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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==
There is relatively little research concerning the effect of tea on athletic performance. However, one study did show that the green tea extract increased fat burning and glucose tolerance during cycling at 60% [[VO2max|V̇O<sub>2</sub>max]] without affecting heart rate [[Heart Rate]] or overall energy consumption<ref name="Venables-2008"/>.
==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"/>.
* A milk intake of 500 to 900 ml/day<ref name="CaffBabyMilk"/>
* The nursing mother's caffeine intake of 200mg/day (one double shot espresso),
We get a resulting caffeine concentration in the milk of 4ug/ml to 8ug/ml<ref name="Stavchansky-"/>, which is a total caffeine intake of between 2mg to 13mg, or 0.6 to 4 mg/Kg body weight. The upper end of that level is quite high. However, the baby's half-life for caffeine is 31-132 hours (average 82 hours)<ref name="Parsons-1981"/>, compared with an adult's 2-10 hour half-life, so the caffeine will build up over time. A 24 hour half live (which is easier to calculate) would result in about a 3mg to 26mg, which is 1 to 8 mg/Kg. I'm guessing that would result in the baby not sleeping well! Conversely, a baby whose mother takes caffeine during pregnancy and is then given formula milk may undergo caffeine withdrawal after birth<ref name="McGowan-1988"/>. Even if the mother breast feeds, the varying levels of caffeine may cause withdrawal symptoms<ref name="Martín-2007"/>. Also, caffeine has been shown to increase fetal heart rate[[Heart Rate]]<ref name="Buscicchio-2012"/>.
=References=
<references>
