What is green tea?
Green tea is made the same plant as black tea, but it has been processed differently. During the preparation of black tea the leaves are exposed to high heat, which causes tannins in the leaf to oxidize which give the tea a dark colour. Green tea doesn’t undergo this process and so the tannins are left unaltered. Tannins are a sub-group of the polyphenol family, and tea contains a number of polyphenol sub-groups, with tannins and catechins being the most abundant. It is these polyphenols which are the source of green tea’s benefits, and as the tannins are oxidised and damaged in black tea, its benefits are less notable.
Green tea and the digestive system
Most polyphenols are not absorbed, and so any benefits they exert on the body usually take place in the digestive system. Catechins have repeatedly been shown to be able to inhibit the growth and development of cancer cells in the colon, which is a very clear health benefit. Other polyphenols have shown similar benefits, but catechins have been one of the most effective. In animal studies, tannins have been shown to stimulate the immune system, particularly parts which protect against pathogens. As the body is an ideal environment for bacteria to grow (both good and bad), protection against pathogenic bacteria in the gut is extremely important.
Tannins also form complexes with nutrients such as iron and proteins which makes them harder to absorb and limits their bio-availability. This is of very little concern to most people though, as these nutrients are quite abundant in modern diets. If you are worried about it though, simply avoid drinking green tea (or taking green tea supplements) around meal times.
Probiotics & Polyphenols
There are a variety of probiotics, many are unique to the host, whereas others can be found in all humans. The most common probiotics are Lactobacillus spp. and Bifidobacer spp, and catechins act as prebiotics for both of these species, which will increase their benefits to the host. These benefits include cancer prevention, aiding the immune system and even lowering cholesterol. Being able to indirectly lower cholesterol in this fashion may be how polyphenols aid the cardiovascular system, as the direct mechanism is unknown.
We also know that Lactobacillus spp. can metabolise tannins into a compound called pyrogallol, and unlike tannins and catechins, pyrogallol is easily absorbed into the gut. Pyrogallol is a very powerful antioxidant, and may help protect against cardiovascular disease by reducing the oxidative stress on the cardiovascular system.
Pathogens & Polyphenols
A common pathogen found in the gut is a bacteria called Helicobacter Pylori (H.pylori), and inhabits roughly 50% of the UK population. H.pylori produces toxins which have been shown to cause cancer development and inflammation which leads a whole host of other diseases. Studies have shown that catechins can prevent the growth of H.pylori which will help to minimise its negative effects on the body. Evidence also suggests that catechins can inhibit the actions of the toxins it produces, which will reduce inflammation and cancer cell production.
Another common pathogen found in the gut is Clostridium difficile (C.difficile), and is also responsible for causing inflammation in the gut. Both tannins and catechins have been shown to have inhibitory effects on C. difficile and the toxins it produces, which will reduce its population and have negative effects on host health.
Can green tea help with weight loss?
One of green tea’s most common claims is that it can aid with weight loss. There have been a number of double blind placebo studies which have demonstrated that the polyphenols in green tea stimulate thermogenesis, which will burn off some calories. So, yes, it will contribute a small amount to weight loss, but nothing compared to the claims some companies make.
Is green tea toxic?
Like all polyphenols, catechins and tannins have a hormetic effect on the body. This means that they are actually very mild toxins, which benefit the body in small dosages, but can have adverse effects at very high dosages. Although it is not fully understood how this works, it is thought that the mild toxins stimulate the immune system and make it more active, causing it to be more responsive to infections and other toxins. It is near impossible to consume too many polyphenols in a natural diet, but highly concentrated green tea supplements are available, which, if consumed in large amounts may have adverse effects. If you are planning to take green tea extract supplements its a good idea to gradually introduce your body to them. Just take one tablet for a couple of days, and if you don’t have any problems (like upset stomach) move onto 2 tablets a day.
Yes green tea is healthy, but marketing has exaggerated its health benefits. Yes – it can make a small contribution to weight loss, but other diet and lifestyle changes will need to be done to achieve any noticeable weight loss. Current research on green tea shows that it can benefit the digestive system in a number of ways – and this reason alone is enough to drink green tea I think. As a drink, there are no negative effects of consuming green tea, but as a supplement it is possible to take too much, which can cause stomach problems.