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:
- From the digestion of dietary carbohydrate. Glucose is formed from the digestion of starch, dextrin, maltose, sucrose and lactose from the foods we eat.
- From the conversion of fructose and galactose. The three monosaccharides—fructose, galactose and glucose—share the same chemical formula. However, they differ in the arrangement of the hydrogen and oxygen units along the carbon chain. During the metabolic process, the liver cells convert absorbed galactose molecules and some fructose molecules. However, fructose is mainly converted to glucose during its absorption through the intestinal walls, where a metabolic interconversion (mutual conversion) occurs.
- From the breakdown of glycogen. When the body’s need for glucose is greater than the supply available in the blood, glycogen reserves in the liver and muscles are broken down and converted to glucose.
- From noncarbohydrate sources. If the body cellsrequire more energy than can be supplied by glucose and glycogen reserves, noncarbohydrate sources can be used to supply glucose. The noncarbohydrate sources used include certain amino acids from protein, glycerol from fat and, indirectly, fatty acids from fat.
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.
- The liver serves as a buffer. As stated earlier in this lesson, absorbed monosaccharides are carried in the portal vein to the liver. This blood in the portal vein may have a very high concentration of sugars, as much as 180 mg per 100 ml of glucose. In the liver, about two-thirds of the excess glucose is removed from circulation. This glucose is converted to glycogen, the storage form of carbohydrate for animals (sometimes called animal starch). At a later time, when the blood sugar level is low, the glycogen is split back into glucose and is transferred out of the liver into the blood.
In essence, the liver serves as a “buffer” organ for blood glucose regulation because it keeps the blood glucose level from rising too high or falling too low.
- Hormones that regulate the blood sugar level. After a meal is eaten, the increased glucose level in the blood (about one-third of the glucose is not removed from circulation by the liver) stimulates the pancreas to produce the hormone, insulin, which promotes the rapid transport of glucose into the cells, thus decreasing the blood glucose level back toward normal. Glucose cannot enter the cells through simple diffusion because the pores of the cell membrane are too small. Therefore, it is transported by a chemical process called facilitated diffusion (also called active transport), in which the glucose combines with a carrier in the cell membrane and is transported to the inside of the cell, where it breaks away from the carrier.
Insulin greatly enhances this facilitated transport of glucose through the cell membrane. In fact, only a very small amount of glucose can combine with the carrier in the absence of insulin, whereas, in the presence of normal amounts of this hormone, the transfer is accelerated as much as 3-5-fold. (Larger than normal amounts of insulin increase the rapidity of glucose transfer as much as 15-20-fold.) As you can see, insulin controls the rate of glucose metabolism in the body by controlling the entry of glucose into the cells.
Three hormones are involved in increasing the concentration of glucose in the blood when necessary: norepinephrine, epinephrine and glucagon. Norepinephrine and epinephrine are secreted by the adrenal glands and glucagon is secreted by the pancreas. These hormones cause liver glycogen to split into glucose, which is then emptied into the blood. This returns the blood glucose concentration back toward normal.
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.
- The transformation of carbohydrate into fat. Fats and carbohydrates eaten in excess of caloric expenditure are deposited in the adipose tissues as fat. It is, therefore, incorrect to label carbohydrates as being “fattening.” Fats eaten in excess of caloric need are also stored as fat. In the diets of many people, however, carbohydrates comprise the foodstuffs most commonly eaten in excess. There are many reasons for this. One reason is because refined sugar and flour are used so heavily and widely in the processing of the foods most widely advertised and distributed to the retail food outlets. Carbohydrates are, as a general rule, less expensive than fat-containing foods (such as cheeses, nuts, many meats, etc.) therefore, they are more likely to be overeaten. In addition, because humans naturally “have a sweet tooth” (because we are biologically frugivores, adapted in nature to eat fruits), we are more attracted to carbohydrates than to fats.
The chemical pathway glucose follows on its way to fat is well understood. You may study this in a good physiology text.
- The vitamin B complex in carbohydrate nutrition. The importance of the B vitamins in carbohydrate metabolism was discovered because of the health problems that resulted from the industrial processing of foods which removed (and still removes today) the B vitamins from their whole food sources where they were packaged by nature side-by-side with carbohydrates. The large-scale introduction of white (refined) rice in the Orient resulted in beriberi, a vitamin B complex deficiency—specifically, a thiamine deficiency. This phenomenon led to the recognition of the existence of this group of vitamins.
Prior to the widespread processing of foods, humans did not suffer as a result of their lack of knowledge about the existence of the B vitamins because in nature there is a union between the vitamin B complex and carbohydrates in foods. This union was broken by the industrial processing of foods.
As will be discussed in greater depth in later lessons, taking vitamin B complex supplements or using so-called “enriched” processed food products will not and cannot substitute for whole foods in their natural state. It is, therefore, very important for health-seekers to consume unprocessed foods—also uncooked, as cooking is an in-home method of food processing that is very destructive of the quantity and quality of vitamins and other nutrients in foods.
B-complex vitamins are also depleted (and/or not synthesized in the body) when various drugs and medications are taken, most notably birth control pills, alcoholic beverages and antibiotics. Other drugs also deplete B vitamin supplies and/or hinder the synthesis of B vitamins in the intestine. A future lesson will be devoted to the effects of various drugs and medications upon nutrition.
Physiology texts also mention the fallacy of regarding any one B vitamin in the complex as more important than another because of the fact that the normal chain of events, physiologically speaking, can be broken by a lack of any one of the B vitamins. The texts also recommend a dietary supplement containing all the factors to “avoid the evils of modern food refinement.” It is appropriate to make a comment here on this subject: It is fully possible, in fact, easily possible, to “avoid the evils of modern food refinement” much more completely and many times more effectively as far as good (healthful) results are concerned than by eating refined foods and taking supplements. Actually, it is not only easily possible and desirable to completely avoid ever eating refined foods, but it is essential for anyone who wants and expects to regain and/or maintain good health. It is not possible to have truly high-level health while continuing to indulge those very practices which undermine it, and eating processed foods and taking food supplements both undermine health.
Please make special note of the above, for it is one of the most important facts you need to completely understand and accept if you are to bring yourself and your clients to a high level of well-being.
- Calcium in carbohydrate metabolism. Like the B-complex vitamins, calcium is essential in the metabolism of carbohydrates. When calcium is present in context with the carbohydrate source (whole foods), there are no problems. But, with today’s high consumption of refined foods, lack of natural calcium in these foods creates a myriad of very serious health problems. Refined sugar and flour, as well as rice, breads, packaged cereals and pastas, have been robbed of the calcium in the plant during processing and refining. Even whole-grain products may completely lack calcium because of the destruction of this mineral during the destructive processes of cooking and baking.
Calcium is taken from the bones and teeth to meet the needs for this important mineral in carbohydrate metabolism. Dental caries, osteoporosis and other bone diseases result.
- 1. Introduction
- 2. Classifications Of Carbohydrates
- 3. The Role Of Carbohydrates In The Body
- 4. How Carbohydrates Are Digested And Used By The Body
- 5. Sources Of Carbohydrates
- 6. Why Starches Are Less Than Ideal Sources Of Carbohydrates
- 7. Why Fruits Are The Ideal Source Of Carbohydrates
- 8. Amounts And Variety Of Carbohydrates Needed By Humans
- 9. Disease Conditions Related To Carbohydrate Consumption
- 10. Questions & Answers
- Article #1: Carbohydrates By Dr. Herbert M. Shelton
- Article #2: Digestion Of Foods By Dr. Herbert M. Shelton
- Article #3: Starches Are Second-Rate Foods By Marti Fry
- Article #4: The “Staff Of Life” By Marti Fry
- Article #5: What’s Wrong With Wheat By Marti Fry
- Article #6: Fruit – The Ideal Food By Dr. Herbert M. Shelton
- Article #7: Are Humans Starch Eaters? By Dr. Herbert M. Shelton