Jennifer L. Temple of the University at Buffalo said the double-blind, placebo-controlled, dose-response study also found boys report caffeine had a positive effect on their athletic performance. Girls didn't report on this issue, Temple said.
The study involved 26 boys and 26 girls ages 12-17 who had previous experience with caffeine but no adverse reactions, did not use hormone-based contraceptives, did not smoke, were not on any medication that could have adverse interactions with caffeine and were willing to visit the laboratory four times for 90 minutes each time.
Participants were instructed not to drink caffeine 24 hours before each visit and to eat nothing or drink nothing but water for 2 hours before each visit. The teens completed questionnaires on dietary and physical activity. Parents provided demographic information.
The study, published in Behavioural Pharmacology, found diastolic blood pressure increased and heart rate decreased as the percentage of caffeine increased in males, but not in females. In addition, boys who were regular "high consumers" of caffeine showed greater increases in blood pressure than low-consuming boys.
"Caffeine is known to increase blood pressure, but the fact that it caused an exaggerated response in high-consuming males was a surprise, since at the time of measurement the amount of caffeine consumed by boys and girls was the same," Temple says.