Drinking several cups of coffee a day may help to prevent depression in women, a study has shown.
Four or more cups of caffeinated coffee cut the risk of clinical depression by 20% when compared with drinking one or fewer cups per week.
The US researchers say they cannot be sure of a causal link between coffee consumption and avoiding depression.
However, they adjusted the results to take account of a multitude of other risk factors, such as medical conditions, marital status, smoking and activity levels.
They also found no similar association with decaffeinated coffee or other sources of caffeine, including soft drinks, tea and chocolate.
A "dose response" was also seen, with two to three cups of coffee per day reducing the relative risk of depression by 15%.
The research involved 50,739 women who participated in the Nurses' Health Study, a major US investigation exploring links between health and lifestyle.
None of the women, who had an average age of 63, had depression at the start of the ten-year study, which began in 1996.
Women were said to suffer from depression if they had been diagnosed with the condition and were regularly taking anti-depressants.
The findings were reported today in the journal Archives of Internal Medicine.
Scientists led by Dr Michel Lucas, from the Harvard School of Public Health in Boston, wrote: "Our results support a possible protective effect of caffeine, mainly from coffee consumption, on risk of depression.
"Further investigations are needed to confirm this finding and to determine whether usual caffeinated coffee consumption may contribute to prevention or treatment of depression."
A previous study from Finland found a reduced risk of suicide with progressively higher levels of coffee consumption.
This association continued until coffee consumption reached eight to ten or more cups a day, at which point suicide risk increased.
"It is possible that persons with more severe forms of depression used very high doses of coffee as a form of self-medication that was, nevertheless, insufficient to elevate their mood," said the US researchers.
They were unable to assess the effect of very high coffee consumption levels because too few of their study participants fell into this category.
The scientists pointed out that caffeine had "well-known psychostimulant effects" including "increased sensations of well-being and energy".
These were known to be dose-dependent but "biphasic", meaning that higher doses could reverse the "pleasant and stimulating" effect of lower doses.
