Dietary oxidative stress and plant extracts main aims of the philosophy of science is the scientific term “predict” and whether it is realistic that animal models can be used to predict human outcomes (Shanks et al. 2009). The prediction is usually related to the recontextualization of knowledge from animal physiology to human physiology. As has been previously defined, recontextu- alization is a process that extracts meaning from the original context, here animal redox biology (decontextualiza- tion), and introduces it into another context, here human redox biology (Bernstein 1990; Nikolaidis and Jamurtas 2009). This process requires a change of meaning (slight or not, depending on the field), leading to some degree of simplification. Differential effects of plant antioxidants on redox status: in vitro and in vivo comparison In the field of sports medicine, there are numerous studies investigating the effects of antioxidants on blood and the redox status of human subjects or animals when they are administered before aerobic exercise. According to its definition, the term “antioxidant” is usually related to its beneficial effects and preventive role against exercise- induced oxidative stress. The literature is rich in studies examining the in vivo effects of antioxidants when they are administered either individually (vitamins, lipoic acid, N- acetylcysteine) or as part of extracts derived from various plants such as grapes, legumes, and green tea. Additionally, there is an increasing number of studies investigating the effects of antioxidants on in vitro models. An in vivo system could be characterized as biologically interesting but experimentally inaccessible, whereas the corresponding in vitro system could be defined as an experimentally accessible but biologically less interesting system (Strand 1999). Generally, in the vast majority of the in vitro studies, extracts derived from different plants possess antioxidant properties monitored by their capacity to scavenge free radicals. Specifically, it has been shown that grape extracts (Fauconneau et al. 1997; Bagchi et al. 2000; Murthy 2002; 17 Stagos et al. 2005, 2007) and extracts or polyphenolic compounds derived from legumes (Beninger and Hosfield 2003; Manna et al. 2004; Heimler et al. 2005; Spanou et al. 2007, 2008; Halliwell 2008) are strong in vitro antioxidant agents and free radical scavengers, but it has also been shown that several grape extracts inhibit catalase and xanthine oxidase activity in vitro, implying a prooxidant action (Spanou et al. 2011). Nevertheless, the case for in vivo experiments in rodents and humans during exercise is rather different. Numerous studies examined the effects of plant extract supplementation on oxidative stress in blood and other tissues, yet the findings are controversial. Interestingly, some studies have demonstrated the antioxi- dant effects of plant extracts (Voces et al. 1999, 2004; Kim et al. 2005), whereas some others have demonstrated the prooxidant effects of different plant extracts (Morihara et al. 2006; Skarpanska-Stejnborn et al. 2008, 2009; Halliwell 2008, 2011). Several studies have reported a consistency between the in vitro and in vivo effects of plant antioxidants after the implication of several oxidative stress stimuli, namely, diabetes (Puiggròs et al. 2009; Cho et al. 2006), exposure in xenobiotics (Hwang et al. 2008; Lee et al. 2005), or exposure in reactive oxygen species (Shi et al. 2006). Nevertheless, other studies have demonstrated that the in vitro activity does not always apply in in vivo models (Cherubini et al. 1999; Lotito 2004). It is noticeable that studies examining such comparison in the context of exercise have not been reported. The lack of consistency in experiments comparing the in vitro and in vivo effects of plant extracts on redox status is not surprising. First of all, the assays used to evaluate the in vitro antioxidant activity implicate the scavenging of free radicals that usually are relatively stable, exist only under artificial experimental conditions, and are largely biologically irrelevant (Halliwell and Gutteridge 2007; Scott 1993; Bartosz 2003; Gutteridge and Halliwell 2010). Therefore, it is difficult to translate the in vitro evidence to in vivo outcomes (Gutteridge and Halliwell 2010; Halliwell et al. 2005). Furthermore, the fact that polyphenolic compounds are degraded in metabolites with smaller molecular weight is partly responsible for their Fig. 1 Simplified figure pre- senting the recontextualization of knowledge between diet components and food, animals, and humans, in vitro and in vivo experiments Diet components Animals In vitro Food Humans In vivo Recontextualized knowledge