Caffeine is a stimulant
A strong cup of coffee has a greater effect on men than women, research shows.
In a study on 668 healthy volunteers, an espresso pepped up men after just 10 minutes. Women also became more alert after the beverage, but less so.
The University of Barcelona researchers say some of this effect might be psychological because decaffeinated coffee also worked to some extent.
Progress in Neuro-Psychopharmacology & Biological Psychiatry reports the work.
The volunteers were asked to drink either a classic espresso containing 100mg of caffeine or a decaffeinated espresso containing 5mg of caffeine.
Then the researchers looked for changes in alertness over the following minutes and hours.
Both men and women saw an improvement in their activity levels after drinking the classic espresso and these effects began after as little as 10 minutes.
According to the investigators, 45 minutes is the time needed for maximum caffeine concentration to be reached in the blood, but levels reach half this concentration after just a few minutes.
And the effect appeared to be greater in the men.
The decaf coffee had a similar, but weaker effect and tended to be more potent in the women rather than in the men.
Lead researcher Ana Adan said: "Numerous studies have demonstrated the stimulant effects of caffeine, but none of these have looked at their effects in terms of the consumer's gender."
Anna Denny of the British Nutrition Foundation said: "This study provides an interesting insight into how the effects of specific foods and nutrients may differ between men and women.
"Research into 'gene-nutrient' interactions is moving forward quickly and we are finding out more about how our genetic make-up affects our requirements for certain foods and nutrients, and how our bodies react to these. In the future this could allow scientists to formulate dietary recommendations based on our genetic make up, as well as our age and body size."
Dr Euan Paul of the British Coffee Association said: "This new scientific study demonstrates interesting differences in the positive effects that caffeine may have on alertness between men and women, an area that has not been heavily researched in previous scientific investigations. We welcome further research to investigate with greater certainty any differences in the stimulant effects of caffeine that may be experienced between gender groups."