Caffeine Affects Girls and Boys Differently After Puberty

Veronica Hackethal, MD

June 16, 2014

Caffeine affects heart rate and blood pressure differently after puberty in girls compared with boys, and the response of postpubertal girls varies across the menstrual cycle, according to a study by researchers at the University of Buffalo, New York, published online June 16 in Pediatrics.

"Although our study was not designed to be a clinical study, we do believe it is relevant that relatively small doses of caffeine (about what you would find in a 24 ounce soda) can cause changes in blood pressure and heart rate," first author Jennifer Temple, PhD, associate professor in the Departments of Exercise and Nutrition Sciences and Community Health and Health Behavior at the School of Public Health and Health Professions, University at Buffalo, told Medscape Medical News. "Our findings concerning gender differences suggest that boys and girls experience caffeine differently [after puberty]."

Past studies in adults and children have suggested that drinking 1 can of caffeinated soda or 2 cups of coffee can decrease heart rate and increase blood pressure, according to background information in the article.

The double-blind placebo-controlled study took place between August 2011 and October 2012. The researchers randomly assigned participants (n = 96) to placebo or caffeine in doses of 1 mg/kg (about the amount of caffeine in a 12-ounce can of soda) or 2 mg/kg. They instructed participants not to consume caffeine for 24 hours before their appointments.

The researchers measured heart rate and blood pressure every 10 minutes for 1 hour after caffeine intake. They checked for menstrual phase by analyzing daily saliva samples for estradiol, progesterone, and testosterone. Participants self-reported 24-hour food intake, caffeine intake, behavioral symptoms, and physical activity. The researchers assigned Tanner pubertal stage on the basis of self-report and parent report.

Sex differences in response to caffeine seemed to become statistically significant after puberty, but not before. Both doses of caffeine significantly reduced heart rate compared with placebo (P = .04 and P = .007, respectively), with postpubertal boys having a greater change in heart rate than postpubertal girls (P < .05). Likewise, both caffeine doses increased systolic blood pressure compared with placebo (P = .009 and P < .0001, respectively), with postpubertal boys having a greater change in systolic blood pressure than postpubertal girls (P < .05). On the other hand, postpubertal girls experienced greater changes in diastolic blood pressure compared with postpubertal boys (P < .05).

Postpubertal girls had significantly greater decreases in heart rate in response to caffeine during the luteal phase (postovulation) and significantly increased systolic and diastolic blood pressure during the follicular phase (preovulation) (P ≤ .05 for all comparisons).

The authors noted that their study sample consisted mainly of children from white, middle-class, well-educated families, which could limit generalizability of the results. Because of financial constraints, the researchers could not double-check that children actually refrained from caffeine before their appointments. Although the study goal remained secret, survey questions about caffeine and instructions asking participants to eliminate caffeine could have given it away. Finally, the study design precluded evaluation of whether previous caffeine use influenced caffeine responses.

On the basis of the study results, according to Dr. Temple, it is not possible to draw conclusions about underlying physiologic mechanisms that could explain the different responses between girls and boys. One possible explanation could be that postpubertal changes in steroid hormones affect caffeine metabolism. Other studies have linked estradiol (a precursor of estrogen) to decreased caffeine clearance, the authors write, suggesting that caffeine clearance slows during postovulation compared with preovulation. Alternatively, increased caffeine consumption in adolescence could also explain the results.

"These data suggest that gender differences in response to caffeine emerge after puberty," the authors conclude, "Future research will determine the extent to which these gender differences are mediated by physiological factors, such as steroid hormones, or psychosocial factors, such as more autonomy and control over beverage purchases."

The study was funded by a grant from the National Institute on Drug Abuse to Dr Temple and by the National Institutes of Health. The authors have disclosed no relevant financial relationships.

Pediatrics. Published online June 16, 2014. Abstract

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