Читать книгу Dietetics for Nurses - Fairfax T. Proudfit - Страница 10

Table of Contents

In the ordinary mixed diet of man, the carbohydrates predominate, being not only the most abundant, but also the most economical source of energy. The term carbohydrate covers all of the simple sugars and those substances which can be converted into simple sugars by hydrolysis; the ones of special interest in this study are divided into three groups, known as, Monosaccharides (C₆H₁₂O₆); Disaccharides (C₁₂H₂₂O₁₁) and Polysaccharides (C₆H₁₀O₁₅).

Monosaccharides.—Glucose, Fructose and Galactose are substances whose monosaccharide molecules contain one sugar radical; hence they cannot be hydrolized to simpler sugars (sugars of lower molecular weight). Those constituting this group of sugars are all soluble, crystallizable and diffusible substances, which do not undergo changes from the action of the digestive enzymes, consequently these sugars will enter the blood stream in their original form, unless attacked by the bacteria which inhabit the stomach and intestinal tract. The monosaccharides are all susceptible to alcoholic fermentation. Each member of the group is utilized in the body for the production of glycogen and for the maintenance of the normal glucose of the blood.

Disaccharides.—Sucrose, Maltose and Lactose are substances yielding, upon hydrolysis, two molecules of simple sugar: each of these sugars is crystallizable and diffusible: all are soluble in water, and to a less degree in alcohol—sucrose and maltose are more soluble than lactose. When attacked by the digestive enzymes, these sugars are changed to monosaccharides.

Polysaccharides.—Starch, Dextrin, Glycogen and Cellulose are substances more complex in character than the above-mentioned groups. They are built up of many sugar molecules, which yield upon complete hydrolysis many molecules of simple sugar. The polysaccharides are insoluble in alcohol, and only soluble to a certain extent in pure water. Some members of this group swell and become gelatinous in the presence of moisture and heat; some become of a colloidal form in water, and will pass through filter paper; others remain unchanged.

A brief description of the various members of these different groups of carbohydrates will assist the nurse in the ways and means of utilizing them in the dietary to the best advantage.

Glucose, which is abundant in the juice of plants and fruits, and to a more or less degree in the blood of all animals (usually about 0.1%) occurs free in nature. This sugar is likewise obtained from many carbohydrates, either through the action of acids, or as the result of the digestive enzymes, and as such becomes the principal form in which the animal body utilizes the carbohydrates ingested. Under normal conditions the glucose in the blood is constantly being burned and replaced; it is only when the body loses to a greater or less degree the ability to burn the glucose that it accumulates in the blood, from which it must escape by way of the urine. There are times, such as when very large quantities of carbohydrates are eaten at once, when glucose will also appear in the urine; but under such circumstances it is generally found to be merely temporary, and for this reason, the condition is known as temporary glycosuria. As a rule, however, the surplus of glucose absorbed, whether it be eaten as such, or is found as the result of enzymic action upon the other carbohydrates, is converted into glycogen and stored in the liver and to a less extent in the muscles. Glycogen is readily reconverted into glucose, which is used by the body for the production of energy. It has been estimated that over half the energy manifested in the human body is derived from glucose, and it is in this form that the tissues of the body will ultimately make use of most of the carbohydrates in food. Practically all of the fruits, and many of the vegetables, are rich in this form of carbohydrate, but grapes contain more than any of the other fruits, while sweet corn, onions, and unripe potatoes contain appreciable amounts.

Fructose.—The second member of the monosaccharide group is more or less associated with glucose in plant and fruit juices, and is used like that substance for the production of glycogen in the body. Eaten as such, or produced as the result of digestive action upon cane sugar, fructose is changed into glycogen, chiefly upon entering the liver, and for this reason will not be found to enter largely into the blood of the general circulation.[3]

Honey is the most abundant source of fructose in nature.

Galactose.—This sugar, unlike the other members of this group, is not found free in nature, but it is produced as the result of hydrolysis of milk sugar, either by enzymes or by acids. Like glucose and fructose, galactose seems to promote the production of glycogen in the body. Certain substances known as galactosides, which are combinations of galactose and some substances other than carbohydrates, are found in the nerve and brain tissues of the animal body.

Disaccharides.—Of the second group of carbohydrates, we are probably more familiar with sucrose, or cane sugar, than with either of the other two, since it is in this form that the greater part of the sugar eaten is purchased.

Sucrose.—By far the greater part of the sugar entering into the average dietary is manufactured from sugar and sorghum canes, and from sugar beets; but appreciable quantities are derived from the sugar maple and sugar palms. Many of the sweet fruits are rich in this form of sugar; pineapples are said to contain at least half of their solids in sucrose; and although other fruits and vegetables do not contain so high a percentage of this sugar, oranges, peaches, apricots, dates, raisins, prunes, carrots and sweet potatoes contain goodly quantities, which are associated with glucose and fructose. Sucrose is readily hydrolized, either by acids or enzymes. The inverting enzyme (invertase) of yeast and sucrase of the intestinal juice, convert sucrose to fructose and glucose, in which forms it is absorbed into the portal blood. It is believed that when sucrose is eaten in very large quantities, it is sometimes absorbed from the stomach. In these cases it does not become available for use in the body, but acts in the same manner as when injected directly into the blood stream, being excreted unchanged by way of the kidneys. According to Herter, sucrose is much more susceptible to fermentation in the stomach than either maltose or lactose; and since it has no advantage over these sugars from a standpoint of nutrition, they are frequently substituted for sucrose in cases where the dangers arising from fermentation must be avoided.

Maltose (Malt sugar) is an important constituent of germinating grains—malt and malt products being formed as the result of enzymic action (amylases) on starch. A similar action takes place in the mouth as the result of the ptyalin in the salivary juices and in the intestines from the action of the starch-splitting enzyme, amylopsin, in the pancreatic juice. The maltose thus formed is further converted into glucose by the sugar-splitting enzyme in the intestinal juice, and in this form it is chiefly absorbed. Maltose is also an intermediate product formed during the manufacture of commercial glucose as the result of the boiling of starch with dilute acids.

Lactose (sugar of milk) is one of the most important constituents in the milk of all mammals. In freshly secreted human milk, lactose occurs in quantities ranging from 6 to 7%, and in the milk of cows and goats from 4 to 5%. Lactose is much less soluble than sucrose, and decidedly less sweet; hence, owing to this latter property, as well as to its lack of susceptibility to fermentation, lactose is frequently used to bring up the sugar content of infant formulas to the desired percentage, and the diets used in the abnormal conditions when additional energy material is needed. During the process of digestion, lactose is hydrolized by the lactase in the intestinal juice, yielding one molecule of glucose and one of galactose. Like maltose, little if any of this sugar is absorbed in its original form, since experiments made with injections of lactose into the blood result in the rapid and almost complete elimination by way of the kidneys. No such results are obtained when even large amounts of lactose are taken by way of the mouth.

Polysaccharides.—This group of carbohydrates is complex in character, built up of many sugar molecules, and upon digestion must be broken down into simple sugars before they can be utilized by the body.

Starch is the form in which the plant stores her supply of carbohydrates. It is found in this form in roots and (mature) tubers, three-fourths of the bulk of which is made up of this material. From one-half to three-quarters of the solids of grains is made up of starch also. Pure starch is a fine white powder, odorless and almost tasteless. It is insoluble in cold water and alcohol, but changes from an insoluble substance to a more soluble one upon the application of heat. Upon hydrolysis starch gives first a mixture of dextrin and maltose, then glucose alone as an end-product. This hydrolysis may be the result of enzymic action, as occurs upon bringing starch in contact with the ptyalin in the saliva, or with the amylopsin in the pancreatic juice; or it may be the result of boiling starch with acid, as is seen in the manufacture of commercial glucose.

Dextrin, as has already been stated, is an intermediate product of the hydrolysis of starch by acid or enzymes.

Glycogen is the form in which the carbohydrates are stored in the body, just as starch is the form in which they are stored in plants. It is found in all parts of the body, but is especially abundant in the liver. Here it is stored in the cell substance rather than in the nucleus. The storage of glycogen in the human body depends largely upon the mode of life and upon the diet. Active muscular work, especially out of doors, uses up the store of glycogen with great rapidity; while rest and a sedentary life promotes its storage. The body readily converts its supply of glycogen into glucose, the form in which the body uses the carbohydrates for fuel.

Cellulose is a woody, fibrous material insoluble in water and to a certain extent impervious to the action of the digestive enzymes. This carbohydrate constitutes the skeleton of plants just as the bones constitute that of the animal body. It is probable that owing to the length of time required for this carbohydrate to be broken down in digestion, much of it escapes oxidation entirely. Hence, it passes down the digestive tract lending bulk to the food mass and thus promoting peristalsis throughout the whole of the digestive tract.

Organic Acids.—Certain of the carbohydrate foods (fruits and green vegetables) contain appreciable amounts of organic acids or their salts; oranges and lemons, for example, are rich in citric acid; grapes contain considerable quantities of potassium acid tartrate, apples and other fruits have malic acid; many of the fruits have succinic acid; a few foods contain oxalic acid, or oxalates. All of these organic acids are burned in the body to produce energy, with the possible exception of the oxalates, which seem to have little, if any, food value. According to Sherman, these organic acids have a lower fuel value, per gram, than carbohydrates, but are reckoned as such in computing a food in which they exist. The function of these acids is chiefly that of neutralizing the acids formed in the body in metabolism. Being base-forming in character, they function after absorption and oxidation in the body as potential bases—the base associated with the acid in their ash combining with carbonic acid to form carbonates, which act as above described.

Bacterial Action upon Carbohydrates of Foods.—The bacteria that act chiefly upon the carbohydrates belong to the fermentative type. The substances formed as a result of this activity are certain acids—lactic, butyric, formic, acetic, oxalic, and possibly alcohol. Certain forms of carbohydrates are more susceptible to bacterial fermentation than others. Herter claims that sucrose and glucose are much more so than lactose, maltose, or starch. The substances thus formed through bacterial activity are not believed to be toxic in character, but merely irritating. However, the irritation arising from excessive fermentation in the stomach may lead to gastric disturbances of a more or less serious nature; hence the amount of carbohydrate taken under certain conditions must be adjusted carefully.

The Effect of Heat upon Carbohydrates.—The changes wrought in the carbohydrates as a result of heat have already been discussed to a certain extent. It is seen that the sucrose (cane sugar) is soluble alike in hot and cold water; the same is true of maltose; but lactose is much more soluble in hot water than it is in water which has not been heated. So far as their digestibility is concerned, the application of heat (boiling) neither increases nor decreases the utilization of these sugars by the body.

With starch it is an entirely different matter. It has been found that the application of heat, either as dry heat, or in the presence of moisture, brings about a definite change in the character of the foodstuff. Pure starch admixed with water and boiled, passes into a condition of colloidal dispersion, or semi-solution, known as starch paste (Sherman). This is graphically illustrated in the cooking of potatoes, in which the starch and water are mixed in nature; and in the cooking of cereals and like starchy foods, to which water is added in preparation for their cooking. In both cases the application of heat adds greatly to the digestibility of the raw material by reason of the change which is wrought in these substances, causing them to be more readily acted upon by enzymes in the digestive juices.

This solubility of carbohydrates in hot water may be utilized in the washing of utensils in which these substances have been prepared; thus saving much time and effort on the part of the nurse in either the diet kitchen or in the home.