Myth Busters #29 - Caffeine Before Exercise Is Bad Because It Dehydrates You.

It is true that caffeine is a diuretic, which means that it promotes fluid loss in the body; however, as you should always be prepared with an adequate supply of water for your training sessions, you should be able to maintain an acceptable level of hydration. The effect which caffeine has on your body can help to boost your exercise benefit, as it increases the activity of your central nervous system, making you more alert and responsive, as well as decreasing the feelings of fatigue. It can help to increase tension development within your muscles, meaning you will be able to lift heavier weights. There is also an effect on the availability of energy sources because it increases fat utilisation, which can be important for long distance training and events; it will delay the use of carbohydrate stores, which in turn results in a delay in the build up of lactate within the working muscles.
While caffeine can be used to increase the benefit obtained from your exercise, however, there is also evidence to show that regular use of caffeine will decrease the response you get. So if you already drink coffee every day, you are less likely to experience a great effect from having caffeine hit before you exercise.