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4: Macronutrient Uptake, Absorption & Transport

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  • The term absorption can have a number of different meanings. Not everything that is taken up into the enterocyte from the lumen will be absorbed, so the term uptake refers to compounds being transported into the enterocyte. Absorption means that a compound is transported from the enterocyte into circulation. Under most circumstances, compounds that are taken up will then be absorbed. After this chapter, hopefully this distinction between these terms will be clear. After later micronutrient chapters, hopefully you will understand the reason for emphasizing this distinction.

    • 4.1: Crypts of Lieberkuhn & Enterocyte Maturation
      There are some additional anatomical and physiological features of the small intestine that are important to understand before before defining uptake and absorption. The crypts of Lieberkuhn (often referred to simply as crypts) are similar to the gastric pits in the stomach. The crypts contain stem cells that can produce a number of different cell types, including enterocytes2. From these stem cells in the crypt, immature enterocyte cells are formed that mature as they rise, or migrate up, the v
    • 4.2: Absorptive Lineup & Cell Membranes
      Having completed digestion in the small intestine, a number of compounds are ready for uptake into the enterocyte.
    • 4.3: Types of Cell Uptake/Transport
      There are a number of different forms of uptake/transport utilized by your body. These can be classified as passive or active. The difference between the two is whether energy is required and whether they move with or against a concentration gradient. Passive transport does not require energy and moves with a concentration gradient. Active transport requires energy to move against the concentration gradient.
    • 4.4: Carbohydrate Uptake, Absorption, Transport & Liver Uptake
      Monosaccharides are taken up into the enterocyte. Glucose and galactose are taken up by the sodium-glucose cotransporter 1 (SGLT1, active carrier transport). The cotransporter part of the name of this transporter means that it also transports sodium along with glucose or galactose. Fructose is taken up by facilitated diffusion through glucose transporter (GLUT). There are 12 glucose transporters that are named GLUT 1-12, and all use facilitated diffusion to transport monosaccharides.
    • 4.5 Glycemic Response, Insulin, & Glucagon
      If only 30-40% of glucose is being taken up by the liver, then what happens to the rest? How the body handles the rise in blood glucose after a meal is referred to as the glycemic response. The pancreas senses the blood glucose levels and responds appropriately. After a meal, the pancreatic beta-cells sense that glucose levels are high and secrete the hormone insulin, as shown below.
    • 4.6: Protein Uptake, Absorption, Transport & Liver Uptake
      There are a number of similarities between carbohydrate and protein uptake, absorption, transport, and uptake by the liver. This section addresses these similarities.
    • 4.7: Lipid Uptake, Absorption & Transport
      Once mixed micelles reach the brush border of the enterocyte, two different lipid uptake mechanisms are believed to occur, but lipid uptake is not completely understood. One mechanism is that individual components of micelles may diffuse across the enterocyte. Otherwise, it is believed that some components may be taken up through unresolved transporters.

    Thumbnail: Insulin is a peptide hormone produced by beta cells of the pancreatic islets, and it is considered to be the main anabolic hormone of the body. Image used with permission (CC BY 2.5; Isaac Yonemoto).