In 1827 "theine" was discovered. It had close to the same effects as caffeine in coffee and we actually know it as black tea caffeine. As time went on it was discovered that it was actually tantamount to caffeine and so the term has been dropped.
By drinking black tea caffeine a person can receive relief from fatigue. It has been shown to be a stimulant, to increase speed in reaction time, increase alertness, and to improve concentration.
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There are also some physical effects. It stimulates the digestive juices, the kidneys, and the metabolism. This means it could help the body eliminate toxins. It also helps the person have an increase in mental alertness, shortens reaction time, and it improves how efficient our reaction time is. Our muscle action is improved, the heart and respiratory system is stimulated, and more oxygen is brought through the body.
A very new study indicates that caffeine can improve brain power performance, reduces the risk of heart disease and many other diseases. Since this is a new study it will need to be confirmed by others before others will jump on its band wagon but for now it looks promising for caffeine. There are still some known bad effects of caffeine if consumed in large quantities. It can lead to heart palpitations, shortness of breath, and insomnia.
There is a difference in the caffeine found in black tea as in comparison to the caffeine found in coffee. That is because there is a lesser amount of caffeine found in black tea. There is about a half or a third as much in black tea as there is in the same amount of coffee. In this way then, you are a lot more unlikely to get too much caffeine when drinking black tea as in comparison to coffee.
There is a confusing aspect in black tea though. When measured in its dry form coffee actually contains less caffeine than tea. When prepared though, coffee contains more caffeine.
The reason for this is somehow related to the processing. It also has something to do with where the tea leaf is grown, the particle size that is used in the tea bag, the way that the tea leaf is cut, and the method and length of brewing or steeping.
It has also been shown that the amount of caffeine per leaf has something to do with where the leaf is located on the plant. The lower the leaf is on the plant the more caffeine it contains. This is because the lower the leaf the more mature it is and the more phytochemicals and substances that naturally occur in the plant.
Black tea has a higher caffeine level than other teas. It is believed to be caused by the way that it is processed. It undergoes full oxidation which does make it have a stronger taste than other teas as well.