Absorption of Carbohydrates
Starch in plants is broken down to glucose, lactose in milk is broken down to glucose and galactose, sucrose and similar double sugars are broken down to fructose and glucose. After carbohydrate digestion is complete, the simple sugars, glucose, fructose and galactose, are absorbed from the small intestines into the bloodstream.
Absorption of single sugars glucose, fructose and galactose into the bloodstream. Fiber is not absorbed.
The gray area on the left represents the small intestines. The blue shape represents glucose, the brown represents galactose and the purple represents fructose. The yellow rectangular shapes represent cells that line the small intestines. Complex carbohydrates, such as starch, are not absorbed into the bloodstream. Carbohydrates must be broken down to simple sugars before they can be transferred from the small intestines, through the intestinal cells, into the bloodstream, as shown above. All cells in the body can use glucose, which means once glucose is in the bloodstream, cells can pick up glucose and immediately use it for energy. Galactose and fructose must first go to the liver, where they are converted to glucose and sent back out into the bloodstream. This process occurs fairly quickly. Fiber, a complex carbohydrate, is not digested in the intestines and is not absorbed into the bloodstream. Fiber leaves the small intestines mostly intact before traveling to the colon, also known as the large intestines. Fiber is partially broken down by the microorganisms in the large intestines to short-chain fatty acids which are mainly used as full for the cells that line the large intestines.
References
- Brody, T. Regulation of Energy Metabolism. In: Nutritional Biochemistry. 2nd ed. San Diego, CA: Academic Press. 1999.
- Brody, T. Nutrients That Resist or Escape Digestion. In: Nutritional Biochemistry. 2nd ed. San Diego, CA: Academic Press. 1999.
- Carbohydrate absorption image created by Christine Dobrowolski, CC BY-NC-SA 2.0.
Last updated April 9, 2016