The small intestinal epithelium plays a crucial role in the digestion/modification of food components, absorption of nutrients and recognition of food-derived signals. It also acts as a barrier against unfavourable materials in food. These intestinal functions may be influenced by food substances. In this paper, the effects of various food substances on the intestinal functions, particularly the absorption functions, are discussed. A new assay method, using a monolayer culture system of human intestinal epithelial cells, is applied to examine food-derived substances which could modulate the tight junction (TJ). Capsianoside, a diterpene glycoside from sweet pepper, was isolated and identified as one of the TJ-modulating substances. Those substances can be expected to facilitate the paracellular absorption of small molecular functional substances such as bioactive oligopeptides. The use of a human intestinal cell culture system also enabled us to make a simple screen test for food substances which affect the intestinal transporter functions. Substances which could suppress the activity of intestinal glucose transporters, for example, can be expected to reduce the dietary glucose uptake in the intestinal epithelium and control the blood glucose level. Polyphenolic compounds from plants, including green tea, were found to have such activity. Modulation of the brush border enzymes by food substances is also discussed.