BOSTON, Sept. 27 (UPI) -- U.S. women nurses who drank four cups of caffeinated coffee a day in a study had a 20 percent decrease in relative risk of depression, researchers say.
Michel Lucas of the Harvard School of Public Health in Boston and colleagues said they found no association between the consumption of decaffeinated coffee and depression risk.
The researchers studied 50,739 U.S. women who participated in the Nurses' Health Study. Participants, who had a mean age of 63, had no depression at the start of the study in 1996 and were prospectively followed up with through June 2006.
Researchers measured caffeine consumption through questionnaires completed from May 1980 through April 2004, including the frequency that caffeinated and non-caffeinated coffee, non-herbal tea, caffeinated soft drinks, caffeine-free soft drinks and chocolate usually consumed in the previous 12 months.
The study, published in the Archives of Internal Medicine, found when compared with women who consumed one cup of caffeinated coffee or less per week, those who consumed two to three cups per day had a 15 percent decrease in relative risk for depression, and those consuming four cups or more per day had a 20 percent decrease in relative risk.
This observational study "cannot prove that caffeine or caffeinated coffee reduces the risk of depression but only suggests the possibility of such a protective effect," Lucas said in a statement. "The authors call for further investigations to confirm their results and to determine whether usual caffeinated coffee consumption could contribute to prevention or treatment of depression."