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=Caffeine and Dehydration=
Caffeine does not impact performance in hot/humid conditions, nor act as a diuretic when running<ref name="CaffDiuretic"/>. Caffeine at 360mg is a diuretic at rest, but not at 180mg or less<ref name="CaffDiureticDose"/>. Caffeine does not cause long term dehydration<ref name="Caff11day"/>, and black tea has been shown to hydrate as well as water<ref name="CaffTea"/>. Caffeine does result in increased sodium excretion in the urine<ref name="CaffSodium"/>, but the significance of this unclear.
* While caffeine could reduce the altitude problems that prevent sleep, the stimulant nature of caffeine may offset these benefits.
Caffeine has a shorter duration of effect at high altitude, possibly due to increased blood flow to the liver, and withdrawal from caffeine would likely make altitude problems more severe<ref name="CafAltitude"/>.
=Glucose Absorption, Insulin Resistance and Glycemic index =
Caffeine changes the way glucose is absorbed, but this effect is different for those at rest compared with those exercising.
[[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"/>.]]
It is claimed that tea is the second most popular drink after water<ref name="Macfarlane"/>, but it does not appear to be commonly used by athletes. Because the level of caffeine can vary dramatically with different types of tea, and different brewing methods, it is difficult to know how much caffeine is in a particular drink of tea. Generally black tea has more than oolong tea which has more than green tea<ref name="Lin-2003"/>. Typically black tea contains 28 to 46mg of Caffeine per 8oz cup<ref name="Bunker-1979"/>. This unpredictability makes tea a problematic source of caffeine, though using the same tea and brewing for the same time will give reproducible levels, but you won't know the absolute caffeine intake.
==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.
* Tea may help reduce dementia<ref name="CaffTeaDementSong12"/>, and tea improves learning in rats<ref name="CaffTeaRelaxJune99"/>.
* Theanine and cystine may help reduce the immune system depression of high intensity training<ref name="CaffTheanImune09"/><ref name="CaffTheanImune10"/>.
=Caffeine, Running and Sudden Death=
While rare, the sudden death of a runner does occur and often makes the news. Running is estimated to have a mortality rate of about 1 in 7,000 runners, or one death per 396,000 man-hours<ref name="Thompson-1982"/>. Sudden death during a marathon is rarer, at less than 1 in 50,000<ref name="MaronPoliac1996"/><ref name="RobertsMaron2005"/>.
==Caffeine and Myocardial Blood Flow ==
A moderate amount of caffeine does not change the myocardial blood flow at rest, but during exercise the caffeine significantly reduces the flow<ref name="Namdar-2009"/><ref name="Namdar-2006"/>. This decrease in myocardial blood flow is worse at altitude<ref name="Namdar-2006"/> and in people with Coronary artery disease<ref name="Namdar-2009"/>. (These changes were seen with 200mg of caffeine, but the studies did not try other amounts to see of the effect varies with dose.) The change in myocardial blood flow is not a risk factor for healthy individuals<ref name="Namdar-2009"/><ref name="Namdar-2006"/>, but for those with Coronary artery disease the combination of caffeine and exercise may exacerbate their condition.
==Caffeine and Irregular Heart Beats==
[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==
Because coronary artery disease may not have any initial symptoms<ref name="MayoCAD"/>, risk evaluation is tricky. Some runners who died suddenly had high cholesterol, high blood pressure or chest pains<ref name="WallerRoberts1980"/>. Other risk factors include diabetes, smoking, family history of heart attacks, obesity, long term alcohol use. If you have any risk factors for coronary artery disease, it would be prudent to talk to a healthcare professional.
=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.
* Any athlete that does not effectively use either caffeine or carbohydrate is at a distinct disadvantage
This comparison indicates to me that caffeine is not against the spirit of the sport, or unethical.
=Newborn babies and Caffeine =
Newborn babies may be subject to higher levels of caffeine from breast milk than one would expect. If we assume:
=References=
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