Consequently, even researchers in the field are tentative in assessing whether exercise really presents oxidative stress problems. The standard technique for measuring free radicals is called electron spin resonance. But this method cannot yet be used on live animals or people. Instead, researchers analyze compounds produced by free radical reactions. For example, measurement of a hydrocarbon called pentane in exhaled breath is a favorite method.
Exercise increases the pentane in breath, at least for a while.
If the lipids being broken down are replaced at the same time, all that is really happening is an increased turnover. This would be similar to the action of certain skin creams promoted to soften wrinkles. They contain compounds that stimulate faster turnover of skin cells so that young ones come to the surface more quickly, hopefully giving a younger, less wrinkly appearance. Biologically this process is called damage because it changes a natural process by speeding it up, but there is no evidence that this is harmful. We don't know.