In part 1 of our posts on caffeine, we discussed the effects and the benefits of caffeine during high-intensity activities that are short in duration. Caffeine has also been shown to benefit endurance events.
Several studies have found that caffeine improves endurance exercise performance, but the factors which are responsible for this are not fully understood. Scientists have thought that one of the possibilities is increased free fatty acid (FFA) oxidation and a resultant sparing of muscle glycogen.
During long duration events athletes use a combination of carbohydrate and fats as their fuel source. Carbohydrates are stored as glycogen (in the muscle and liver), and glucose (in the blood). The preferred fuel source for endurance events is muscle glycogen. However, muscle glycogen stores deplete after approximately 90 minutes of exercise; when this occurs, athletes have to rely on fats as the dominant fuel source. The problem with using fats as a dominant fuel source is that they are slower to break down, and as a result the athlete must slow down their intensity of exercise. So it would be a great advantage to rely on muscle glycogen stores for longer and try to ‘spare’ these stores for the end of the race in order to maintain a high intensity. This is the proposed benefit for caffeine intake: sparing of muscle glycogen.
A few studies have found that caffeine administration, 60 min prior to exercise; significantly increases plasma FFA levels both prior to and during exercise. Caffeine administration did not alter any of the other variables examined, including heart rate, rating of perceived exertion, plasma levels of glucose, lactate, epinephrine, and norepinephrine. Researchers from this study concluded that caffeine administration in athletic, habitual caffeine consumers increased plasma FFA levels but had neither metabolic nor neuromuscular effects that would be of potential ergogenic benefit in endurance running.
The mechanisms of the benefits of caffeine for endurance events are still not properly understood, although the proposed benefits are ‘sparing’ of muscle glycogen and an increase in plasma free fatty acids. This increase in free fatty acids, however, has not been shown to have an ergogenic benefit in endurance training. Therefore, caffeine has a greater benefit with high intensity activities that are short in duration.