## 1 Introduction  
Food lipid oxidation is an issue of great importance in field of the food technology because it causes food degradation with economic and health repercussions. This can occur during food processing and storage [1, 2]. A common strategy to avoid oxidation consists of the incorporation of compounds with antioxidant abilities in foods. Many studies have been published on the incorporation of antioxidants in lipid foods, and have evaluated their antioxidant efficiency under different processing conditions [3, 4, 5]. In recent years, it has been shown that food lipid oxidation can also occur during digestion, and some studies have addressed the effect on digestion of the enrichment of lipids with compounds able to act as antioxidants. It should be noted that food oxidation during digestion can give rise to the formation of toxic compounds that can be directly absorbed. For this reason, the study of this issue could be considered even more important than the oxidation of food during processing or storage. Among compounds having antioxidant abilities are phenolic compounds. These are usually secondary metabolites present in fruits and vegetables in very low concentrations; the healthy propertiesof such foods have been attributed to phenolic compounds. These compounds are considered to be free radical scavengers, and their antioxidant capacity has been related to the number and arrangement of their hydroxyl groups [6]. Due to their potential antioxidant abilities and to the beneficial health effects attributed to them [6; 7], some studies have focused on the enrichment of lipid foods with different natural or synthetic phenolic compounds in order to prevent oxidation when they are submitted to thermal processes or to gastrointestinal digestion. In this context, the object of the present research is to study the efficiency of different polyphenols such as epicatechin, resveratrol and caffeic and gallic acids in the inhibition of the oxidation of linoleic acid in authentic fluid from rat small intestine [8] by monitoring the oxidation levels according to the concentration of hexanal. The effect of some hydrophilic and lipophilic phenolic compounds on the oxidation efficacy of lipids during in vitro digestion of high- and low-fat beef meat was also studied by monitoring the concentration of malonaldehyde, 4-hydroxy-nonenal and hexanal [9]. Likewise, the antioxidant effect of 2,6-di-_tert_-butyl-hydroxytoluene (BHT) in the oxidation of cod liver oil during in vitro digestion was demonstrated [10]. Furthermore, several studies have analyzed the effect that enrichment of certain oils with alpha-tocopherol has on their oxidation when they are submitted to digestion conditions, with controversial results [9; 11; 12; 13]. Finally, a comparative study of the effect of the enrichment of corn oil with alpha- and gamma-tocopherol showed that the former acts as prooxidant, which is in agreement with some of the aforementioned studies, whereas the second acts as antioxidant [14]. However, phenolic compounds, in addition to exhibiting antioxidant or prooxidant abilities, may also take part in other reactions during digestion because they come in contact with both all food components and with enzyme-containing digestive juices. It should be noted that phenolic compounds can react with proteins, and also with enzymes [15; 16], reducing their activity and provoking negative effects on digestion by diminishing the extent of the hydrolytic reactions. In fact, it has been proven that tea polyphenols are able to inhibit the pancreatic lipase activity, thereby reducing the gastrointestinal lipolysis [17], and thus, the absorption of lipids. Likewise, it has been described that alkyl gallates are able to inhibit the activity of amylase, thereby reducing the absorption of carbohydrates [18; 19].