Editing Exercise physiology (section)

=== Dehydration ===
[[Dehydration]] refers both to hypohydration (dehydration induced prior to exercise) and to exercise-induced dehydration (dehydration that develops during exercise). The latter reduces aerobic endurance performance and results in increased body temperature, heart rate, perceived exertion, and possibly increased reliance on carbohydrate as a fuel source. Although the negative effects of exercise-induced dehydration on exercise performance were clearly demonstrated in the 1940s, athletes continued to believe for years thereafter that fluid intake was not beneficial. More recently, negative effects on performance have been demonstrated with modest (<2%) dehydration, and these effects are exacerbated when the exercise is performed in a hot environment. The effects of hypohydration may vary, depending on whether it is induced through diuretics or sauna exposure, which substantially reduce plasma volume, or prior exercise, which has much less impact on plasma volume. Hypohydration reduces aerobic endurance, but its effects on muscle strength and endurance are not consistent and require further study.<ref>{{Cite journal |last=Barr |first=SI |year=1999 |title=Effects of dehydration on exercise performance |journal=Canadian Journal of Applied Physiology |volume=24 |issue=2 |pages=164–72 |doi=10.1139/h99-014 |pmid=10198142}}</ref>  Intense prolonged exercise produces metabolic waste heat, and this is removed by [[sweat]]-based [[thermoregulation]]. A male [[marathon]] runner loses each hour around 0.83 L in cool weather and 1.2 L in warm (losses in females are about 68 to 73% lower).<ref name="cheuvront">{{Cite journal |vauthors=Cheuvront SN, Haymes EM |year=2001 |title=Thermoregulation and marathon running: biological and environmental influences |journal=Sports Med |volume=31 |issue=10 |pages=743–62 |doi=10.2165/00007256-200131100-00004 |pmid=11547895 |s2cid=45969661}}</ref> People doing heavy exercise may lose two and half times as much fluid in sweat as urine.<ref>{{Cite journal |last=Porter |first=AM |year=2001 |title=Why do we have apocrine and sebaceous glands? |journal=Journal of the Royal Society of Medicine |volume=94 |issue=5 |pages=236–7 |doi=10.1177/014107680109400509 |pmc=1281456 |pmid=11385091}}</ref> This can have profound physiological effects. Cycling for 2 hours in the heat (35&nbsp;°C) with minimal fluid intake causes body mass decline by 3 to 5%, blood volume likewise by 3 to 6%, body temperature to rise constantly, and in comparison with proper fluid intake, higher heart rates, lower stroke volumes and cardiac outputs, reduced skin blood flow, and higher systemic vascular resistance. These effects are largely eliminated by replacing 50 to 80% of the fluid lost in sweat.<ref name="cheuvront" /><ref>{{Cite journal |last=González-Alonso |first=J |last2=Mora-Rodríguez |first2=R |last3=Below |first3=PR |last4=Coyle |first4=EF |year=1995 |title=Dehydration reduces cardiac output and increases systemic and cutaneous vascular resistance during exercise |journal=Journal of Applied Physiology |volume=79 |issue=5 |pages=1487–96 |doi=10.1152/jappl.1995.79.5.1487 |pmid=8594004}}</ref>