When tea or coffee is consumed black, i.e. without milk, then it affects the body differently than when consumed with milk. When combined with milk, the effect of caffeine is weakened. When isolated, caffeine is a bitter crystalline compound. It acts on the cells in our brain, the nerve cells, the heart and the intestine. In other words, it stimulates the heart and the central nervous system.
Coffee improves circulation in the brain by dilating the vessels in the brain.
As a result, nerve cells are also stimulated. Coffee has a mild laxative effect by working on the movement of the intestine. Consumption of coffee results in increased secretion of gastric juices. However, different people react differently to the same amounts of coffee. Obviously the levels of tolerance are different.