“Absolutely not,” says Donald Hensrud, medical director of Mayo Clinic's Healthy Living Program. “You have to enjoy life, and if you enjoy tea, keep on enjoying it. It’s all good. There are health benefits to coffee, to black tea and to green tea.” But there can also be problems associated with higher doses of caffeine, he notes. The amount in more than two cups of coffee a day, for example, can interfere with conception and increase the risk of miscarriage. And, he says, because individuals metabolize caffeine at different rates, slow metabolizers may be more susceptible to side effects such as heartburn, insomnia, heart palpitations and irritability.
Other studies have tried to pinpoint exactly what in caffeine is helping to halt the onset of these diseases. A 2014 study (14) found that caffeine blocks various tau receptors (a protein that contributes to brain cell degeneration). Another study found that caffeinated coffee increased GCSF (15) - a substance greatly decreased in Alzheimer’s patients - levels in the bloodstream.