Drinking several cups of coffee a day may help to prevent depression in women, new research suggests.

It found that four or more cups of caffeinated coffee slashed the risk of clinical depression by 20% when compared with drinking one or fewer cups per week. The research, published today in the Archives of Internal Medicine, questioned 50,739 women who participated in the Nurses' Health Study, a major US investigation exploring health and lifestyle. The study tracked the health of the women from 1980 to 2004 using detailed questionnaires to record their coffee consumption. Just over 2,600 of the women developed depression over this time period. Analysis showed that more of these women drank little or no coffee rather than being frequent coffee drinkers.

It is not clear why coffee might have this beneficial effect, but the authors believe that caffeine in coffee may alter the brain's chemistry. Researchers found no similar association with decaffeinated coffee or other sources of caffeine, including soft drinks and tea.

Caffeine is the world's most widely used stimulant of the central nervous system with approximately 80% consumed in the form of coffee. However, studies analyzing the relationship between coffee or caffeine consumption and depression risk are few and far between.

The authors said further study was necessary: "In this large longitudinal study, we found that depression risk decreases with increasing caffeinated coffee consumption. Further investigations are needed to confirm this finding and to determine whether usual caffeinated coffee consumption can contribute to depression prevention."