You probably know that a mix of carbs, protein and fat is essential for proper body function and sufficient energy. However, your body can’t use these macronutrients without chemically breaking them down into usable molecules, which are used for generating energy, making enzymes and producing hormones. Your body has a mechanism and set of enzymes for digesting the carbs, protein and fat molecules in your food. Chemical digestion occurs in several steps beginning in the mouth and ending with absorption in the small intestinal cells.
Digestive System Anatomy
Although you may think the stomach is the primary site for digestion, the multistep process involves several organs and structures. The second you bite into your sandwich, your mouth and pharynx begin the mechanical and chemical breakdown of your food. Your stomach further digests and churns your food, producing a thick mixture of water, partially digested food, acid and digestive enzymes called chyme. However, the carbohydrate, protein and fat molecules in the chyme are not ready for absorption into your bloodstream until they finish digestion in the small intestine. Enzymes from the pancreas and bile from the liver are also essential for digestion.
Carbohydrates consist of one or more sugar molecules linked by chemical bonds. Your saliva has an enzyme called amylase that begins the breakdown of starches, which can contain 1 million sugar units assembled in a chain. These shortened chains travel down to your stomach, where acidic gastric juices and pancreatic enzymes further break them down into chains containing two to nine sugar units. These sugar units continue their journey into your small intestine, where enzymes attached to the cell membrane break down the sugars into single sugar molecules. Your intestinal cells take up these single sugars and transport them through the bloodstream to different tissues throughout your body.
Most of the fats you eat consist of three fatty acids that are linked. Your digestive system must break these apart into individual free fatty acids, which your body can use for building cell membranes, making certain hormones or absorbing fat-soluble vitamins. Your body can also break down these acids for energy. Enzymes called lipases at the back of your tongue and in your stomach break the bonds between the small fatty acids, and these free fatty acids travel to the small intestine where cells in the intestinal wall absorb them into your bloodstream. Bile salts from your gallbladder and enzymes from your pancreas are secreted into your small intestine, where they work with other enzymes and hormones to break the links between long fatty acids. These long free fatty acids are packaged into transport molecules and absorbed by cells in the intestinal wall.
The breakdown of dietary protein by your digestive system produces amino acids, which provides the building blocks for essential compounds in your body such as cell proteins, hormones, enzymes and genetic material. Your mouth doesn’t have enzymes to digest proteins, so they don’t begin digestion until they reach the stomach. Stomach cells produce acid and an enzyme called pepsin, which work together to break the protein into smaller proteins and some single amino acids. These travel to the small intestine, where they encounter another set of enzymes secreted from your pancreas and intestinal wall cells. These enzymes continue the work of the pepsin enzyme of the stomach, deconstructing the proteins into single amino acids. These amino acids are transported across the intestinal wall cell and travel through the bloodstream, supplying amino acids to the different parts of your body.
- City College of San Francisco: Human Anatomy and Physiology: Digestion
- Basic Medical Biochemistry: A Clinical Approach, 3rd Edition; Michael Lieberman, Ph.D. and Allan D. Marks, M.D.
Gina Battaglia has written professionally since 2006. She served as an assistant editor for the "International Journal of Sports Medicine" and coauthored a paper published in the "Journal of Strength and Conditioning Research." Battaglia completed a Doctor of Philosophy in bioenergetics and exercise science at East Carolina University and a Master of Science in biokinesiology from the University of Southern California.