Both organic and inorganic nanoparticles are often added to food and beverage products to modify their quality attributes, such as their look, feel, flavor, or shelf life. However, there is still a relatively poor understanding of how these nanoparticles behave inside the human gut after ingestion, particularly their impact on macronutrient digestion and nutraceutical absorption. In this thesis, the alterations in the physicochemical and structural properties of several model organic and inorganic nanoparticles were determined as they passed through a simulated in vitro gastrointestinal tract (GIT). First, the behavior of food-grade titaniumn dioxide (TiO2)nanoparticles under simulated oral conditions were characterized. Second, two nature-derived organic nanoparticles, anionic nanocellulose and cationic nanochitin, were fabricated. These nanoparticles can stabilize lipid droplets and form Pickering emulsions. The impact emulsifier type (molecular versus particle) on the gastrointesinal fate of vitamin D3-fortified emulsions was examined by measuring their physicochemical properties, microstructure, digestibility, and bioaccessibility using an in vitro GIT model. Third, the impact of nanochitin on the gastrointestinal fate of a model emulsion was examined. The addition of nanochitin to the emulsions reduced lipid digestion and β-carotene bioaccessibility. Fourth, nanocellulose, lipid nanoparticles, and TiO2 particles were added to plant-based milks as examples of functional organic and inorganic nanoparticles. The various kinds of nanoparticles were well dispersed in simulated gastrointestinal fluids, which was attributed to protein adsorption to their surfaces and the mechanical action of the GIT fluids. Fifth, it was found that calcium affected the digestion and bioaccessibility of vitamin-loaded lipid nanoparticles in plant-based milks using an in vitro gastrointestinal model. High levels of calcium (soluble or insoluble forms) reduced the bioaccessibility of vitamin D, which was attributed to the formation of insoluble calcium soaps in the small intestine. Overall, these results are useful for understanding the impact of different kinds of nanoparticles on the behavior of foods within the gastrointestinal tract. This information can help assess the potential functionality and toxicity of ingested nanoparticles. These findings are also useful for the development of functional foods, such as fortified plant-based milks with improved physicochemical and bioaccessibility properties.