Caffeine in sports Flashcards
Caffeine
Caffeine is the most widely consumed psychoactive substance.
It belongs to a group of lipid soluble purines, such as adenine and guanine, found naturally in 63 plants, including the coffee plant, guarana, cocoa and yerba mate.
Even in regular coffee, the amount of caffeine in a drink can differ depending on the preparation method.
75% of US 7-12 year olds consume caffeine daily.
90% of adults consume caffeine daily
Caffeine is absorbed rapidly and can reach maximum values in plasma within 1h of consumption.
It can increase both physical and cognitive performance.
In 1984, caffeine was on the International Olympic Committee (IOC) banned list. A doping offense was defined as having urinary caffeine concentrations exceeding a cut-off of 15 μg/ml. However, only 0.6-1% of athletes will have values higher than this, despite consuming it at doses much higher than those required to increase performance.
In 1985, the threshold was reduced to 12 μg/ml
In 2000, the World Anti-Doping Agency (WADA) added caffeine to its banned list at 12 μg/ml.
Caffeine was removed from the WADA Banned List on Jan 1 2004 and instead placed on the ‘monitoring’ list. Monitoring is more done for safeguarding or data collection rather than for regulating caffeine intake. This may be because WADA may not have the resources to test all athletes for caffeine intake since it is so widely consumed.
Caffeine is available in a wide number of formats including coffee, energy drinks, powder, pills, capsules, gels and gum.
There is widespread evidence of performance increases across different forms of exercise.
There is an increased interest in fast acting gum. This is absorbed in the buccal cavity, so blood concentrations peak much quicker, offering a quick boost. It is also readily accessible and it is easier to monitor your own intake.
Effects of caffeine
Caffeine causes ergogenicity, an increased capacity for bodily or mental labour especially by eliminating fatigue symptoms.
The potential mechanisms of ergogenicity is:
- Glycogen sparing/increased fat oxidation. This allows readily available glycogen to be kept in the body for longer and used when activity needs to be increased, e.g. at the end of a race.
- Increased glycogen resynthesis
- Neuromuscular function at the SR level
- Adenosine receptor antagonism, which results in dulling of pain and perceived exertion
Enhanced fat oxidation
The hypothesis is that caffeine activates the adrenal gland and release adrenaline, resulting in the breakdown of triglycerides into free fatty acids.
However, most studies have NOT observed increased concentration of FFAs and decreased respiratory exchange ratio (RER). Even if plasma FFAs increase, this doesn’t necessarily mean that there is an increase in fatty acid oxidation.
An increased concentration in FFAs seems to take place mainly at rest just before exercise or at very low intensities, so this is not applicable to most sports performance. This may be because at high intensity, adrenaline increases anyway, so the contribution of adrenaline doesn’t make much of a difference to plasma adrenaline levels.
Studies showing that caffeine does NOT decrease the RER and/or increase FFAs outnumber those that do show a change.
Even in the presence of increased FFAs, increased oxidation will not necessarily take place. Graham et al (2000) found no significant differences in oxidation in participants exercising following the consumption of caffeine vs placebo.
Fat oxidation is trivial in some situations where caffeine improves performance.
Glycogen sparing
The evidence on whether caffeine ingestion spares muscle glycogen stores is inconclusive.
Early studies such as the one by Ivy et al. (1979) supported that caffeine spared glycogen, however few other studies support these theories.
It is commonly stated that caffeine results in glycogen sparing.
In a depletion study, volunteers exercised until glycogen was depleted, then stores were repleted and caffeine was consumed. This found:
- No differences in glycogen utilisation in glycogen loaded subjects
- No increase in Plasma FFA compared to placebo despite adrenaline increases
Data suggest that caffeine lowers the threshold for exercise-induced β-endorphin release. More endorphins are released, which reduce feelings of pain and exertion.
Adenosine receptor antagonism
Caffeine improves endurance performance largely by blocking adenosine A2A receptors.
The A2A receptors form a mutually antagonistic complex with dopamine D2 receptors.
Stimulation of the CNS reduces pain, makes exercise feel easier by reducing perceived exertion, and delays muscle fatigue. Adenosine inhibits this stimulation, causing fatigue.
Adenosine regulates arousal through depressive influence on adrenaline and noradrenaline release.
There is therefore an increased production of catecholamines when caffeine is ingested due to adenosine antagonism.
Caffeine is metabolised within the liver. Its metabolites can also bind to adenosine receptors. This increases the duration of action.
Neuromuscular function at the SR level
Caffeine may facilitate neuromuscular function at the sarcoplasmic reticulum (SR) level.
Ryanodine receptors (RyR1) are essential for excitation-contraction coupling.
RyR1 releases calcium ions required to activate the contractile proteins. Ca2+ is required for myosin-actin binding.
Ca2+, ATP, and Caffeine regulate RyR1 through a network of allosteric interactions.
This increases RyR1 calcium release and reduces reuptake. This results in stronger muscle contractions.
Post-exercise glycogen resynthesis
Glycogen resynthesis replaces energy in the muscles.
This requires an energy source in the form of carbohydrates.
Sugars are taken up into muscles through GLUT-4 transporters.
Caffeine increases GLUT-4 translocation through AMPK signalling.
Co-ingestion of caffeine (Caff) with carbohydrates (CHO) increases rates of muscle glycogen resynthesis during recovery from exhaustive exercise.
Skeletal muscle glycogen content was monitored following cycling to fatigue in participants that received CHO alone or with the addition of Caff. The caffeine group had a higher muscle glycogen level.
Phosphorylation of calcium/calmodulin-stimulated protein kinase (CaMK) was similar after exercise and after 1 h of recovery, but after 4 h CaMKT phosphorylation was higher in the Caff group than the CHO group (P <0.05).
Phosphorylation of AMP-activated protein kinase (AMPK) and Akt was similar for both treatments at all time points.
Caffeine and endurance sports (60+ min)
Many cyclists drink a sports drink for the first two thirds of a road race, then switch to defizzed Coke. Why? Does it matter if intake is pre or during exercise?
A study found that consuming caffeine pre or during exercise made no difference - both increased performance by 3.4% compared to placebo. Coke alone towards the end of the race showed a 3.1% increase in performance from placebo.
Performance enhancing effects occur way below the IOC limit.
Another study found that the best performance increase occurred in a high carbohydrate and caffeine group, so taking both at the same time is beneficial in endurance exercise.
A trial found that both a low (100 mg, ~1.5 mg/kg) and moderate (200 mg, ~3 mg/kg) dose of caffeine ingested late in exercise improved performance, reducing the time taken to complete a cycling race.
The moderate dose was more effective than the lower dose → if you consume caffeine during exercise rather than before, a moderate dose offers added benefit.
In sports performance at the Olympic level, not taking caffeine may put you at a disadvantage.
As long as habitual intake is lower than the dose taken for sports performance, effects are expected to be similar in those that regularly consume caffeine vs those that do not consume it.
- It may be more accurate to assess effects of further increases while still allowing habitual intake before the study, as the majority of the population consumes caffeine daily.
- Nausea and migraine can also occur if individuals with high habitual intake suddenly stop taking caffeine - withdrawal may convolute results more.
Caffeine and sustained high-intensity sports (1-30 min)
Caffeine enhances performance in sustained high-intensity sports, but there is no need to take more than 6 mg/kg.
This is likely through the adenosine response resulting in increased power.
Caffeine in a 1 km race showed a 3.1% increase in performance. This is a huge difference as the difference between first and second place is only 0.31%
Conclusions
Caffeine may enhance performance of a variety of sports, including endurance and sustained high intensity sports
Benefits are seen at low-moderate caffeine doses
The effects of caffeine may be dose-dependent at concentrations <3 mg/kg, but not at concentrations >3 mg/kg
A variety of protocols of use can work, including before exercise, during exercise or before the onset of fatigue.
Responses are variable and may include side-effects
Athletes should experiment during training to find if a protocol is beneficial for their event, as responses are variable and there may be side effects.
