Girls and boys seem to respond differently to caffeine after puberty, suggests a small study into a largely unexplored area of pediatrics that comes as young people consume more caffeinated products.

The amount of caffeine in about two cups of coffee is known to slow heart rate and increase blood pressure in the short term, in both adults and children.

Kids Caffeine

In a Buffalo study published in Pediatrics, the effects of caffeine seemed to differ depending on where girls were in their menstrual cycles. (Jack Dempsey/Associated Press)

Researchers in Buffalo recognized that questions remain about whether and how cardiovascular responses to caffeine differ after puberty, so designed an experiment to try to answer them, given previous studies' findings that children and adolescents are increasingly consuming caffeine products.

In the Buffalo study, funded by the U.S. National Institutes of Health, a total of 50 boys and girls eight to nine years old, and 46 aged 15 to 17 had their heart rates and blood pressures measured before and after they were given two doses of caffeine and placebo.

"This study shows that the gender differences in cardiovascular response to caffeine emerge after puberty and there are some differences in postpubertal girls across the menstrual cycle," Jennifer Temple of the department of exercise and nutritional sciences at the University of Buffalo and her co-team conclude in Monday’s Pediatrics.

As part of the study, parents filled in a questionnaire about income, education, profession, race and ethnicity.

The children and teens had their heights and weights recorded. They also filled in a questionnaire about mood, behaviour and stage of puberty.

The children and teens also gave saliva samples so their hormones levels could be tested.

Taking in caffeine resulted in more pronounced effects on heart rate during the luteal or premenstrual phase of the menstrual cycle, the researchers found. The blood pressure effects were heightened during the follicular phase.

It’s possible that the changes in steroid hormones like testosterone and progesterone that occur during puberty change how caffeine is metabolized, the researchers said. Changes in caffeine consumption as kids get older could also explain the gender differences.

Soda top caffeine choice

The caffeine doses ranged from 1 mg/kg, or about the amount of caffeine in a 355 ml can of soda, to 2 mg/kg. The drinks were lemon-lime-flavoured soda, orange juice or lemonade containing caffeine or placebo.

As expected, those surveyed in the older group were much more likely to have caffeinated beverages than those in the younger group:

  • Soda (93 per cent versus 78 per cent).
  • Tea (87 per cent versus 64 per cent).
  • Coffee (35 per cent versus 8 per cent).
  • Energy drinks (28 per cent versus 6 per cent).

The study was designed to be blinded, but some of the participants might have guessed that it was about caffeine from the questions and instructions to abstain from caffeine for 24 hours before coming to the lab, using the honour system. Researchers also asked what the participants ate and did for physical activity.

The scientists weren’t able to tell how previous experience with caffeine may have influenced the responses they observed, something they plan to study down the road.

The American Academy of Pediatrics recommends against caffeine consumption for children and teens because of potentially harmful effects from the mild stimulant. There isn’t high-quality data on what might be a safe amount for kids.

Based on average body weights, Health Canada recommends these maximum daily caffeine intakes for children in different age groups:
•    45 milligrams for children four to six years old (about one can of cola each).
•    62.5 milligrams for children seven to nine years old (about 1.5 cans of cola each).
•    85 milligrams for children 10 to 12 years old (about two cans of cola each).

The American Beverage Association, which represents makers of soft drinks and energy drinks, maintains caffeine has been safely added to drinks for a more than a century.  

With files from The Associated Press