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Studies have shown caffeine can improve performance by ~2%<ref name="CaffMeta"/>. Though much greater improvements (~12%) have been shown in laboratory conditions, these are not likely to be seen in real world race conditions<ref name="CaffMeta"/>. That ~2% represents 3.5 minutes on a 3 hour marathon, nearly 5 minutes on a 4 hour marathon. For the 5K, that represents 25 seconds on a 21 minute 5K, or 18 seconds on a 15 minute 5K. Caffeine tends to benefit fitter individuals more<ref name="CaffMeta"/>. Caffeine may improve performance by reducing the [[Rating of Perceived Exertion|RPE]]<ref name="CafRPE"/> and muscle pain<ref name="CafMusclePain"/>.

==Green Tea and Performance==

==Coffee and Performance==

When you take caffeine as coffee, it may not improve performance as it does when taken in other forms. One study showed that caffeine in coffee does not give a performance improvement where isolated caffeine does<ref name="CaffCoffee"/> and another showed no benefit from caffeinated coffee over decaffeinated coffee<ref name="CaffCoffeeButts25"/>. Some studies show a performance benefit from caffeinated coffee<ref name="CaffCoffeeWiles"/><ref name="CaffCoffeeCostill"/><ref name="CaffCoffeeCasal"/>, but did not compare with caffeine alone. This may be because coffee contains hundreds if not thousands of compounds besides caffeine<ref name="CaffMetaAndPerf"/><ref name="CafCoffeeCancer"/>. Some of these extra compounds of been shown to affect glucose metabolism<ref name="CafT2DMSysRevDam05"/>. It is been observed that decaffeinated coffee can reduce the absorption of glucose<ref name="CaffDecaffCoffeeGlucose"/>.

=How much?=

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=Caffeine and Dehydration=

The general recommendation to avoid caffeine at altitude does not seem to have a scientific basis. In fact, a number of the effects of caffeine have the potential to be beneficial at altitude<ref name="CafAltitude"/>:

* As noted above, caffeine does not produce dehydration, and there is little evidence that dehydration contributes to altitude sickness.

* The risk of a brain or lung edema at altitude is a serious concern, and there is the possibility that caffeine may help by constricting blood vessels without restricting oxygenation.

* Caffeine may help alleviate the headache that is common with altitude sickness.

==Tea==

[[File:Caffeine Tea Brewing Time.jpg|right|thumb|200px|Brewing time and levels of Caffeine (CF), the catechins epigallocatechin gallate (EGCG), epicatechin gallate (ECG), epigallocatechin (EGC) and epicatechin (EC)<ref name="ShishikuraKhokhar2005"/>.]]

==Gels ($2-8/100mg)==

Gels are available with and without caffeine, and the caffeinated variety normally has 25mg, though some go as high as 100mg. The higher caffeine concentrations often include a coffee extract, which may limit the benefits. Given that gels typically cost $1-2 per gel, the caffeine cost is $2-8/100mg. See [[Comparison of Energy Gels]] for more details.

The caffeine levels in soda vary widely, with some common values shown below.

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Typically soda makes a poor sports drink as it contains too much sugar, is too carbonated and additives like phosphoric acid can upset the digestive system. However, ultrarunners used to regularly use soda that had been left to go flat and then often diluted with water<ref name="CaffDeFizz"/>. This is something to practice in training before using during a race.

There are many different types of energy drink available on the market today. Frequently they are a mixture of many different substances that are believed in some way to be stimulating. Energy drinks should be used with caution, as the substances they contain may not have been scientifically evaluated, especially in the combinations provided. Energy drinks often contain carbohydrate, but the specific types of carbohydrate and the concentration used may not be appropriate to athletic events. The caffeine content of energy drinks is often 200-300mg per can<ref name="CaffDatabase"/>, which is quite high, but lower than a Starbucks Venti brewed coffee which typically has 400+mg<ref name="CaffStarbucks"/>.

==Red Bull ($3-6/100mg)==

==Energy Shots==

Energy shots are more convenient than energy drinks due to their small size and portability. However, they concentrated form can easily cause stomach upsets in addition to the problems noted in energy drinks above.

[http://en.wikipedia.org/wiki/Cardiac_arrhythmia Cardiac arrhythmias] (irregular heartbeats) are a group of conditions where the heart does not beat normally and can be a life threatening emergency. There is a common belief that caffeine is linked to some types of arrhythmias, but this does not seem scientifically supported for <ref name="RashidHines2006"/><ref name="Katan-2005"/><ref name="Pelchovitz-2011"/>. In fact, an animal study showed that moderate caffeine reduces the risk of atrial fibrillation<ref name="RashidHines2006"/>. A study giving recent heart attacks 450mg of caffeine showed no change in the risk of arrythmia<ref name="Myers-1990"/>. Giving 200mg of caffeine before a bicycle stress test to patients who have malignant ventricular arrhythmias made no difference<ref name="Graboys-1989"/>. However, there are anecdotal reports of people having arrhythmia triggered by caffeine. Note that a caffeine overdoes can result in heart problems<ref name="Pelchovitz-2011"/>.

==Evaluating the risks==

=Caffeine and health =

For most people, caffeine is generally taken as tea or coffee, to the bulk of the scientific evidence around the health impact of caffeine is based on these beverages.

<ref name="Pasman-1995">WJ. Pasman, MA. van Baak, AE. Jeukendrup, A. de Haan, The effect of different dosages of caffeine on endurance performance time., Int J Sports Med, volume 16, issue 4, pages 225-30, May 1995, doi [http://dx.doi.org/10.1055/s-2007-972996 10.1055/s-2007-972996], PMID [http://www.ncbi.nlm.nih.gov/pubmed/7657415 7657415]</ref>

<ref name="CaffRedBull">Nutrition Facts and Analysis for Energy drink, RED BULL, with added caffeine, niacin, pantothenic acid, vitamins B6 and B12 http://nutritiondata.self.com/facts/beverages/7399/2 </ref>

<ref name="Lin-2003">YS. Lin, YJ. Tsai, JS. Tsay, JK. Lin, Factors affecting the levels of tea polyphenols and caffeine in tea leaves., J Agric Food Chem, volume 51, issue 7, pages 1864-73, Mar 2003, doi [http://dx.doi.org/10.1021/jf021066b 10.1021/jf021066b], PMID [http://www.ncbi.nlm.nih.gov/pubmed/12643643 12643643]</ref>

<ref name="ShishikuraKhokhar2005">Yoko Shishikura, Santosh Khokhar, Factors affecting the levels of catechins and caffeine in tea beverage: estimated daily intakes and antioxidant activity, Journal of the Science of Food and Agriculture, volume 85, issue 12, 2005, pages 2125–2133, ISSN [http://www.worldcat.org/issn/0022-5142 0022-5142], doi [http://dx.doi.org/10.1002/jsfa.2206 10.1002/jsfa.2206]</ref>

<ref name="Buscicchio-2012">G. Buscicchio, M. Piemontese, L. Gentilucci, F. Ferretti, AL. Tranquilli, The effects of maternal caffeine and chocolate intake on fetal heart rate., J Matern Fetal Neonatal Med, volume 25, issue 5, pages 528-30, May 2012, doi [http://dx.doi.org/10.3109/14767058.2011.636104 10.3109/14767058.2011.636104], PMID [http://www.ncbi.nlm.nih.gov/pubmed/22502981 22502981]</ref>

<ref name="CaffStarbucks">The Complete Guide to Starbucks Caffeine http://www.energyfiend.com/the-complete-guide-to-starbucks-caffeine </ref>

<ref name="Stern2011">Judith Stern, Coffee and muscle cramps, Neuromuscular Disorders, volume 21, issue 5, 2011, pages 375, ISSN [http://www.worldcat.org/issn/09608966 09608966], doi [http://dx.doi.org/10.1016/j.nmd.2011.03.007 10.1016/j.nmd.2011.03.007]</ref>

<ref name="Dubowitz2011">Victor Dubowitz, Muscle cramps and coffee, Neuromuscular Disorders, volume 21, issue 3, 2011, pages 235, ISSN [http://www.worldcat.org/issn/09608966 09608966], doi [http://dx.doi.org/10.1016/j.nmd.2011.01.004 10.1016/j.nmd.2011.01.004]</ref>