Let me first make the notation of the article that has given me all this very cool information that I TRUST to be accurate.
Kruskall L. Caffeine and Exercise Performance.
ACSM Health and Fitness Journal. 2009. vol 13, no. 6
The most important thing to say is that there is more evidence to support the assertion that caffeine enhances performance than there is to say that stretching does! Caffeine is considered a drug by some and has had some naysayers, but there is certainly a safe and effective dose of caffeine, just as there is of alcohol. Caffeine occurs naturally and is also synthesized. It is found in plants, foods and other products. Like alcohol, some amount of caffeine can actually be health promoting. Though there is evidence to support an association between caffeine consumption and some disease prevention, the article I read today was not about that.
Many people are interested in the research about caffeine and physical performance, caloric expenditure and fat burning. I can tell you that there IS research to support an increase in performance, but not of calorie burn or metabolism and not of fat burn. Where science does show some changes in metabolism with caffeine, it is in certain teas which have caffeine along with a substance commonly known as EGCG. [bottom line here is that you will be wasting your money if you buy products claiming to boost your metabolism or fat burning potential]
And though I said that there is a safe and even beneficial dose of caffeine, there is also a level which decreases performance and makes us feel not so good. Without getting into the science of it all, it is true that caffeine is an ergogenic aid, if nothing else. This means that it helps us to increase both our physical and mental labor or output. It may do this by helping us to focus, reducing fatigue and making it feel like we are not working as hard as we really are. A few system responses are responsible for this. Caffeine can stimulate the central nervous system, which includes our brain and it inhibits a neurotransmitter that makes us feel tired. It may also affect our muscular contractions.
People who use caffeine on a regular basis do have some tolerance and might need more caffeine to boost their performance than people who use it just before an event. However, it is important that one start using caffeine in their training, and not wait to the event date.
I noted the following advice as particularly important for anyone. If you are going to get off caffeine you should taper yourself off because stopping abruptly can have a negative impact on performance and can generally make you feel crappy.
Caffeine helps with endurance and anaerobic ( very hard - intense- breathless) activity and though it may help with resistance training, because it can increase heart rate and blood pressure, I do NOT recommend caffeine boosting before lifting. (caffeine is not associated with heart attack, that has been disproved over time)
There is a threshold as I've said. It might be wise to stay in the range that the research found helpful and that is about 5 to 6 mg of caffeine per kilogram of body weight. You can determine your kilogram weight with this calculator... and then multiply that by five. You can look up caffeine contents on line, not all products actually list them. Even an 8 oz cup of coffee can vary in content from 65-135 mg so you won't have more than an estimate.
Again, caffeine does not help you to increase your metabolism or burn fat, but it may help you to run, swim and cycle better or more comfortably. Personally, oh YES<>