Caffeine is available in a wide variety of common drinks such as coffee, tea, chocolate, and energy drinks. Caffeine's effect on exercise performance is the subject of much research and some controversy. Overall, the effects of caffeine are seen as positive. Many studies have shown that moderate caffeine consumption can improve endurance and enhance athletic performance, particularly in terms of prolonged and intermittent exercise.
The effects of caffeine on weight training are not as clear. Some studies indicate that supplements with caffeine, taken before resistance training (RT) increase upper body muscle strength, but not lower body. Another study showed that caffeine increase the amount of weight lifted, while yet another reported no change in muscle strength nor endurance. Finally, most researchers agree that caffeine does reduce pain during RT and delayed onset muscle soreness.
RT also elicits a series of hormonal changes in the body. Testosterone, insulin, and growth hormone stimulate muscle growth, and cortisol inhibits protein synthesis and promotes muscle degradation. Little is known about caffeine's effect on these responses. A study published this summer in the Journal of Sports Science and Medicine examined the effect of caffeine on acute hormonal response (insulin, growth hormone, testosterone and cortisol) to RT.
In this randomised, double-blind study, participants were randomly assigned to a caffeine group or a control group, and they filled out exercise history questionnaires and various physical parameters such as BMI, height, and weight were measured. All consumed the same diet and performed the same RT programme over a period of 3 weeks, and one group was given a regular caffeine supplement and the other group a placebo. Participants were given regular blood tests to determine the hormonal response.
The results showed that caffeine ingestion before RT results in a decreased response of growth hormone to RT as compared to those who did not ingest caffeine. The other hormones, insulin, cortisol, and testosterone, were not significantly affected by caffeine consumption. This result is surprising, since many other studies showed that caffeine caused an increase in testosterone. The researchers speculate that increased blood glucose or increased free fatty acids in the blood, both caused by RT, may be responsible for the decrease in growth hormone after RT, but suggest that further research is required to fully understand the mechanism behind the changes in growth hormone, and its ultimate effects on the outcome of RT.