Nutrient Digestion, Absorption, and Sensing

The primary issue in the digestion and absorption of lipids is their hydrophobicity. The majority of body surfaces, such as the enterocyte brush border, and aqueous media, such as the intestinal lumen, are hydrophilic. Thus, the major challenge that must be overcome in order for physiological digestion and absorption of fats and lipids to proceed is that lipids must traverse and interact with media, interfaces, and surfaces that will only poorly dissolve them. Carbohydrate digestion primarily produces the monosaccharides glucose, galactose, and fructose. They are actively transported across the brush border into the enterocyte and, finally, into the portal circulation. Proteins may be transported as di- and tripeptides as well as free amino acids. Control of metabolism at a molecular level is fundamental to cellular homeostasis. This process, termed nutrient sensing, regulates cellular energy expenditure in a nutrient-dependent manner.