Tea - good for your health

Tea comes from the tea plant, camelia sinensis. The name reflects its likely country of origin, which is China (Sinae - latin). It was originally consumed as a medicinal drink and it is still widely used for its tonic effects today.

Types of tea

The different types of tea: black, white, oolong, green and yellow, reflect the processing method. Particular cultivars of the tea plant are responsible for the different flavours from different tea varieties such as Assam and Darjeeling, which are both black teas. 

Generally green tea is the least processed, with the leaves being steamed and then left to dry. Green tea is the least oxidised of the teas. White and Oolong teas are processed more, but oxidation by enzymes in the tea leaves is stopped by fixation (a heating process that destroys the oxidative enzymes). Black tea is the most processed of the teas and usually after steaming and withering the leaves are macerated and cut to activate oxidative enzymes, which convert chemicals present in the fresh leaf called catechins into thearubigins and theaflavins.

The tea found in teabags is normally processed using a cut, tear, curl - CTC method, which was developed in the 1930s using machines with contra rotating rotors.

Black

The most common type of tea in the US and UK. It comes in many varieties, reflecting the regions in which it is grown. For instance Assam, Ceylon, Darjeeling, Kenya and Nepal. English Breakfast tea is a mixture of Assam and Kenyan leaves. Until recently, you would have been hard pushed to find anything other than black tea in most supermarkets. However, nowadays it is increasingly easy to find green tea and occasionally white tea.

Earl Grey and Lady Grey tea are both black teas. In both cases an oil extracted from the rind of a bergamot orange is added. Lady Grey may have lavender of Seville orange flavour added also. Lapsang Souchong is a black tea made from the outer, not so prized leaves of the tea plant. Unlike all other teas, the leaves used to make Lapsong Souchong are smoked. The tea is thought of as a tea for Westerners in China. It was Winston Churchill's favourite brew.

Black tea is best made using a teapot, which is pre-heated by swishing some boiled water in it before putting in the tea leaves. The water should be poured on shortly after boiling as much below this will result in less flavour. Brew it for betwen 1-5 minutes depending on your strength preference.

Oolong

Oolong tea, or literally "Black dragon" tea from the Chinese can be brewed multiple times and the flavour is said to improve until the 3rd brewing. Use water slightly cooler than boiling at 93-97C to improve the flavour. There is a complex way of making oolong tea that is known as the Gongfu style(1). This may be the best way to get the most flavour.

Oolong tea is produced by leaving it out in the sun and wind to dry, where it withers and twists into the shapes that got it the name "black dragon tea". This ensures that it is oxidised. Its oxidation level can vary betwen low and high. It has an intermediate level of caffeine between black and green teas. 

White

White tea comes from the young leaves and buds of the tea plant. These are allowed to wither in the sun before being lightly processed to avoid too much oxidation. Optimal withering is achieved if the leaves are left for 1 day at a temperature of 30C, and humidity of 65%. Fine white hairs attached to the buds are sometimes present in a freshly made cup of white tea. The taste is meant to be fuller and sweeter than an equivalent green tea.

Green

Green tea has a reputation as the most healthy type of tea. This may be justified, as it is the tea with the least amount of processing and this leaves many of its original phytochemicals intact. The world has gone crazy about the health benefits of green tea. See left a green tea kit-kat. 

Many people don't realise that green tea has different brewing requirements than black tea. It is best steeped in water between 65-85C and should be left to brew for between 30 seconds to 3 minutes. This prevents it developing a potentially bitter taste.

There are kettles that allow you to set the temperature at which you want the water(2). Failing that, if you take freshly boiled water from the kettle and pour it into the cup, into the empty teapot, back to the cup, put the tea leaves in the teapot and pour the water from the cups into the teapot again, you should by this stage have water at around 80C. Just use a thermometer if you want to get it exact.

Sencha is a Japanese green tea. Gunpowder tea is a green tea made from tea leaves that have been rolled up and yellow tea is slightly more oxidized than green tea.

Health benefits of tea

Chemicals in tea

Tea contains 3 classes of chemical that can influence health.

Polyphenols

Polyphenols are molecules that contain a 6-sided ring in their structure as pictured to the left. 

One type of polyphenol that is well researched and present in large amounts in green tea, are a type of polyphenol called catechins. There are 6 types of these: catechin, gallocatechin, epicatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate (EGCG). The final one, EGCG is the most abundant in green tea making up nearly 30% of the dry weight of the leaves.

Other types of polyphenol, more prevalent in black tea are called thearubigin and theaflavin. These are the oxidised products of catechins, and found in black teas which are well oxidized. They are responsible for the red tinge you get in a cup of tea. Theaflavin is the polyphenol pictured.

Tannins, which are also present in tea as well as some fruit juices and beer, are also polyphenols. Tannins, such as gallic acid act as an astringent, that is they constrict body tissues, giving the dry mouth feel you get after drinking tea.

Alkaloids

Caffeine, theobromine and theophylline provide the stimulatory effects of tea. They are also responsible for the bitter taste.

Amino acids

Finally there is the amine, theanine. This is supposed to produce a calming effect by boosting levels if the inhibitory neurotransmitter, GABA in the brain.

Effects on health

Tea has been associated with a number of health effects, with green tea being considered especially healthful. 

There have been many studies of an epidemiological nature (associating tea drinking with health effects). Remember, epidemiological studies cannot be considered reliable as proof of any effect of health. There have also been a number of animal experiments and test tube experiments that have suggested that tea could be beneficial for some conditions.

Cancer 

A lot of research has looked at the effect of the catechins in green tea on cancer cells. It does appear that in the test tube they can be effective at zapping cancerous cells(3). Epidemiological studies back up these findings for certain types of cancer such as gastric, breast and ovarian(6).

Heart disease

There are a number of studies that show that tea drinking may help reduce cholesterol levels and to a lesser extent blood pressure. More significantly perhaps, there are studies that show an increase in blood vessel diameter, reduced proliferation of cells in the artery walls and and reduced blood clotting, with green tea administration(4). These results suggest that tea may reduce the risk of heart attack and stroke as was found in a number of epidemiological studies from Japan(4).

Obesity

Green tea does seem able to firstly boost fat burning, and secondly reduce inflammation, which may also help reduce fat levels. These effects have been demonstrated in laboratory mice(5). The effect seems strongest when both the caffeine and catechins in tea are present.

The extent of any weight loss is likely to be limited, with some studies showing around 0.5kg of weight loss over a 12 week trial in humans(6). This weight loss would vary between individuals and may increase if green tea was drunk for longer periods than 12 weeks. However, as with all weight loss strategies a total change in the diet is normally necessary to make significant progress.

DrDobbin says:

For most people tea drinking is not a bad thing. Between 1-4 cups a day will not overload most people with caffeine. At the same time it will provide a ton of beneficial flavanoids which could improve your long term health. In the short term you may find it both refreshing and calming.

Whether you have it with milk is your own choice, although I have to say I'd only take milk with a black tea. 

References

1) http://teaguardian.com/how-to-make-tea/gongfu-tea-1.html#.UkmTEobkuSo

2) http://gizmodo.com/5938541/this-kettle-heats-water-to-the-exact-temperature-you-want

3) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3509513/

4) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3123419/

5) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3569937/

6) http://www.maturitas.org/article/S0378-5122(12)00270-8/fulltext