Everyone has his or her own tolerance for caffeine. Some experts say that particularly sensitive people are probably reacting to the coffee's tannin. People who suffer adversely from coffee should pay close attention to their body and its signals. (File Photo)

BEIJING, Sept. 11 -- Some people sit in their office and drink 10 cups of coffee a day. Others feel their heart racing after one espresso.

    Everyone has his or her own tolerance for caffeine. Some experts say that particularly sensitive people are probably reacting to the coffee's tannin. People who suffer adversely from coffee should pay close attention to their body and its signals.

    "Caffeine affects the central nervous system... the metabolism," says Antje Gahl of the German Nutritional Association (DGE).

    Depending on the amount ingested, it can excite the heart and circulation, but also the stomach, intestines and kidneys. That's because caffeine blocks receptors that are responsible for picking up adenosine, which is produced by the body and promotes relaxation.

    Thus, coffee usually has a desired effect when people drink it in the mornings and afternoon. "They wake up, become concentrated and can make decisions faster," says Hans Biesalski, a professor at the Institute for Biology, Chemistry and Nutritional Science at the Hohenheim University in Stuttgart.

    Some people get the shakes after drinking coffee. In such cases, some quick exercise can help. It lets the body work off the energy that comes from the coffee.

    Different people filter caffeine out at different speeds, says Gisela Olias of the German Institute for Nutritional Research in Potsdam. "For some people, it takes three to five hours. For others, it can take up to 12," says Gahl.

    Biesalski says this is the reason for the different tolerances. "We're all individuals," he says, adding that he himself can easily stomach coffee after his evening dinner and has no problem falling asleep, but becomes ill after drinking tea.

    A 125-millilitre cup of green or black tea contains 20 to 50 milligrams of caffeine sometimes called theine in tea according to the DGE. The same size cup of coffee contains 80 milligrams, while a 50-millilitre cup of espresso only contains 50 milligrams. The amount of caffeine in a cup of coffee depends on the preparation and roasting of the beans.

    Espresso contains less caffeine because the beans are roasted more strongly. Additionally, the tannin, which causes so many problems for some people, is destroyed in the process. Finally, the water used for espresso is under more pressure and pushed through the coffee powder faster. "That means less dissolved tannin," says Biesalski.

    These factors can make regular coffee more palatable if it's prepared in an espresso machine, says Biesalski. "Anyone who doesn't like coffee after dinner because it keeps them from sleeping is well-advised to try espresso."

    People with sensitive stomachs should also check out miniature Italian coffees. "Chlorogenic acid is the strongest acid in coffee. It's only broken down 30 percent during normal roasting, but up to 70 percent in stronger roasting processes," says Olias.

    That means extremely roasted espresso beans have smaller concentrations of this acid, which encourages hydrochloric acid production in the stomach.