Caffeine and Exercise

Caffeine/Stimulants

Caffeine is the most widely used stimulant in the world. Ninty-percent of the adult population consider themselves as daily coffee users drinking on average two cups of coffee daily.   This is because caffeine consumption is completely legal, socially acceptable, and is consumed daily by a large majority of the world population. Daily morning staples, such as coffee and tea, as well as soda pop and energy drinks contain caffeine. Much research has been conducted to determine how caffeine consumption effects one’s body systems and processes during exercise. Many studies support the findings that caffeine enhances both physical and cognitive performances. Physically, caffeine specifically improves aerobic performance— extending the time that an activity can be sustained prior to fatigue. Mixed results exist as to whether caffeine significantly improves performance in resistance training. Several mechanisms work on different systems of the body to produce the overall enhancement of exercise performance that caffeine causes.

A neuromuscular mechanism that aides in causing caffeine’s ergogenic effects is its influence on the ryanodine receptors in the sarcoplasmic reticulum of muscles. The function of these receptors is to open to allow Ca²⁺ to flow out of the sarcoplasmic reticulum. Ca²⁺ facilitates the contraction of muscle fibers. Ca²⁺ binds to troponin, which moves tropomyosin aside so that myosin can bind to the actin myofilaments and muscle contraction can occur. Therefore, the effect that caffeine has on this system is to make Ca²⁺ more readily available, which allows for stronger contractions of muscles than is typical at a given level of stimulation.

Caffeine has also been shown to enhance exercise performance by reducing pain thresholds. Many studies have looked at caffeine's effect on the aspect of fatigue during a muscle contraction and attributed the increase force maintenance to decreaced pain perception. One study found that plasma b-endorphin levels almost doubled after two hours of cycling with caffeine consumption, while the control group had no increase. Therefore, caffeine's effect to lower pain perception is beneficial to endurance exercise performance.  An additional performance enhancement provided to the endurance athlete by caffiene is related to the pulmonary system.  When researching cross-country runners, one randomized and double-blinded study found a significant difference in the measurement of tidal volume, alveolar ventilation, and rating of perceived exertion between those that were given caffeine before performing submaximal exercise and those who were not.  Caffeine causes bronchodilation, which likely leads to the increase in tidal volume and alveolar ventilation.  As tidal volume and alveolar ventilation rise, an individual's respiration during exercise becomes more efficient and therefore less exertion is required and the perceived effort needed to complete the activity is reduced.  Despite strong evidence indicating caffeine's positive effects on the pulmonary system (such as the aforementioned study), collectively the research remains to be conclusive on the topic and therefore more experimental studies need to be done.

Caffeine also acts on the nervous system. Specifically, it affects normal neurotransmitter release, increasing both the amount of noradrenaline (NA) and dopamine (DA) released in the brain during exercise. Dopamine has widespread functions, but a few of these include influences on motivation, cognition, reward, motor control, and mood. Many studies have shown that an increase in DA release results in enhanced endurance. One such study showed that muscle pain perception and perceived exertion was much lower in a group that received caffeine prior to resistance training, as opposed to when those same subjects were administered a placebo on a separate date. When the individuals ingested the caffeine, they performed significantly more repetitions before failure than when they were given the placebo.

Although research supports the ergogenic effects of caffeine on endurance, it has been less conclusive as to whether caffeine has the same type of effects on resistance training. Data from several studies supports an increase in resistance training performance following caffeine ingestion; however, researchers have been unable to isolate the exact physiological mechanism responsible for this. Therefore, the increased performance could potentially be due only to the decreased pain & exertion perception of the subject [as previously explained].

One other substantial mechanism by which caffeine positively effects athletic performance is by increasing the rate that lipolysis occurs. Lipolysis is the process human bodies use to break apart fats and produce ATP (our primary usable form of energy) to fuel our body. Each triglyceride (fat molecule) that is broken down produces approximately 300-400 ATP (depending on how many carbons form the specific fat being used). The increase in fats being used to produce energy results in a decrease in carbohydrates used for that same purpose. This greatly increases efficiency because carbohydrate molecules produce far fewer ATP than triglycerides.

There are many factors in the methods of caffeine studies that cause confounded results. One factor that must be considered is the dose of caffeine administered. The many studies use different doses when investigating the effects that caffeine has on exercise. Researchers use doses of anywhere from 2-8 mg/kg, but the methods of most studies call for doses of 5-6 mg/kg. Doses of 2-5 mg/kg improve athletic performance by approximately 3%, whereas doses of 5-7mg/kg improve performance by approximately 7%. Other factors include the form of caffeine used, other ingredients in the caffeine source and their possible physiological effects, diet, time and intensity of exercise tested, and length of time prior to exercise that the caffeine is administered. These variables, in addition to many others, increase the complexity of research of caffeine and exercise performance.

The adverse effects of caffeine consumption in athletes who use it conservatively are minimal, if at all present. However, if the consumer is sedentary, or if the caffeine intake exceeds 7mg/kg, many negative side effects occur. In a sedentary person, caffeine interferes with the role of insulin, and therefore, also effects the metabolism of fats and carbohydrates. Most sources of caffeine are also high in glucose, which is a combination that leads to a decline in glucose disposal. Therefore, in the sedentary person, decreased glucose disposal often leads to obesity, which can cause many diseases, such as type 2 diabetes and metabolic syndrome. If caffeine intake exceeds 7mg/kg (even in the active individual), side effects such as nausea, jitters, headaches, and tachycardia present themselves. Additionally, these large doses do not improve athletic performance any more than the 7% improvement caused by doses of 5-7 mg/kg.

Another negative side effect that can occur from caffeine intake is its effect on Polycystic Kidney Disease. Polycystic Kidney Disease is the most common kidney disease in adults. It is an inherited disease that currently has no cure. Cysts are formed in the kidneys and grow in number and size overtime. The disease can lead to many more health problems and eventually kidney failure. There has been suggestive research that caffeine plays a role in PKD. A nucleotide known as cAMP stimulates the growth of cysts and the secretion of cyst fluid. One study showed that caffeine promotes the accumulation of cAMP in the kindeys, which leads to an increase in the size and number of cysts. The study noted that caffeine has this effect on the kidneys in only those who have PKD. If a person has PKD, they should avoid drinks such as coffee, tea, soda, or pre-workout drinks that contain a high amount of caffeine. Water could work as a substitute and also gatorade after a workout to replenish their electrolytes.

Five mechanisms of action for caffeine 
1. Antagonism of adenosine
2. Increased fatty acid oxidation
3. Caffeine acts as a nonselective competitive inhibitor of the phosphodiesterase enzymes
4. Increased post-exercise muscle glycogen accumulation
5. Mobilization of intracellular calcium