The level of F₂-isoprostanes (F₂-IsoP) in blood or urine is widely regarded as the reference marker for the assessment of oxidative stress. As a result, nowadays, F₂-IsoP is the most frequently measured oxidative stress marker. Nevertheless, determining F₂-IsoP is a challenging task and the measurement is neither free of mishaps nor straightforward. This review presents for the first time the effect of acute and chronic exercise on F₂-IsoP levels in plasma, urine and skeletal muscle, placing emphasis on the origin, the methodological caveats and the interpretation of F₂-IsoP alterations. From data analysis, the following effects of exercise have emerged: (i) acute exercise clearly increases F₂-IsoP levels in plasma and this effect is generally short-lived, (ii) acute exercise and increased contractile activity markedly increase F₂-IsoP levels in skeletal muscle, (iii) chronic exercise exhibits trend for decreased F₂-IsoP levels in urine but further research is needed. Theoretically, it seems that significant amounts of F₂-IsoP can be produced not only from phospholipids but from neutral lipids as well. The origin of F₂-IsoP detected in plasma and urine (as done by almost all studies in humans) remains controversial, as a multitude of tissues (including skeletal muscle and plasma) can independently produce F₂-IsoP.
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