Effects of caffeine on physiological responses to exercise in young boys and girls.

PURPOSE: To describe the influence of caffeine on physiological responses to exercise in young children and determine whether sex differences in these responses exist.

METHODS: Twenty-six healthy 7- to 9-yr-old boys and 26 healthy 7- to 9-yr-old girls volunteered to participate in a double-blind, randomized, double crossover study design. Each child randomly received both the placebo (PL) and caffeinated (5 mg x kg(-1)) drink (CAF) twice each on four separate days. Following a 1-h wash-in period and resting measures, each child rode a cycle ergometer at 25 and then 50 W for 8 min each, while HR, blood pressure (BP), and oxygen consumption (VO2) were measured.

RESULTS: HR (bpm) was significantly (P < 0.05) lower at rest and at 25 and 50 W in CAF versus PL in both boys and girls. Diastolic BP (mm Hg) was significantly (P </= 0.05) higher at rest, 25 W in both boys and girls, and at 50 W in boys, in CAF versus PL. Systolic BP (mm Hg) was significantly (P </= 0.05) higher at rest in both boys and girls, at 25 W in boys, and at 50 W in girls. During exercise, VO2 (L x min(-1) or mL x kg(-1) x min(-1)) and RER were not different in CAF versus PL in either boys or girls.

CONCLUSIONS: A moderate dose of caffeine (5 mg.kg) does not affect metabolism (VO2 or RER) in young children at low-moderate intensities of exercise. However, CAF causes a significantly lower HR (bpm) and higher BP (mm Hg) in both young boys and girls.