The relief from fatigue that tea provides is a big reason for its popularity. This is due to caffeine, and caffeine has been a matter of controversy. It is a stimulant that has been shown to speed reaction time, increase alertness, and improve concentration. The physical effects include stimulation of digestive juice, the kidneys, and the metabolism in ways that possibly help eliminate toxins. An increasing of mental alertness, shortening of reaction time, and improving efficiency of muscle action is brought about by caffeine’s stimulation of the heart and respiratory system, bringing more oxygen to the brain.
There has been much concern in the United States recently about the possible dangers of caffeine. As regards tea, it should be noted that all types of tea contain less caffeine than coffee. The caffeine content of some of our teas is available here.
Caffeine tolerance varies greatly among individuals, and an excess of it is toxic. Some research has shown a possibility that caffeine can interfere with fetal development, including lowering birth weight and contributing to skeletal and other abnormalities. Until they reach the age of seven or eight months, babies cannot get rid of caffeine metabolites, and traces of caffeine can appear in breast milk too. Due to these concerns, pregnant and nursing mothers should limit or avoid any beverage with caffeine, including tea.
Caffeine content is also affected by the length of the infusion in water. Black tea infused for 5 minutes yields 40-100 milligrams, whereas a 3-minute infusion produces 20-40 milligrams, or half as much. Twenty cups of green tea yield 240 milligrams, or about 12 milligrams per cup.
Because tea bags contain broken leaves of smaller size, they produce an infusion with more caffeine than loose tea does. This is also true of very fine loose tea.