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[[File:Caffeine.jpg|right|thumb|500px|Common sources of Caffeine.]]
Caffeine is widely consumed by the general public and athletes, and moderate levels can improve athletic performance. Caffeine does not cause dehydration, but it can interfere with sleep. Caffeine in coffee does not seem as effectiveat improving performance, so other sources should be used. Caffeine increases blood pressure, and should be avoided during exercise by those with high blood pressure.
=Introduction=
Man has been searching for ways of improving athletic performance since at least 400 BC, when the hearts of lion were believed to impart benefits<ref name="CaffLionHeart"/>. Today, caffeine can improve performance in endurance running, and three of every four elite athletes take caffeine when competing<ref name="CaffUse"/>. Caffeine is one of the most widely used drugs in the world<ref name="CaffWorld"/>, with average daily intakes worldwide of 70mg/day, but higher in the US (~200mg/day) and the UK (~400mg/day)<ref name="CaffDependence"/>. Caffeine has many effects on many different tissue types, directly and through its metabolites, as well as stimulating adrenaline release<ref name="CaffMetaAndPerf"/>.
| 80 per 8.3oz Can
|}
=Caffeine, Blood Pressure and Heart Rate=
Caffeine increases blood pressure at rest and under stress, including exercise stress. The effect of caffeine on heart rate is unclear, with both increases and decreases observed in studies. Generally caffeine decreases heart rate at rest and moderate intensity exercise, but increases it at maximal workloads.
==Confounding factors==
Understanding the effect of caffeine on blood pressure and heart rate is complicated by a number of factors. Do the subjects regularly use caffeine or are they caffeine naive? Do they have normal blood pressure or hypotension? How does exercise or non-exercise stress change the effect of caffeine? Should we look at acute or long term changes? There may be other underlying factors that influence the response to caffeine, such as nicotine consumption<ref name="Perkins-1994"/>, or taking caffeine with food<ref name="HasenfratzJaquet1991"/>.
==Acute Changes at Rest ==
At rest, caffeine increases blood pressure, but it may lower or slightly raise heart rate.
* Systolic blood pressure is increased by 3-7 mmHg<ref name="Sondermeijervan Marle2002"/><ref name="Hartley-2004"/><ref name="TurleyGerst2006"/>.
** A meta-analysis showed that coffee increased systolic blood pressure by 2.4 mmHg, with each additional cup of coffee increasing by a further 0.8 mmHg<ref name="Jee-1999"/>.
* Diastolic blood pressure is increased by 2-4 mmHg<ref name="Sondermeijervan Marle2002"/><ref name="Hartley-2004"/><ref name="TurleyGerst2006"/>.
** Coffee increases diastolic blood pressure by 1.2 mmHg, with each additional cup of coffee increasing by a further 0.5 mmHg<ref name="Jee-1999"/>.
* The change in blood pressure is due to an increase in peripheral vascular resistance rather than increased cardiac output<ref name="Pincomb-1996"/><ref name="Pincomb-1985"/><ref name="SungLovallo1990"/>.
* Most<ref name="Pincomb-1996"/><ref name="Sung-1995"/><ref name="HartleySung2000"/>, but not all<ref name="Lovallo-1991"/> studies show that blood pressure increases are generally greater in people with borderline hypotension.
* The rise in blood pressure is seen while the subjects went about their normal activities<ref name="Green-1996"/>.
* At rest, heart rate decreases with caffeine<ref name="Whitsett-1984"/><ref name="Sung-1995"/><ref name="TurleyGerst2006"/> though one study showed no change<ref name="Sondermeijervan Marle2002"/>.
* Most studies show that [[Heart Rate Variability]] is increased at rest<ref name="Yeragani-2005"/><ref name="Kolodiĭchuk-1991"/><ref name="Nishijima-2002"/>, but one study showed no change in habitual caffeine users<ref name="RauhBurkert2006"/>, and another showed a reduction in the caffeine naive<ref name="Sondermeijervan Marle2002"/>.
* Caffeine withdrawal can reduce blood pressure by 5-6 mmHg<ref name="Phillips-Bute1997"/>.
* A review of the available research indicated that 7 studies showed that habitual caffeine users had a lower Blood Pressure response to acute caffeine compared with caffeine naive individuals, but 21 studies found no difference<ref name="GreenKirby1996"/>. One study showed a reduced increase in blood pressure after regular coffee drinking for four weeks<ref name="Ammon-1983"/>.
* Older individuals may have a greater rise in blood pressure<ref name="Izzo-1983"/>.
* Taking caffeine with food dampens the effect of the caffeine, probably because it decreases the absorption rate<ref name="HasenfratzJaquet1991"/>.
==Acute Changes During Exercise==
Caffeine raises blood pressure during exercise, increasing the possibility of excessively high blood pressure. Caffeine can increase or decrease heart rate during exercise, possibly lowering it during lower intensity exercise and increasing it at highest intensities.
* The effect of Caffeine on heart rate during exercise is ambiguous, with some studies showing an increase in heart rate<ref name="Bell-1998"/><ref name="Mcnaughton1987"/><ref name="Sasaki-1987"/><ref name="Sung-1995"/><ref name="Bell-2002"/>, while others show a decrease<ref name="Sullivan-1992"/><ref name="TurleyGerst2006"/><ref name="Gaesser-1985"/><ref name="McClaranWetter2007"/><ref name="SungLovallo1990"/>. Examining the studies in more detail however, and it appears that the increase in heart rate may be mostly at the highest intensities, with caffeine reducing heart rate at the lower intensities. The effect does not appear different for those that are caffeine habituated or those that are caffeine naïve.
{| class="wikitable"
! Study
! Subjects
! Caffeine
! Exercise
! Caffeine's effect on Heart Rate
|-
| Bell 1998<ref name="Bell-1998"/>
| Caffeine Habituated
| 5mg/Kg
| 85% [[VO2max|V̇O<sub>2</sub>max]]
| Increased at 5 min but not 10 min
|-
| McNaughton 1987<ref name="Mcnaughton1987"/>
| Caffeine Naïve
|
| Incremental to exhaustion
| HR Increased
|-
| Bell 2002<ref name="Bell-2002"/>
| Both Habituated and Naïve
| 5mg/Kg
| 80% [[VO2max|V̇O<sub>2</sub>max]]
| HR Increased in both groups
|-
| Sasaki 1987<ref name="Sasaki-1987"/>
|
| 200mg
| Incremental to exhaustion
| HR Increased
|-
| Sung 1995<ref name="Sung-1995"/>
| Caffeine Habituated
|
| 30 min cycling
| HR Increase in hypotensive, not normotensive
|-
| Sullivan 1992<ref name="Sullivan-1992"/>
|
| 3.3mg/Kg
| 45% [[VO2max|V̇O<sub>2</sub>max]]
| No change
|-
| Turley 2006<ref name="TurleyGerst2006"/>
|
| 5mg/Kg
| 25 and 50watt cycling (children)
| No change
|-
| Gaesser 1985<ref name="Gaesser-1985"/>
| Caffeine Habituated
| 200mg
| Incremental to exhaustion
| HR lower at moderate work rates, no change at high work rates
|-
| McClaran 2007<ref name="McClaranWetter2007"/>
| Caffeine Naïve, Trained
| 1.5 or 3.0 mg/Kg
| Incremental and maximal
| HR lower at submaximal (30-63% [[VO2max|V̇O<sub>2</sub>max]]) but not maximum workloads
|}
* The effect of caffeine on blood pressure is more consistent, with most studies showing an increase<ref name="Sung-1995"/><ref name="Sullivan-1992"/><ref name="TurleyGerst2006"/>.
* Blood pressure during exercise can become excessive (> 230 for systolic or > 120 for diastolic). Caffeine increases the risk of excessive blood pressure in normotensive individuals<ref name="SungLovallo1990"/>, but the risk is greater in people who are hypertensive <ref name="Sung-1995"/>. There are recommendations that hypertensive individuals avoid caffeine during exercise<ref name="PincombWilson1991"/>.
* [[Heart Rate Variability]] goes down with exercise, but this decrease is reduced with caffeine<ref name="Yeragani-2005"/><ref name="Nishijima-2002"/>.
* Caffeine did not alter [[Maximum Heart Rate]] in one study<ref name="SungLovallo1990"/>.
==Caffeine Changes During Non-Exercise Stress==
Non-exercise stress includes things like mental arithmetic, rapid information processing, final exams. An analysis of 21 studies that looked at caffeine and non-exercise stress found that<ref name="GreenKirby1996"/>:
* Heart rate changes:
** 13 showed no change in heart rate, both at rest and under stress.
** 3 decrease in heart rate at rest, but an increase under stress.
** 3 no change in heart rate at rest, but an increase under stress.
** 2 showed a decrease both at rest and under stress.
* Nearly all studies showed an increase in blood pressure for both the at rest and stress conditions.
=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.
==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"/>.
==Caffeine and Blood Pressure==
As noted above, blood pressure during exercise rise to an excessive level (> 230 for systolic or > 120 for diastolic). Taking caffeine can increase the risk of excessive blood pressure in people with normal blood pressure<ref name="SungLovallo1990"/>, but the risk is greater in people who already have high blood pressure<ref name="Sung-1995"/>. Some authorities recommendation that individuals with high blood pressure do not take caffeine with exercise<ref name="PincombWilson1991"/>.
==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.
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<ref name="Nishijima-2002">Y. Nishijima, T. Ikeda, M. Takamatsu, Y. Kiso, H. Shibata, T. Fushiki, T. Moritani, Influence of caffeine ingestion on autonomic nervous activity during endurance exercise in humans., Eur J Appl Physiol, volume 87, issue 6, pages 475-80, Oct 2002, doi [http://dx.doi.org/10.1007/s00421-002-0678-1 10.1007/s00421-002-0678-1], PMID [http://www.ncbi.nlm.nih.gov/pubmed/12355185 12355185]</ref>
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</references>
