Caffeine is both a drug and a nutrient. A cup of coffee in the morning is used to wake up a cup in the evening is often used to work or study late. Athletes often use it to improve endurance performance, but it is also used in power sports for explosive power. There is a substantial amount of evidence that caffeine has ergogenic effects, especially for endurance performance but how exactly does caffeine work?
There was a theory that caffeine improves fat metabolism but is that really the way it works? The short answer is no. There is little evidence that the small effects that caffeine may have on fat metabolism in some situations is responsible for the performance effects. So, what is the true relationship between caffeine and athletic performance?
What are the proposed mechanisms?
Caffeine has been thought to improve performance in a variety of sports and exercise types through three possible main mechanisms:
Blocking adenosine
Increased muscle calcium release
Effects on catecholamines
We will discuss each of these mechanisms briefly below. First we will describe the mechanism, then we will discuss the evidence for each.
Blocking adenosine: how does caffeine affect the brain?
Caffeine has a chemical structure that is similar to a molecule called adenosine. This molecule is produced naturally in the body and is responsible for feelings of tiredness, fatigue and even pain sensation when it binds with its receptors in the brain. Simply put: more adenosine means more fatigue. Because of the structural similarities between caffeine and adenosine, caffeine can stop adenosine from binding to these receptors, reducing the sensations of both tiredness and pain. This effect of caffeine is often referred to as a central effect: an effect on the brain.
Because of the structural similarities between caffeine and adenosine, caffeine can stop adenosine from binding to these receptors, reducing the sensations of both tiredness and pain.
Caffeine increases calcium release from muscle cells
Caffeine increases the release of calcium in muscle cells. This release is responsible for the contraction of muscle. It was thought that caffeine supplementation might be able to increase the force production of muscle because of this.
Caffeine may affect catecholamine levels
The early theory of how caffeine improved exercise performance was that caffeine stimulated fat metabolism and spared muscle glycogen. Now we know that this is unlikely the main explanation. Although caffeine may have small effects on fat metabolism (at least in some situations), this is not the reason for the performance effects. Caffeine may stimulate the breakdown of triglycerides (the storage form of fat). The fatty acids that are released can be used for energy. In addition, caffeine can cause an increase in the family of hormones called catecholamines, which includes the ‘fight or flight’ hormone adrenaline (also called epinephrine). The increase in catecholamines can also increase the breakdown of fat. If the condition are right and more fatty acids are available, they will be used for energy during exercise (but not in all situations). If more fat is used this could lead to reduced breakdown of glycogen – the stored form of carbohydrate in the body – meaning it is available for energy later, perhaps delaying fatigue or exhaustion and leading to improved performance.
What is the evidence for these mechanisms?
Using brain scanning techniques, it has been shown that caffeine molecules in the brain can directly block the adenosine from working. Many studies have shown that caffeine taken before exercise can reduce the sensations of pain and exertion during a variety of exercise types. This reduced pain sensation with caffeine is thought to allow athletes to exert themselves harder and for longer in training or competition, and a greater willingness to tolerate the discomfort of exercise. Many studies also show effects on performance, in particular endurance performance around 1 hour in duration. Taking caffeine before training may also reduce the muscle soreness in the days after the exercise.
The evidence for calcium increasing the strength of muscle cell contraction is quite strong. However, most of the experiments showing this were done using muscle cells (often from animals) that had been removed and measured in a petri dish. The amounts of caffeine used in the studies were far higher than could be achieved (or safe) in humans. Caffeine may affect the release of calcium in the amounts used by humans, but it likely isn’t a major contributor to exercise performance in the majority of sports events.
Caffeine may affect the release of calcium in the amounts used by humans, but it likely isn’t a major contributor to exercise performance in the majority of sports events.
Although some studies showed that caffeine can increase catecholamines and the use of fat for energy, this isn’t always the case. Many studies have shown that performance can be improved in short duration exercise, where glycogen sparing wouldn’t benefit performance. Interestingly, many of those studies that did not see increased in fat use for energy, still saw performance improvements with caffeine ingestion, meaning that cannot be the primary mechanism for caffeine. Furthermore, studies that have measured the amount of glycogen in the muscle after endurance exercise didn’t see higher levels at the end of exercise after taking caffeine compared to a placebo.
Not all studies have shown increases in fat oxidation and even if these were observed they did not find significant degrees of glycogen sparing.
So, how does caffeine really work?
It appears that the main mechanism by which caffeine can improve performance in a wide variety of sports is through its effect on blocking the action of adenosine, resulting in reduced fatigue and dulled sensation of pain. It also improves alertness and reaction time. It is possible that caffeine could increase the strength of muscle contraction, but if it does occur it probably doesn’t have a large effect. Caffeine doesn’t appear to cause glycogen sparing despite its effects on adrenaline and fat burning.
It is important to note that too much caffeine van have the opposite effects and that the effects of caffeine are highly individual. Careful experimentation is probably the best way to find out what works for each individual.
References
McLellan TM, Caldwell JA, Lieberman HR. A review of caffeine's effects on cognitive, physical and occupational performance. Neurosci Biobehav Rev. 71:294-312, 2016
