4. How Carbohydrates Are Digested And Used By The Body
4.1 Introduction to Digestion
Before discussing carbohydrate digestion in particular, let's give a little attention to digestion in general. Complete and thorough digestion of foodstuffs is extremely important for good health. A tremendous amount of toxin elimination and accumulation puts a great stress and burden upon the organism and results in a large variety and number of diseases. This happens both directly, from the presence of accumulated toxic substances that the body was unable to eliminate, and indirectly, from a decrease in the body's digestive capabilities due to overworking the digestive system and depleting the body's supply of vital energy.
It is, therefore, important for us to do everything we can to insure thorough and complete digestion of all foods eaten. This can be done by eating primarily (or only) easily digested and uncomplicated foods such as fruits; by eating compatible combinations of foods; by eating moderate amounts of foods; by eating at well-spaced meals; by abstaining from drinks during or too soon before or after meals; and by refraining from eating while under stress or emotionally upset.
One of two things happens to foods that do not get thoroughly or completely digested: 1) Sugars may ferment or 2) proteins may putrefy (rot). These processes result from bacterial activity which breaks down (decomposes) undigested or undigestible foods in preparation for their elimination from the body. The "trick" to, getting nourishment (nutriment) from the foods you eat is to see to it that they, get digested quickly, before the bacteria (present within every healthy digestive tract) have a chance to decompose them. The results of bacterial decomposition are toxic and do not provide nourishment. Foods that don't digest relatively soon after ingestion will ferment or putrefy and contribute to body toxicity and disease.
Keeping the above facts about digestion in mind, let's take a look now at carbohydrate digestion.
4.2 Salivary Carbohydrate Digestion
Disaccharides and polysaccharides must be digested before the body can use them, while monosaccharides do not require digestion. For this reason, as well as for other reasons (to be discussed in depth later in this lesson), our best source of carbohydrates is from fruits. Fruits require much less of the body's energies and render primarily monosaccharides that, as stated, need no digestion.
Digestion is both a mechanical process (chewing) and a chemical process (enzymic actions). The class of enzymes that hydrolyze carbohydrates are broadly known as carbohydrases. We will be concerned in this lesson with carbohydrases known as amylases.
While the digestion of all types of foods (proteins, carbohydrates, fats, etc.) begins in the mouth with the mechanical process of mastication, certain carbohydrates—namely, starches and dextrins—are the only food types whose chemical digestion begins in the mouth. Here an enzyme known as salivary amylase or ptyalin, secreted by the parotid glands, is mixed with the food during the chewing process and begins the conversion of glycogen, starch and dextrins into the disaccharide maltose.
What happens when the starches, dextrin, and glycogens that were not converted to maltose in the mouth and what happens to the maltose when these carbohydrates reach the stomach depends upon several factors—what other types of foods are eaten with the starch, how much food is being eaten and how fast, the emotional condition of the eater and the condition of the eater's digestive system. If a relatively uncomplicated starch such as potatoes or yams is eaten alone or with nonstarchy vegetables, and no proteins (as meats, cheese or milk, or even nuts or seeds or acids (as tomatoes, lemon or lemon juice or vinegar—as in salads or salad dressings) are consumed with the starchy food, salivary amylase (ptyalin) can and will continue the digestion of starches and dextrins in the stomach for a long period.
For thorough digestion and consequent good health, this continuation of starch digestion by ptyalin in the stomach is a necessity. Therefore, for good health, it is important to consume starchy foods at separate meals from protein foods and acids. (This and other facts relative to the topic of food combining for good digestion will be discussed in depth in later lessons.)
Briefly stated, ingestion of protein foods causes a secretion of hydrochloric acid in the stomach, and hydrochloric acid destroys ptyalin; that is, it destroys the amylase activity and substitutes acid hydrolysis. Physiology texts state that "if this acid hydrolysis was continued long enough it could reduce all the digestible carbohydrates to the monosaccharide stage. However, the stomach empties itself before this can take place."
The acids of tomatoes, berries, oranges, grapefruits, lemons, limes, pineapples, sour grapes and other sour fruits and the acid of vinegar will, like hydrochloric acid, destroy our only starch-splitting enzyme, ptyalin. Therefore, these foods also inhibit starch digestion. For good digestion and consequent good health, acids should not be eaten at the same meal with starches.
Another factor that can impair salivary starch digestion is the drinking of water or other liquids with or too soon before or after meals. Water or other liquids do not aid in the digestion of foods. On the contrary, they interfere with digestion by diluting the digestive juices and cause them and their enzymes to be passed through the digestive tract too quickly for digestion to occur.
To summarize this aspect of starch digestion, taking proteins, acids, water or other liquids with starches interferes seriously with their digestion by the salivary amylase, ptyalin. This first stage of starch digestion is of great importance because there is a great likelihood that the food will be acted upon by bacteria and ferment before it reaches the intestine where further starch digestion can take place. Digestion, rather than fermentation and its resulting toxic byproducts, is much more likely to occur soon after the food is put into the mouth than further along in the digestive tract.
From the above, you can see why thorough mastication of food is so important when starches are eaten. No one who seeks health should eat starches in a hurry, nor should they have them with a beverage or with proteins or acids, for good digestion of foods is imperative for good health.
A special note should be made here about glycogen—animal starch. Glycogen should not be consumed by health seekers because much disease results from the ingestion of animal flesh and animal products. This will be discussed in depth in later lessons. For the purposes of this lesson, suffice it to say that glycogen ingested cannot be digested in the stomach because, of the hydrochloric acid that will be secreted to digest the protein, which is the primary nutritive component of foods that contain glycogen. Therefore, whatever glycogen that is not converted to a disaccharide by the salivary amylase, ptyalin, must be converted in the intestine. The likelihood of the glycogen reaching the intestine without fermenting before it can get there is small. This is just one of the many hazards of consuming animal flesh and animal foods.
4.3 Starch Digestion in the Intestine
Now that we have discussed starch digestion by the enzyme ptyalin, let's get into starch and sugar (disaccharide) digestion in the intestine.
Whatever carbohydrates make it to the intestine quickly enough to escape fermentation by bacterial action will be acted upon in the first part of the small intestine, the duodenum, by pancreatic amylase. This enzyme, secreted by the pancreas, converts any remaining dextrin and starch to maltose. The reason this amylase can act in the intestine is because of the more alkaline medium which prevails there. As stated earlier, amylase must have a somewhat alkaline medium to do its job and is destroyed by acids.
At this stage in the digestive process, that is, after the polysaccharides (starch, dextrin and glycogen) have been converted to the disaccharide maltose, maltose and the other disaccharides (sucrose and lactose) must be converted to monosaccharides since, as stated earlier, the body can absorb and use sugars only as monosaccharides. This is accomplished by the amylases maltase (to convert maltose), sucrase (to convert sucrose) and lactase (to convert lactose). These amylases are secreted by the wall of the small intestine and are capable of splitting the particular sugars for which they were designed to the monosaccharide stage.
4.4 Carbohydrate Absorption
Even though some substances (water, ethyl alcohol, small amounts of monosaccharides) may be absorbed into the bloodstream through the mucosa (mucous membrane) of the stomach, most absorption of the soluble products of digestion occurs in the small intestine. There the absorptive surface is increased about 600 times by villi, which are fingerlike projections in the lining of the small intestine. Each individual villus contains a network of capillaries surrounding a lymph vessel, and each cell on the surface of the villus is made up of smaller units called brush border cells or micro villi.
Substances or nutrients pass through the intestinal membrane through the process of osmosis in one of two ways: 1) diffusion or 2) active transport. Substances and nutrients in the intestinal tract that are in higher concentration than across the membrane in the blood and lymph pass through by diffusion. This is a simple osmotic process in which no energy has to be expended. Fructose is absorbed by diffusion.
Active transport is the osmotic process used when substances or nutrients are absorbed from an area of lower concentration across a membrane to an area of higher concentration. This process requires energy for the absorption, as well as a "carrier" to transport the substance. The carrier substance is thought to be a protein or lipoprotein (a combination of a protein and a fat). Glucose and galactose are absorbed into the bloodstream by active transport. Monosaccharides are absorbed by the capillaries, which empty into the portal vein, which in turn carries them directly to the liver.
4.5 Carbohydrate Metabolism
Metabolism is the term used to describe the many chemical changes that occur after the end products of digestion have been absorbed into the body. There are two phases of metabolism: 1) anabolism, which is the chemical reaction by which absorbed nutrients are utilized for replacement of used or worn-out body substances (maintenance) and to create new cellular material (growth), and 2) catabolism, which includes the chemical reactions whereby cellular materials are broken down into smaller units. An example of anabolism is the use of monosaccharides to build up stores of muscle and liver glycogen, and an example of catabolism is the breaking down of these glycogen stores to supply energy to the muscles during physical exersion. Anabolism and catabolism occur simultaneously in the body cells.
4.6 Sources of Glucose
The body's immediate needs determine whether carbohydrates that have been digested and absorbed are used for immediate energy, converted and stored as glycogen or changed to fat and stored in adipose tissue.
Glucose is the principal sugar used by body cells and tissues. It is, therefore, important to know the sources of this nutrient. It may come from carbohydrates or from noncarbohydrate sources. Following are the four primary sources of glucose:
4.7 Regulation of Blood Glucose Concentration
The liver, the pancreas and the adrenal glands play roles in keeping the blood sugar level at a normal concentration of around 90 mg. per 100 ml.
4.8 How Energy is Derived From Glucose
Energy is derived from glucose in one of two basic ways: 1) by oxidation and 2) by glycolysis. By far the major amount of energy from glucose is released in a series of reactions in the cells in the presence of oxygen; but some energy from glucose is released by a process called glycolysis. This is an involved process which does not require the presence of oxygen. (A detailed explanation can be found in a physiology text such as Physiology of the Human Body by Arthur C. Guyton, M.D.)
4.9 Carbohydrates in Relation to Other Nutrients
Not only are fats converted to carbohydrates for energy when carbohydrate intake is inadequate, but when carbohydrates are consumed beyond need, the excess is converted to fat and stored in adipose tissue. Also, the B-complex vitamins and the mineral calcium are known to play an integral part in carbohydrate metabolism.
Home > Lesson 7 - Carbohydrates - Fuel For The Human Body
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