If you are frequently tempted to buy a pastry treat at your favourite coffee shop, there is a good reason - and it is not just your lack of willpower.

A new study on coffee has found that caffeine can affect the way we perceive sweetness and may make us crave sweets more strongly.

Caffeine gives us an energy jolt because it blocks receptors in our brain for adenosine, a chemical that can make us feel sleepy.

Previous research established that adenosine also helps us taste sweet flavours.

For this study, a team of scientists at Cornell University gave participants a cup of lightly sweetened coffee and did not tell them whether it contained caffeine.

The purely decaf cups contained quinine, to make sure both types of coffee had the same level of bitterness.

The participants were "unable to estimate the caffeine content of their sample", the study said, which confirmed "the strong placebo effect to coffee consumption".

But even though many of the participants guessed that there was a normal amount of caffeine in their cups, those who drank caffeine perceived their coffee to be less sweet than those who unknowingly drank decaf.

When the participants were asked to taste and rate a sucrose solution more than 15 minutes later, the caffeinated participants still reported tasting lower levels of sweetness.

That finding means that the dulled palate for sweets "is a noticeable effect and that it does stick around after you have finished consuming", said Dr Robin Dando, director of the Cornell Sensory Evaluation Facility and an assistant professor in the university's Department of Food Science, who was a member of the research team.

Because many people drink more than one cup of coffee, "this may have a cumulative effect".

The research team did not measure how long the effect lasted.

They found caffeine had no apparent effect on our perception of bitter, sour, salty or umami tastes.

Dr Dando's previous research had already found that when you chemically block people's ability to taste sweet flavours, it makes them crave more sugar and seek out higher-calorie treats.

Based on his collective research, we now know that drinking a caffeinated cup of coffee, which has the same blocking effect, makes people want cookies or cake more than they otherwise would.

"It has always been coffee and doughnuts, or coffee and some type of sweet... we've been doing this a long time, this link between sugar and coffee. But now we understand more of the mechanism," said Assistant Professor Lauri Wright, director of the doctorate in clinical nutrition programme at the University of North Florida, who is also a spokesman for the Academy of Nutrition and Dietetics.

"This is one more reason to be moderate with our caffeine intake," she added.

Coffee companies could certainly capitalise on these findings to promote the sales of post-coffee snacks or to make their drinks sweeter.

Caffeine has health benefits too. Studies have shown that it reduces the risk of heart disease and Type 2 diabetes. But the amount of sugar adults consume in their coffee and throughout the day could curtail some of these effects.

The Academy of Nutrition and Dietetics recommends that adults consume no more than 400mg of caffeine - about three cups of coffee - per day.

For the overcaffeinated, fortunately, the study offers an easy way to cut back.

Participants reported getting an energy boost whether they were given the decaffeinated coffee or the caffeinated version.

They could not tell the difference.

That placebo effect works in a blind study, but it is hard to trick yourself into drinking decaf.

Prof Wright recommends making your daily coffee with half- caffeinated beans and half-decaf "to gradually start weaning yourself down to moderate levels".

THE WASHINGTON POST