The Final Stages Of Carbohydrate Digestion

The Final Stages Of Carbohydrate Digestion

  Last update November 11, 2021, article reviewed & updated multiple times since June 7, 2001.

  What You Need to Know

  • During the height of absorption, the sugar concentration in the blood rises.
  • To make complex carbohydrate glycogen, the liver puts the simple sugars back together by rejoining them, using water.
  • As the quantity of sugar in the body is reduced by the expenditure of energy, it is replenished by the intake of food.

In Review

As we have discussed in previous articles, digestible carbohydrates must be brought to the simple sugar stage before they can be absorbed. And, as we have learned, Carbohydrate Digestion is done in the mouth, stomach, and small intestines via the action of enzymes in a process called hydrolysis, which involves breaking down by the removal of water. When the sugars have reached the small intestines, no matter what carbohydrates are the source, they have all become simple sugars. The sugars diffuse through the selective membranes of the small intestines and then enter the blood.

The Small Intestines

Let’s take a look at the anatomy of the small intestines. The small intestines are variously called small intestines, small intestine (with no final S), upper bowel, and small bowel. It is the part of the digestive tract that is located just below the stomach. It is, of course, a tube, but the walls of the tube are folded many, many times.

To get a picture of what these folds are like, carefully slide the paper wrapper off a straw. The paper cylinder is what a straight tube would look like. Now, take the wrapper off a second straw by pushing it down the straw so that it bunches up on itself. Pull the wrapper off the straw so you can see all the folds. Notice how many more surfaces there are on the bunched-up wrapper, up and down the folds. In addition to all the folds of the small intestines, each of the folds has multiple tiny finger-like projections called villi (one is a villus; more than one are villi). Since the tiny blood vessels, the capillaries, are located just on the inside the membrane walls of the multi-folded, villi-loaded small intestines, there are a great number of places for the sugars to transfer across into the capillaries, and into the blood.

How Sugars Affect The Blood

Blood Sugar
The blood from the capillaries is carried directly into larger blood vessels that lead to the liver. During the height of absorption, the sugar concentration in the blood rises. This higher concentration of sugar increases the density, or ‘thickness’ of the blood. The increased density is called increased specific gravity.

As the specific gravity increases, it tends to pull water from the body tissues to dilute the blood. If this were allowed to continue, it would result in increased heart action beyond that which the heart can tolerate, and in tissue dehydration. To keep these things from happening, some of the sugar is withdrawn from the blood by the liver and converted to a new complex carbohydrate called glycogen.

What Does The Liver Do?

Think back to the first article in this series on “Just What Are Carbohydrates?” Recall that I said that the word carbohydrate means ‘one of many different combinations of carbon, hydrogen, and oxygen, whose various sections join and break apart by taking on, or releasing water.’ To make the complex carbohydrate glycogen, the liver (with help from some specific enzymes) puts the simple sugars back together by rejoining them, using water. The liver does this trick with the substance known as insulin, which is secreted by the pancreas. The process is called glycogenesis (GLY-co-genesis; glyco- means sugar; -genesis means creation).

The glycogen is stored in the cells of the liver, in the muscles, and to a lesser extent, in various other tissues in the body. And, if glycogen is not needed in those places because they are “full,” the glycogen converts to body fat.

How Blood Sugar Levels Are Regulated

Blood Sugar
Carbohydrates furnish the main source of energy for the brain and nervous system, the main source of energy for muscular activity, and provide the energy to maintain body temperature. Sugar must be available at every moment, since it is constantly being used. In order for sugar to be immediately available, a supply of it is always present in the blood. This is what is known as blood sugar, or blood glucose.

As the quantity of sugar in the body is reduced by the expenditure of energy, it is replenished by the intake of food. Since we do not normally eat constantly, but rather eat at intervals, the level of sugar in the blood is kept in line by several hormones that tie up and release the sugar as needed to maintain the level. In an ongoing process, the liver makes glycogen from the sugar, and makes sugar from the glycogen. The reconverting of glycogen to glucose is called glycogenolysis (GLY-co-gen-OL-ysis), which means the breaking apart of glycogen. This is done with the help of another friendly enzyme, glycogen-ase.

The liver stores and then doles out the sugar as needed, maintaining the level much like a thermostat regulates house temperature. The control of the thermostat is done by a balance of hormones: insulin (which directs the liver to store sugar), glucagon (which directs the liver to release sugar), and hormones from the thyroid, the pituitary, and the adrenal glands. Low blood sugar levels trigger the release of glucagon from the pancreas. High blood sugar levels trigger the release of insulin.

Our muscles use the glycogen stored there to perform muscle work, but the work doesn’t use up all of the intermediary stages of the breakdown of the glycogen. Some of the substance that is created by the use of sugar in the muscles still has some usable energy in it. This substance, called lactic acid, is carried by the blood back to the liver, where it is used to make more glycogen, some of which is stored again in the muscles until needed there.

All the various tissues of the body use the sugars, and at the very end, there is nothing left, except carbon dioxide and water. The carbon dioxide is expelled in the breath, and the water is reused, or excreted in the urine. Such a great system!

In Conclusion

So, there we have the essentials of what is supposed to happen in carbohydrate metabolism. But, it doesn’t always work that way. Enter diabetes, and other diseases and mistakes of carbohydrate utilization. We will be talking about those things in the weeks and months to come, but first, we are going to continue with what is ‘normal’ in human nutrition. Next time, we will begin the series “Just What Are Proteins?”

Please join me.

The Science of Low-Carb & Keto Diets
    The Science of Low-Carb & Keto Diets

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  • This article is number 5 of 24 articles describing The Science of Low-Carb & Keto Diets. Visit this link or click the above image for all of the articles in the series.
Article History

  • Last Update November 9, 2021
  • Updated January 15, 2018
  • Updated May 30, 2010
  • Updated August 6, 2003
  • Original Article June 17, 2001
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About Dr. Beth Gruber
Dr. Gruber is a graduate of the Southern California University of Health Sciences and has been in private chiropractic practice in Long Beach, California since 1964. She also received both a Bachelor’s Degree and a Master’s Degree from California State University at Long Beach. She has written on health-related subjects for over 30 years, for several different publications. She lives in Southern California with her husband of 33 years. Both she and her husband follow and live the low-carb lifestyle full time.

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