably catechins. Green tea is alleged to boost weight loss, reduce cholesterol, combat cardiovascular disease, prevent cancer and Alzheimer's disease.
A CLOSER LOOK AT THE SCIENCE: Let's examine further whether this myriad of ascribed health claims to green tea are founded.
Green Tea and Cancer
As green tea is drunk widely across Asia, many correlate this with the observable trend of lower rates of cancer in this part of the world. But does this observation hold any truth? Some laboratory studies have shown that extracts from green tea can halt the growth of cancer cells. This could be explained by green tea's antioxidant properties. However, while these lab results hold promise, further evidence from human interventions is required to prove them.
The evidence from human studies is ambivalent. A meta-analysis - an overview of studies - published in 2014 suggested that green tea could reduce the risk of developing mouth cancers. Other clinical studies suggest that green tea consumption could reduce lung and bladder cancers. However, in 2009, the Cochrane Library published a review encompassing 51 studies, which involved over 1.6 million participants.
The authors from this high quality review concluded that the evidence of a link between green tea and cancer was weak and highly contradictory.
Green Tea and Weight Loss
It has been proposed that the antioxidants catechins in conjunction with caffeine contained in green tea could have a role in promoting the burning of calories, also referred to as speeding up the metabolism. In turn, this could promote weight loss. Green tea weight loss products such as herbal remedies are extracts of green tea which typically contain a higher concentration of catechins and caffeine compared to regular green tea.
A 2012 review examining 18 studies involving 1,945 participants found no significant effect of consuming green tea and weight loss promotion.
Green Tea and Cholesterol-Lowering Effects
There is increasing evidence that both green and black tea could have a beneficial role in preventing cardiovascular disease. A high quality review from 2013 found that daily consumption of green or black tea - as a drink or a capsule- could be involved in the lowering of cholesterol and blood pressure. Again, this is likely to be as a result of the antioxidant content. The authors of this review caution interpretation as the majority of the studies were short-term. As such, further long-term, high-quality studies are required to confirm this finding.
Furthermore, another review from 2011, found that green tea consumption enriched with antioxidant catechins could lead to small cholesterol reductions, which is a main risk factor in heart disease and stroke. Nevertheless, it remains unclear what levels of tea consumption would be required to bring about the observed health effects.
Green Tea and the Prevention or Progression of Dementia & Alzheimer's Disease
Evidence of a link between green tea and Alzheimer's disease is weak. A 2010 laboratory study using animal cells found that green tea enriched with antioxidant catechins protected against the nerve cell death indicative of dementia and Alzheimer's disease. However, whether these results can be reproduced in human studies remains to be seen.
As such, for the time science does not conclusively demonstrate a link between green tea consumption and Alzheimer's disease.
Sophie's professional opinion: In the Far East green tea has been regarded by many as the elixir for life, ascribing to it a plethora of beneficial health attributes. Although many of these are a bit far-fetched, the scientific evidence is pointing towards a potential role for the antioxidant components of green tea towards combating cardiovascular disease; this is probably due to green tea's cholesterol-lowering ability. However, this warrants stronger evidence from clinical trials to confirm these findings. The evidence for the preventative effects of the majority of other diseases, such as cancer, are weak or non-existent.
As such, tea appears safe to consume in moderation (approximately 3- 5 glasses a day). Remember that green tea contains caffeine and can act as a stimulant and may keep you awake if you drink it before going to bed!
Sophie Bruno is a Registered Dietitian living and working in Brussels (Belgium). Read Sophie's foodie blog which will enable you to learn, increase your knowledge & cultivate yourself in the field of nutrition & health directly from Brussels