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Most of the people do not need to worry about that.The only problem is that it dispels the need to sleep and hence excessive amounts can lead to sleep deprivation, which is not good in its own way. Hence you can call it a negative calorie food, which utilizes more calorie in getting digested than it provides to the body. However, this is not a very huge calorie deficit that will cause a great amount of weight loss.
Sometimes tea causes gas but that depends from person to person. It is also said that tea after a meal destroys the nutritional content of the food. This is a myth and absolute nonsense. In fact having red tea without milk acts as a cleasing agent. Tea with milk can also be considered as a small meal sine it contains milk, sugar and other ingredients and can also be filling. But that applies only old people.