The effect of caffeine on the exercise responses of six women habituated to caffeine
(> 600 mg/day) was examined during 1-h running at 75% V̇O2 max on a motorized treadmill. Each subject completed a placebo (PL) and a caffeine
ingestion (CC) trial while maintaining normal caffeine intake. The subject then abstained
from caffeine for 4 days and again ran after receiving caffeine (CW). Caffeine dosage
for all trials was 5 mg/kg body weight. Ingestion of caffeine after withdrawal (CW)
resulted in the greatest physiologic effects. Exercise oxygen uptake was significantly
elevated by 0.17 l/min over the PL and CC trials (P < 0.05). The CW trials resulted in an overall R value of 0.79±0.04 compared with 0.85±0.08 for the PL and 0.83±0.04 for the CC trials.
Caffeine had its greatest effect on the resting free fatty acid levels after withdrawal:
1104±425 µEq/l compared with 543±288 for the PL and 839±526 for the CC. Postexercise
lactates were similar for all trials. Post-exercise plasma norepinephrine and dopamine
were the highest after the CW trials. The results suggest that habitually high caffeine
users acquire a tolerance to caffeine which reduces its effects during prolonged exercise.
Furthermore, to magnify the effect of caffeine, habitual users should withdraw from
caffeine use for about 4 days.
free fatty acids - dopamine - norepinephrine - caffeine - females
