18 different in vitro and in vivo effects on redox status (Shirai et al. 2006). Besides, polyphenols are metabolized as typical xenobiotics, and such metabolism alters and possibly decreases their antioxidant capacity (Halliwell 2008). In conclusion, the effects of polyphenols on the redox status in vivo cannot be simply extrapolated from their activities in vitro. The in vitro antioxidant activity of a plant extract seems to differ from its in vivo effect. There is also a common aspect that the contribution of diet antioxidants to protection against oxidative stress or even against diseases (cancer, diabetes, cardiovascular diseases) is a multifactorial case. Specifically, diet antioxidants may be beneficial only in humans whose diet and lifestyle are of poor quality and who are deficient in vital micronutrients and vitamins against overwhelming oxidative damage or the detrimental effects of several diseases related to excessive reactive species production (Hercberg et al. 2007; Halliwell 2009). The process of recontextualization is also applied in the effort of scientists to predict the in vivo effects of plant antioxidants using in vitro models. The recontextualization is based on the need of redox biology to detect the real meaning of experimental evidence applied in vitro. It is a very difficult task which requires much caution in order to obtain real and not overestimated, simplified, or false results. Concluding remarks It is now established that the function of reactive species is highly related to their concentration. The detrimental effects of oxidative stress are linked to the disruption of normal signaling pathways, damage of macromolecules, and disruption of homeostasis. It should be noted that oxygen is not evenly distributed in tissues and cells; therefore, there are sites that are under physiological oxidative stress. It is a difficult task to set a threshold between pathological and physiolog- ical oxidative stress. This is in biological accordance with the fact that antioxidants under specific conditions may have a prooxidant action. A commonly used experimental model altering cell or tissue redox status is exercise. This is an example indicating the necessity of reactive species in physiological processes that are beneficial for human health. A characteristic example of this fact is that the benefits of regular exercise on health are accompanied by repeated episodes of oxidative stress. The field of redox biology has been rapidly developed over the last 30 years. The relative literature is rich in studies examining the effects of antioxidants using animal and human models as well as both in vitro and in vivo models. It is obvious that the studies in A.S. Veskoukis et al. animal and in vitro models aim to extrapolate their findings to human and in vivo models, respectively. This attempt is usually related to the recontextualization of knowledge, which is presented schematically in Fig. 1. The recontextualization is based on the need of redox biology to detect the real meaning of experimental evidence. This process usually requires a change of meaning (slight or not, depending on the field), leading to some degree of simplification. Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which per- mits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. 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