Drinking coffee may have a protective effect against depression, according to a U.S. study.
Caffeine is the world's most frequently taken psychoactive substance, but its potential effects on depression aren't well understood.
To learn more, Michel Lucas from the Harvard School of Public Health in Boston and his colleagues studied more than 50,000 women who participated in the Nurses' Health Study from 1996 to 2006.
"Because of its observational design, this study cannot prove that caffeine or caffeinated coffee reduces the risk of depression but only suggests the possibility of such a protective effect," the study's authors concluded in Monday's online issue of the Archives of Internal Medicine.
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Postmenopausal women who consumed four cups of caffeinated coffee or 550 milligrams a day or more had a lower risk of depression compared with those who consumed one cup or less per week, the researchers said.
The findings were consistent with earlier observations that suicide risk is lower among those with higher coffee consumption.
During the study period, 2,607 new cases of depression were identified.
The large sample of participants were given diet questionnaires were given seven times during the course of the investigation, which are strengths of the study.
But the researchers said they were unable to exclude the possibility that mild symptoms of depression were the common reason for low caffeine consumption and depression or that women who are sensitive to the stimulant lowered their consumption.
The authors called for more research to confirm their results and to determine whether caffeine consumption could contribute to the prevention or treatment of depression.
The association between caffeine consumption and depression link was only seen in nonsmoking women. It's known that caffeine is metabolized faster in smokers, the researchers said.
No link was found between decaffeinated coffee and depression or other caffeinated drinks.
According to survey data from the U.S. Department of Agriculture, about 90 per cent of adults in the U.S. said they consumed caffeine, and average consumption ranged from 166 milligrams a day to 336 milligrams a day.
The study was funded by the U.S. National Institutes of Health, and the National Alliance for Research on Schizophrenia and Depression. Lucas received a postdoctoral fellowship from Quebec's health research funding agency.