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Carbohydrates in nutrition. From what has been said above it will be clear that the various digestible carbohydrates of the food, having been split by the digestive ferments to monosaccharides, are absorbed into the blood. Any surplus is stored temporarily in the form of glycogen, chiefly in the liver, though to some extent in the muscles. The glucose which circulates in the blood is burned in the muscles and other active tissues as fuel, the burned glucose being constantly replaced by new glucose derived from the stored glycogen, so that under ordinary conditions the carbohydrate of the food is entirely burned as fuel. When more carbohydrate is received than is burned, the surplus is stored as glycogen, but only to a limited extent, the total amount of glycogen which the human body can store being estimated at less than one pound or only about as much carbohydrate as might be contained in the food of one day. A surplus of carbohydrate, in addition to being stored as glycogen, may also be converted into fat, and this transformation of carbohydrate into fat can be carried on to a very large extent and with almost no loss of energy. The energy value to the body of average carbohydrate in the food is 4.0 Calories per gram, or 1814 Calories per pound.
Organic Acids Some foods contain considerable quantities of organic acids or their salts. Oranges and lemons, for instance, are rich in citric acid; grapes contain potassium acid tartrate; apples and other fruits contain malic acid; and many fruits contain succinic acid. Fermented foods may contain appreciable quantities of lactic acid as in sauerkraut and sour milk, buttermilk, etc., or acetic acid as in vinegar. A few foods contain oxalic acid or oxalates, but these are probably of little if any food value and may be injurious.
With the exception of oxalic acid, these organic acids appear normally to be burned in the body, and doubtless their energy is used in practically the same way as the energy of the carbohydrates. The fuel values of some of these acids have been determined as follows: acetic acid, 3.5 Calories per gram; citric acid, 2.5 Calories per gram; lactic acid, 3.7 Calories per gram; succinic acid, 3.0 Calories per gram; tartaric acid, 1.7 Calories per gram. While these values are somewhat lower than those of the carbohydrates, it is not uncommon in reckoning the fuel value of a food to count the organic acid as carbohydrate, especially as in routine analyses the acids are often not sought nor are the carbohydrates determined directly, but all of the material not found to be moisture, protein, fat, or ash is often considered to be carbohydrate for the purposes of ordinary estimations of food values.
Fats The fats are all glycerides; that is, substances consisting of combinations of glycerol (commercially called “glycerin ') with fatty acids. Many of these fatty acids belong chemically to the same series with acetic acid. The chief members of this series occurring naturally in fats are butyric acid, CH302 ; caproic acid, C6H12O2; caprylic acid, C3H1602; capric acid, C10H2002; lauric acid, C12H2402; myristic acid, C14H2802; palmitic acid, C16H3,02; stearic acid, C18H3602.
Butyric acid is a liquid which mixes in all proportions with water, alcohol, and ether, can be boiled without decomposition, and is readily volatile in steam.
With increasing molecular weight, the acids of this series regularly show increasing boiling or melting points, decreasing solubility, and become less volatile. Those up to capric acid are liquids at ordinary temperatures; those above are solids. The higher the molecular weight, the harder the solid. Stearic acid is a hard paraffin-like crystalline solid insoluble in water and only moderately soluble in alcohol and ether.
The properties of the fats themselves depend upon and run parallel with those of the fatty acids.
In addition to the fatty acids of the series to which acetic, butyric, and stearic acids belong, all of which are saturated compounds, there are several unsaturated fatty acids, capable of combining chemically with hydrogen, oxygen, or halogens by direct addition. The most important of these contain eighteen
carbon atoms to the molecule and therefore resemble stearic acid in molecular size.
The most important of these unsaturated fatty acids are: oleic acid, C18H34O2; linoleic acid, C18H3202; linolenic acid, C18H3002. All of these acids and their glycerides are liquid at ordinary temperatures. Commercial fats consisting mainly of the glycerides of these acids are therefore liquids and are usually called oils. The chief chemical difference between olive oil and lard is that the former contains more olein (glyceride of oleic acid) and the latter more of palmitin and stearin (glycerides of palmitic and stearic acids). Olein or linolein (glyceride of linoleic acid) may be converted into stearin by direct chemical union with hydrogen, and this is now done on a commercial scale for the hardening of fatty oils so as to give them the consistency of lard (Chapter X).
The body fat of man and of the animals commonly used as food consists of glycerides of palmitic, stearic, and oleic acids. Since palmitin and stearin are solids, while olein is a liquid, the hardness or softness of these fats is principally due to the proportion of olein which they contain. Butter fat contains all of the fatty acids listed above in the series from butyric to stearic acid and is distinguished from the other food fats principally by this fact. Olive oil consists chiefly of palmitin, stearin, and olein, but contains much more olein and much less stearin than the ordinary solid fats. In cottonseed oil, sesame oil, and other seed oils used as food, the quantities of palmitin and stearin are still smaller and, in addition to large quantities of olein, considerable quantities of linolein and in some cases even linolenin may occur.
“Simple” and “mixed” glycerides. For convenience we speak as though the oleic acid radicles in a fat were present simply in the form of olein; the stearic in the form of stearin; the palmitic as palmitin ; etc. As a matter of fact this may be true, and glycerides which contain only one kind of fatty acid radicle in the molecule like simple olein, stearin, or palmitin are called “simple glycerides ”; and it may also be true that two or more kinds of fatty acid may combine with the same molecule of glycerol forming what is called a “mixed glyceride.” “Mixed glyceride " is thus used as a technical term to mean something more than merely a mixture of glycerides. If one molecule of glycerol be combined with one molecule each of oleic, stearic, and palmitic acids, the resulting mixed glyceride, oleo-stearopalmitin, is different from a mere mixture of simple olein, palmitin, and stearin. Ordinary natural and commercial fats are mixtures of both simple and mixed glycerides.
Fats in foods and nutrition. In food analysis, fat is usually determined by extraction with ether. All ether-soluble substances are therefore likely to be counted as fat. In this way some small quantities of materials of less food value are likely to be counted along with the fat of the food. A more serious source of misunderstanding arises from the fact that some food fats are important sources of fat-soluble vitamin while others are not. As will be shown more fully in Chapter X, the familiar edible fats and oils of commerce are rather similar in their chemical composition and digestibility and therefore in their fuel value; but they are very different in their food value because of the great differences in their fat-soluble vitamin content.
The fat of the food after digestion and absorption is again found in the blood in the form of glycerides or neutral fat which disappears partly by being burned in the muscles and other active tissues where it is used as fuel for the same purposes as carbohydrate; and if in excess of the fuel requirements of the body, the fat obtained from the food may also be stored in the tissues. The body fat obtained thus directly from the food may show somewhat different characters from the fat which has been formed in the body from carbohydrate, but its nutritive relations appear to be exactly the same. In either case, the fat thus stored in the body may be drawn upon for use as fuel at any future time when the energy requirements of the body demand it.
The energy value to the body of average food fat is 9.0 Calories per gram, or 4082 Calories per pound.
Proteins (Nitrogen Compounds) Among the nitrogenous constituents of foods, the proteins usually so far predominate that the term “protein ” is often used as practically synonymous with the nitrogen compounds of food materials. For this reason, and because the great majority of proteins contain from 15 to 18 per cent, averaging about 16 per cent, of nitrogen, the protein content of food materials is usually estimated by determining nitrogen and multiplying the percentage of nitrogen found by 6.25.
The proteins are very complex substances and in no case is the chemical constitution of a natural protein fully and exactly known. It has, however, been determined that the typical proteins are essentially anhydrides of amino acids. Thus the relation of the protein molecule to the amino acids, from which it is derived and into which it can be resolved, is analogous to the relation of starch to glucose. There is, however, this striking difference: that the molecules of monosaccharide derived from the complete hydrolysis of the starch are all alike (glucose), whereas the complete hydrolysis of a protein always yields several different amino acids, usually from twelve to fifteen.
The names of the amino acids commonly met as products of hydrolysis of proteins are: glycine (glycocoll), alanine, serine, valine, leucine, proline, phenylalanine, tyrosine, aspartic acid,
The names of the amino acids are sometimes spelled without the final e; for example, glycine as glycin, alanine as alanin. The names of proteins should always be spelled without the final e to distinguish them from amino acids and some other groups of nitrogen compounds; thus always gelatin (never gelatine).