I'm not promoting the use of caffeine as a sports stimulant just presenting a summary of research behind the use of it for endurance athletes. For me personally, I'm kind of a coffee afficionado and have found its use beneficial in training.
Caffeine optimizes muscle fuel
Science has shown that caffeine causes your body to burn a higher percent of fat which is really a secondary energy fuel. When doing so it's sparing a small percentage of glycogen stores in your muscles. If you’re preserving some of your glycogen stores, your body is less likely to fatigue when training or competing anaerobically or at high intensities. The sparing effect is believed to occur during the first 12 - 20 minutes of the exercise. If you’re sparing glycogen early it can be utilized in the later stages of exercise in theory extending your endurance.
Caffeine and pain sensation
We're all very aware that endurance sports can be associated with a high level of stress on the body. Athletes are constantly trying to mentally and physically manage the pain brought on by the intensity of high level endurance sports. Caffeine is believed to stimulate an endorphin like release which aids in decreasing pain perception. It also seems to lower an athlete’s perception of effort for the same amount of actual physical output.
Research indicates that caffeine stimulates your adrenal glands to release adrenaline into your system, providing a rapid physical boost. Strength and speed can elevate for a limited period of time, while the body’s ability to sense pain decreases.
Caffeine is certainly a case of more is NOT better. High levels can actually have dramatic setbacks. The levels of Caffeine that are beneficial are dependent on factors such as weight and your regular intake as you can build a resistance to the stimulation. Virtually all research has concluded that moderate amounts are best for performance enhancement. Extended events such as an Ironman or ultra-races have a greater risk of dehydration, nausea and abdominal cramps making it very important to consider the side effects of caffeine as it's a mild diuretic. Be aware though that not everyone benefits from caffeine and some will experience side effects even at moderate doses. Like all sports nutrition strategies make sure you try caffeine in training before using it in a race situation.