Macronutrients: Digestion, Absorption, and Metabolism of These Essential Energy-Producers

                                                               © Galina Barskaya | Dreamstime Stock Photos

To understand the importance of macronutrients (carbohydrates, fats, and protein), or energy-yielding nutrients, it's important to understand how the body transforms them to energy. Carbohydrates begin their digestion in the mouth where saliva turns the starch into maltose. Once swallowed and upon entering the stomach, the salivary enzyme is broken down further. The digestion process comes to a halt once the carbohydrate reaches the small intestine. This is where the "pancreas delivers a starch-splitting enzyme that breaks it down into disaccharides and small polysaccharides" (Sizer & Whitney, 2012). Monosaccharides are taken for absorption by other enzymes. The carbohydrates are then split once more before they are absorbed. "The absorbed carbohydrates travel in the bloodstream to the liver, which can convert fructose and galactose (two of the sugars that have now been made by the breakdown process) to glucose" (Sizer & Whitney, 2012). Glucose gives our bodies energy.
Another macronutrient, protein, starts its digestion in the stomach where it is denatured by acid. Enzymes in the stomach and then small intestine digest the protein into its building blocks, amino acids, dipeptides, and tripeptides (Sizer & Whitney, 2012). The "cells of the small intestine complete digestion, absorb amino acids and some larger peptides, and release them into the blood stream for use by the body's cells" (Sizer & Whitney, 2012). The cells use the absorbed amino acids and peptides for energy to carry out their various tasks.
The last of the macronutrients, fats, or lipids, begin their digestion in the stomach where they are separated from the other components of the food. Once the fat enters into the small intestine, bile, a powerful stomach acid emulsifies it. Enzymes in the small intestine digest this emulsified fat and the cells within the intestines absorb them (Sizer & Whitney, 2012). The fat is then stored in the body as a reserve. After the body has used carbohydrates and proteins for its main energy sources, it uses the reserved fats. According to the article "The Catabolism of Fats and Proteins for Energy", marathon runners often say that running  the last six miles is more difficult than the first 20, one theory is that the body has used up all of its sugars and now it has switched to fats (2012).

Sizer, F. & Whitney, E. (2012).  Nutrition: Concepts and Controversies, MyPlate

Update (12th ed.).  Mason, OH: Cengage Learning.

"The Catabolism of Fats and Proteins for Energy". (2014) antranik.org. Retrieved from 

http://antranik.org/the-catabolism-of-fats-and-proteins-for-energy/