# Upgrading _In Vitro_ Digestion Protocols with Absorption Models
## 1 Introduction  
In order to determine the nutritional quality of a foodstuff, besides composition data, it is important to determine nutrient bioavailability [1]. Bioavailability is the quantity of the ingested component that becomes available at the site of action, after the following phases: digestion, absorption by intestinal cells, transport into the circulation, and transport from the circulation to the site of action [1]. Bioavailability is determined through _in vivo_ assessments [1]. Due to ethical and economic reasons, the number of _in vivo_ assays should be reduced as much as possible; thus to serve this purpose, _in vitro_ digestion studies were designed. Although numerous dynamic _in vitro_ digestion systems are available [2], the static COST Infogest _in vitro_ digestion protocol [3] is still one of the most popular methods to mimic food digestion, as it is time effective, less costly and less labor-intensive [4]. This method is standardized, the protocol is reproducible, and suitable for carrying out easy inter-laboratory comparisons of the results. This protocol simulates the oral, gastric, and intestinal phase of digestion, and is used to measure the components released from the matrix and is accessible (bioaccessibility) but does not model absorption (bioavailability).  
_In vivo_, the intestinal epithelium regulates the absorption of nutrients and electrolytes, acts as a physical barrier, and blocks pathogens, toxic metabolites, and allergenic compounds [5]. The small intestine, due to the presence of integral membrane proteins called brush border membrane enzymes (peptidases, alkaline phosphatase, disaccharides(maltase, sucrose-isomaltase, lactose)), also participates in the degradation of macronutrients [6; 7]. The absorption through the intestinal epithelium is a decisive component of bioavailability. Furthermore, not only the intestinal epithelium itself but also the intestinal epithelium, the gut microbiota, and their interactions play a key role in the bioavailability of food ingredients. Thus, the reproduction of the intestinal epithelium becomes important for digestive studies.  
The intestinal epithelium is a complex tissue with multiple cell types (Section 1.1). Intestinal epithelium _in vitro_ models should contain the different cell types in correct proportion and three-dimensional orientation. This is useful as the interaction between different cells can be studied [8].  
So far, there is no standardized protocol for the _in vitro_ investigation of the absorption of food components through the intestinal barrier [8]. Numerous methods exist and the goal of the study should be exactly determined before choosing the proper method [9]. This review describes the most important information on methods and gives suggestions which may be applied to investigate the absorption of food components for the required purpose. Digestion-absorption is a very complex biochemical process system and the accurate reproduction of their specific physiological conditions is very difficult. It is important to mention that although methods have disadvantages as well as advantages, they are nevertheless of great help in understanding absorption processes.