From what ive read, being sunburnt affects your body's ability to cool itself and causes it to lose fluids as well. Plus you'll look like an idiot. I wear sunblock, not sunscreen anytime i do longer races/training in the sun. Huge difference. Im sure there are studies to back all of this up, im just too lazy to find them for you :)

After a quick search this was pretty interesting(PE part). http://www.fitness.gov/.../Digest-June2007.pdf
"

Sunburn—When exercising in a hot environment,

sunburned individuals exhibit an impaired ability

secrete sweat. In addition, sunburn increases ratings

perceived exertion and thermal sensation, which

that sunburned individuals feel more hot and fatigued

when exposed to a given exercise-heat stress.2"

I don't know about that, but within hours of the sun overexposure inflammatory mediators are released into the body. It can even cause fever, lethargy, etc. Could this be quick enough to effect the run? I don't know. I would put a VERY WATERPROOF sunscreen on before the swim, and the bike and the run. Antioxidants-fruits and veggies, green tea in large amounts before and after the exposure might mitigate this. (I'm speculating here.) The temperature has nothing to do with it, BTW.

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Dick

Take everything I say with a grain of salt. I know nothing.