Caffeine and Exercise

Introduction 

Caffeine is a naturally occurring, central nervous system (CNS) stimulant and is the most widely taken psychoactive stimulant globally. This drug is most commonly sourced from the coffee bean but can also be found naturally occurring in certain types of tea and cacao beans. It is also an additive to soda and energy drinks. The primary goal of caffeine consumption is to combat fatigue and drowsiness, but there are many additional uses.

• 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 soft drinks and energy drinks, contain caffeine.
• Ninety-percent of the adult population consider themselves daily coffee users, drinking on average two cups of coffee a day.  

Many studies support the findings that caffeine enhances both physical and cognitive performances.

• A number of meta-analyses have demonstrated that caffeine’s ergogenic effects on exercise performance are well-established and well-replicated, appearing consistent across a broad range of exercise modalities.
• Several mechanisms work on different systems of the body to produce the overall enhancement of exercise performance that caffeine causes.. 

Caffeine Sources

Most dietary caffeine is still consumed as tea and coffee, and the latter accounts for 55% of per capita intake in the UK (Scott et al, 1989). Despite (or perhaps because of) its ubiquity, caffeine is rarely thought of as a problematic drug. Doctors do not often ask patients about its use and enquiry into caffeine consumption is not usually included in psychiatric assessment.

On average,

• A cup of brewed coffee contains 100 mg of caffeine, compared with 75 mg for instant coffee and 50 mg for tea (Food Standards Agency, 2001);
• A can of Coca Cola contains 30 mg.
• Stimulant drinks such as Red Bull (80 mg of caffeine per can)
• Pharmaceutical caffeine may be bought over the counter (for example as ProPlus tablets)
• Also contained in numerous proprietary analgesics, cold and ’flu remedies, diet pills and diuretics; Anadin Extra, for example, contains 90 mg of caffeine per dose.

It is assumed that it is safer for healthy adults when taken under 400 mg per day. The amount of this caffeine is roughly equal to consuming 4 cups of coffee.

Neuromuscular Effects

1. Ryanodine receptors (RyR)

A neuromuscular mechanism that aides in causing caffeine’s ergogenic effects is it's influence on the ryanodine receptors (RyR) in the sarcoplasmic reticulum of muscles.

• RyR functions as a calcium release channel in the sarcoplasmic reticulum, as well as a connection between the sarcoplasmic reticulum and the transverse tubule
• Caffeine makes the RyR more sensitive to either the action potential (skeletal muscle) or calcium (cardiac or smooth muscle), thereby producing calcium sparks (microscopic release of calcium) more often (this is partially responsible for caffeine's effect on heart rate).
• Caffeine makes Ca²⁺ more readily available, which allows for stronger contractions of muscles than is typical at a given level of stimulation.
  

2. Nervous System.

Caffeine 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 as it lowers pain perception.

• A 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.
• Another study offered an explanation of the mechanism causing the reduced pain threshold. The researchers found that plasma ß-endorphin levels almost doubled after two hours of cycling with caffeine consumption, while the control group had no increase.

Research has shown that pure caffeine can help endurance athletes

• Run faster and cycle for longer.
• Footballers to sprint more often and over greater distances
• Basketball players to jump higher
• Tennis players and golfers to hit the ball with greater accuracy.
• Weightlifters lift more weight.

An increasing number of studies have also shown that coffee can be used as an alternative to caffeine to improve cycling and competitive running performance, and produce similar results similar to pure caffeine.

Cardiopulmonary Effects

An additional performance enhancement provided to the endurance athlete by caffeine is related to the cardiopulmonary 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 who 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; therefore, less exertion is required and the perceived effort needed to complete the activity is reduced. Another, more recent, double-blind randomized trial also found results supporting the claim that caffeine ingestion prior to exercise lowers an individual’s rating of perceived exertion. The study also found that caffeine ingestion prior to sets of resistance exercises to fatigue had no effect on resting and peak heart rates prior to or during the bouts of exercise. Despite there being strong evidence indicating caffeine's positive effects on the pulmonary system and lack of effects on heart rate, collectively the research remains inconclusive on the topics and, therefore, more experimental studies must be conducted.

One example of the conflicting evidence concerning the cardiovascular and pulmonary effects of caffeine is a study performed to specifically study the effects of Red Bull© on the systems of the body. Red Bull© contains glucuronoactone, taurine, B vitamins, and sugar in addition to caffeine. Results of the study showed significant increased arterial blood pressure (ABP), heart rate (HR), blood glucose levels, respiration rate, and respiratory flow rate (RFR) at rest and during exercise, as compared to the control group. Clearly, these physiological effects would have a negative impact on athletic performance. At various points before, during, and after energy drink consumption (500mL of Red Bull©), plasma adrenaline and noradrenaline levels were measured. The significant increase in these levels is a product of sympathetic nervous system activation. The study claimed that long term energy drink consumption could have detrimental cardiovascular and respiratory effects.

Metabolic Effects

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 from the specific fat are 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%. Another factor that often confounds results is the form of caffeine used, because other ingredients in the caffeine source cause changes in the resulting physiological effects. Other factors include 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 (often resulting in hyperinsulinemia and hyperlipidemia); therefore, caffeine 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 syndromes. 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 it's effect on Polycystic Kidney Disease (PKD). PKD 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 over time. 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. Instead, people with PKD should drink water, and can replenish their electrolytes post-workout by drinking Gatorade.

Summary

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