Caffeine increases stress, according to preliminary results of an intervention study undertaken by scientists at Bristol University, UK.

The study involved 420 people including both caffeine and non-caffeine consumers, who were given 100mg doses of caffeine, followed up by 150mg an hour later. "We have genetic information looking at variation in adenosine receptors [which can affect mood]," said Peter Rogers, professor of biological psychology at the university, who leads the study.

"A plausible idea is that some people may have a genetic variant that makes them react negatively to caffeine," he said.

The study was launched after a recent survey conducted by Rogers suggested a positive link between caffeine intake and stress.

The Bristol Dietary Caffeine and Health Survey of 6,000 adults, found that caffeine consumers had higher levels of depression, anxiety and stress than non-consumers.

It was possible that people who were more stressed turned to coffee for relief, he accepted. "But if this is the case, it is clear that attempts to self-medicate are largely unsuccessful."

The link between caffeine consumption and mood remained significant regardless of gender, age, level of social economic deprivation, alcohol intake and smoking, he added.

However, it's not all bad news for caffeine lovers. Rogers also believes that long-term caffeine consumption may provide chronic protection against age-related cognitive decline and Parkinson's disease.