Women who drink four cups of coffee a day are 20 per cent less likely to become depressed than women who rarely drink coffee, US researchers said on Monday.
Caffeine is the most frequently used central nervous system stimulant in the world, and coffee consumption accounts for about 80 per cent of caffeine use.
Drinking coffee offers a boost of energy and a lift in well being, said Alberto Ascherio of Harvard School of Public Health.
"This short-term effect is what drives the consumption of caffeine," said Ascherio, whose study appears in the Archives of Internal Medicine.
"Here we are looking at long-term chronic use of caffeinated coffee," Ascherio said in a telephone interview.
His team studied more than 50,000 women enrolled in a health study of nurses. The women had an average age of 63, and none were depressed when they enrolled in the study.
Ascherio's team measured coffee consumption based on data on the women for 14 years dating back to 1976. They then classified the women according to how much coffee they drank and followed them for an additional 10 years.
"We found that those women who regularly drink four or more cups of coffee a day have 20 per cent lower risk of developing depression than those who rarely or never drink coffee," Ascherio said.
The team focused specifically on coffee, but they had similar findings when they looked at overall caffeine consumption, including caffeinated soft drinks and chocolate. They found that women who were in the top fifth of caffeine consumption had a 20 per cent lower risk of depression than women in the bottom fifth.
The team built a two-year gap or latency period between when they measured caffeine consumption and their assessment for depression to make sure they were not just capturing women who were too depressed to be regular coffee drinkers.
Ascherio said there have been very few studies that look at the long-term effects of coffee consumption. One smaller study in Finland showed men who drank a lot of coffee were less likely to commit suicide.
And Ascherio's own team has shown that drinking a lot of coffee may be protective against Parkinson's disease in both men and women.
He said it is not yet clear how coffee might protect against depression, but there are some hints.
Animal studies have shown that caffeine protects against certain neurotoxins. And brain receptors that respond to caffeine are concentrated in the basal ganglia, an area that is important for both depression and Parkinson's disease.
Ascherio said low-dose, chronic stimulation of these receptors may make them more efficient.
He stressed that the study does not prove that coffee lowers depression risk - only that it might be protective against depression in some way.
And many more studies will be needed to show whether coffee can be used to prevent depression, Ascherio said.