A study of more than 40,000 individuals found a statistically significant 21% increased mortality in those drinking more than 28 cups of coffee a week and death from all causes, with a greater than 50% increased mortality risk in both men and women younger than 55 years of age. Investigators warn that younger people in particular may need to avoid heavy coffee consumption. No adverse effects were found in heavy coffee drinkers aged over 55. . . . Younger men had a trend towards higher mortality even at lower consumption, but this became significant at about 28 cups per week where there was a 56% increase in mortality from all causes. Younger women who consumed more than 28 cups of coffee per week also had a greater than 2-fold higher risk of all-cause mortality than those who did not drink coffee.The more than 40,000 subjects were tracked by the study over sixteen years. According to a non-study source "more than 60% of American adults drink coffee every day, consuming on average just over three cups a day."
From here citing Junxiu Liu, Xuemei Sui, Carl J. Lavie, James R. Hebert, Conrad P. Earnest, Jiajia Zhang, Steven N. Blair. Association of Coffee Consumption With All-Cause and Cardiovascular Disease Mortality. Mayo Clinic Proceedings, 2013; DOI: 10.1016/j.mayocp.2013.06.020