Bioaccessibility and associated concepts: Terminology in the context of in vitro food digestion studies

In vitro gastrointestinal models are widely used to study food digestion, in combination with analytical methods to determine the physicochemical and biochemical fate of food compounds. The in vitro bioaccessibility determined with these models is often used as an indicator of the in vivo bioavailability. However, the bioaccessibility concept is not used consistently within the scientific literature, leading to confusion and making it difficult to compare the results from different studies. The aim of this article is to provide standardized definitions of in vitro digestibility and bioaccessibility, detailing the main processes involved, including physical release, solubilization, and biochemical/metabolic reactions. The terminology of complementary cellular, ex vivo , and animal/human in vivo experiments is also given. Application of the in vitro terminology to different nutrients is discussed, including lipids, proteins, carbohydrates, vitamins, and other bioactive compounds. The proposed definitions unify most concepts related to the gastrointestinal fate of ingested food compounds. • Definitions of digestion processes measured in vitro are not consistent in literature. • Bioaccessibility is the result of combined processes which should all be considered. • General definitions of terms describing food digestion processes in vitro and in vivo. • Application of the vocabulary to nutrients and specific food bioactives is clarified.