Water Water Everywhere...
With my background doing research on the effects of hydration status during exercise in the heat, it is only natural that we return to this topic every summer with the onset of warm weather. We have seen, in general, the problems brought about by sudden or prolonged heat waves every year in both the athletic and general population.
More: Cycling Hydratioin Myths
In the former, we've seen many examples of athletes succumbing to heat exhaustion, from Gabrielle Andersson-Schiess at the first Olympic women's marathon in 1984 to Paula Radcliffe at the Athens marathon in 2004. Much more seriously, we've witnessed major increases in deaths coinciding with massive heat waves in Europe (2003) and Chicago (1995).
To counteract the health and performance problems with hyperthermia, the primary approach has been to maintain proper hydration status. Though there remains some dissenters, the majority consensus amongst scientists is that maintaining adequate hydration both prior to and throughout exercise is the best method of preventing performance decrements in the heat. This has been outlined in detail in position statements by the American College of Sports Medicine (ACSM) , the pre-eminent international exercise physiology society.
The main mechanism of protection is to keep blood volume high, in order to decrease the strain imposed by the heart pumping blood, which fuels both the active muscles and the skin to dissipate heat. You're faced with a double whammy—you're losing fluid from your blood to sweat, and at the same time, your skin blood vessels are opening more and requiring greater blood flow to get rid of the heat produced by exercise. You may have seen this problem happen in the phenomenon of "cardiovascular drift," where your heart rate rises over time even though you're riding at the same power output.
Aerobic Versus Anaerobic
The major caveat to the above "consensus" statement is that it mainly applies to prolonged endurance exercise, where the exercise is long enough for the above problems of decreased blood volume and increased skin blood flow demands to become important. But what if your main event is a short track event, or even a short age-group criterium? With the events short in overall duration and the focus on brief and very high-intensity efforts, do the same considerations apply?