THE DIGESTIVE SYSTEM: DIGESTION AND ABSORPTION

CARBOHYDRATES, PROTEINS, AND FATS:  

How the Body Digests, Absorbs, and Metabolizes.

The human body is an extremely complex machine.  With any machine, like a car for example, we need to properly care for it and maintain it in order for it to function properly.  The human body will not function without energy and nourishment.  We provide these things to our body through eating food.  We eat every day, multiple times a day, but rarely do we think about or even know what exactly happens to the food after we swallow it.

The purpose of the digestive system is simple. We need a process that can take the food, or calories, we eat and break it down to the point where we can use it to carry out the body’s life processes.  Digestion allows food to be used by the body for energy, provides nutrients through absorption, and helps provide nourishment replenishment as we lose vital nutrients every day.

Let's take a look at how the body digests, absorbs and metabolizes the three macronutrients of carbohydrates, proteins, and fats.  

Carbohydrates:

First, let us look at the chemical process involved in breaking down carbohydrates.  The enzyme called carbohydrase is the main enzyme involved in this process.  In the stomach, enzymes and hydrochloric acid are used to slow the digestion while the carbs continue to be reduced.  As the contents move to the small intestine, carbohydrase is released to break starch fragments into simple sugars called glucose, which is the absorbed by the body.  Fiber is considered a carbohydrate but the body does not absorb the majority of fiber.  The pancreas is a major organ that is involved in the absorption of carbs.  After carbohydrates are digested and broken down to its simplest form of glucose, the pancreas releases insulin which acts as key to carry glucose into cells of the body.  

Fat:

In the mouth, the fatty food is mixed with saliva and enzymes to start the breakdown.  In the stomach, mechanical digestion continues to reduce fat.  As the fat contents move to the small intestine, the liver secretes bile, which is then stored and released by the gall bladder into the intestine. The bile churns the fat and prepares it to be further broken down by the enzyme lipase.

Protein:

Protein takes the longest to break down due to the complex composition of this nutrient.  As protein enters the mouth, saliva begins the chemical breakdown.  As protein contents enter the stomach, the digestion process begins as gastric juice is mixed with hydrochloric acid and enzymes.  As these chemicals are mixed, they activate the main protein enzymes called protease, which breaks down the chemical bond of proteins.

References:

Martin, B. (2006).  How Digestion Works.  Total Health, 28(3), 44-51.  Retrieved from

         EBSCOhost.

Sizer, F. & Whitney, E. (2013).  Nutrition: Concepts and Controversies (13^th ed.). Mason, OH:

         Cengage Learning.