Corn-starch, treated with weak sulfuric acid, changes the starch into glucose. The ordinary glucose or corn-sirup is not all changed by this process, into pure glucose, but contains some maltose and other gummy compounds; hence it will not crystallize or granulate into pure sugar. After the acid has changed the starch into glucose it (the acid) is neutralized with an alkali. A crude compound is thus formed, which settles to the bottom of the tank, and from which the glucose can be easily separated. Commercial glucose is now very extensively used in the manufacture of various food products, especially confectionery. Pure glucose is a wholesome food, but there is some danger that the commercial product may (due to carelessness in manufacturing, or to the use of cheap and impure acid) contain various mineral poisons. Government testing of glucose and similar manufactured products is, in the writer's opinion, fully as essential as the government inspection of packing-house products.

Just as glucose may be manufactured from starch treated with dilute acids, so maltose may be made by treating starch with malt. The brewing of beer depends upon the chemical changes induced in starch by malt. Barley is ordinarily used for this purpose. The barley is sprouted in a warm, damp room, and a process of starch digestion begins, which is necessary in order that the young barley sprouts may grow. This changes the starch into maltose. The digestive principle developed in the barley-malt may be utilized to malt other grains by mixing them with the sprouted barley.

If this process of malting is stopped at the proper time, and the sugar dissolved, and extracted, a product is formed consisting chiefly of the sugar maltose. This is the basis of malt extract, malt honey, and many similar foods put on the market, which are claimed by the manufacturers to have wonderful dietetic and curative values.

2 GLYCOGEN

Glycogen is commonly called animal-starch. It exists in the liver in small quantities. All carbohydrates are digested in the alimentary canal and absorbed into the blood in the form of simple sugars of the glucose group. When these sugars reach the liver they are again built up into a complex carbohydrate very similar to starch in composition. This glycogen or animal-starch is stored in the liver until the body has need of it, when it is changed into glucose and given back to the body in the form of energy. (See "Metabolism of Carbohydrates," Lesson VI, p. 202).

3 CELLULOSE

Cellulose, from the standpoint of human nutrition, is not a food product, being insoluble by the digestive juices, but it is very important in the digestion and the alimentation of other foods. Its chief purpose is to excite stomach and intestinal peristalsis. All plant products in their natural form contain some cellulose, though the percentage is very small in such grains as rice and barley. The bran of wheat or of corn is chiefly cellulose. Wood is almost pure cellulose.

Cellulose can be digested by strong acids into simple carbohydrates, in the same way that starch may be. Sugar can be manufactured from wood or rags, but the process is yet too expensive to be applied commercially. Some of us may live to see the time when the chief food of mankind will be manufactured from scrap lumber and waste paper. Bacteria have the power of digesting cellulose. The bacterial action or fermentation in the human intestines may cause a small amount of cellulose to be digested, but the quantity is of no consequence from a nutritive point of view.

4 GUMS

The gums include a group of rather complex carbohydrates which are intermediate between starches and sugars. From plants are derived many varieties of gums which have various commercial uses in the market, such as gum arabic.

I have already spoken of the formation of dextrin from starch. Dextrin has no particular dietetic qualities that do not exist in starch. It is, in fact, starch arrested at an intermediate point of digestion.

Pectins are a group of gummy substances found in fruits, especially green fruits which are in the process of being formed into sugar. These pectins form the basis of fruit jellies. Green grapes, as every housewife knows, will make better jelly than ripe grapes. This is because the pectins in ripe grapes have been transformed into sugar. The pectins in fruit are in most cases wholesome enough, though it would seem the better part of wisdom to eat all fruits in the ripened state, after Nature has completed her work.

5 INULIN

Inulin is a compound closely related to starch, and upon digestion with acids, yields levulose just as starch yields glucose. It is of no particular interest to the food chemist, as it exists in but very small quantities in starch, and has no distinct dietetic value.

FATS AND OILS

The fats and oils in food products, whether of plant or animal origin, contain the elements carbon, hydrogen, and oxygen. These fats are formed by uniting the fatty acids with glycerin, which belongs to the alcohol group. The particular fat that is formed takes its name from the acid which enters into its composition; thus stearic acid unites with glycerin to form the fat stearin.

The following table gives the names of a few of the more common fatty acids and their corresponding fats:

A fat from any source will usually contain several of these chemical compounds. The ordinary animal fats, such as tallow and lard, are formed chiefly of the two fats stearin and olein. The different proportions of these fats will determine the melting point or hardness of the mixed product. Olein is a liquid at ordinary temperature, while stearin is solid. The reason that tallow is a firmer fat than lard or butter is because it contains a larger per cent of stearin.

Olive-oil, cottonseed-oil, and other vegetable oils contain large per cents of olein, which accounts for their being liquid at ordinary temperature.

Butyrin is a fat found in small quantities in dairy butter, and does not exist in cottonseed-oil and other fats from which oleomargarin is manufactured. This is the reason that artificial butter lacks the flavor of the dairy product, and this is remedied to some extent by churning the fats of the cottonseed-oil and tallow with fresh cream, which imparts a small quantity of the butyrin and similar compounds to the oleomargarin and gives the characteristic flavor of butter.

Besides the more common fats herein mentioned there are many other fats that exist in certain vegetable oils in small proportions. These fats give the oils their characteristic properties, and may render them unfit for food. Some oils are active poisons, such as croton-oil, which is the most powerful physic known. The power of all physics and cathartic drugs is measured by the active poisons they contain.

When fats are heated to a high temperature they decompose and form various products, some of which are irritating and poisonous to the human system. In the manufacture of packing-house and cottonseed products the stearin is often separated from the olein. The granular appearance of pure leaf lard is due to crystals of stearin. In the packing-house stearin is separated from the tallow in large quantities. The stearin is used to make candles, etc., while the olein is used for food purposes in this country in the form of oleomargarin, while in Europe it is used under its right name as a cooking product. It is equally as wholesome, if not more so, than lard.

Fats may become rancid; this is caused by the decomposition of fat due to its uniting with the oxygen of the air. Rancid fats and nut-kernels can be restored and made edible by heating them in an oven until the oxidized fat is neutralized by the heat.

PROTEIDS OR NITROGENOUS FOOD SUBSTANCES

The food substances which contain nitrogen are commonly called proteids, or, if these compounds are considered together, the name protein may be given the group. Protein is not a single compound, but includes all substances which contain the element nitrogen in such combinations as are available for assimilation in the human body.

Protein is the most important group of nutrients in the animal body. The proteid substances in the body must be formed from proteids taken in the form of food, because only proteid foods contain the element nitrogen. All proteids contain nitrogen, but all nitrogen does not contain protein. All proteids, therefore, are nitrogenous compounds.

The animal body does not possess the power of combining elementary nitrogen with other elements. Bacteria have the power to utilize the nitrogen of the air to form mineral salts or nitrates. Plants have the power to unite the nitrogen derived from these nitrates with carbon, oxygen, and hydrogen. In this way organic nitrogen, or proteids, are formed. The animal body may digest these proteids, however, and transform them into other proteid compounds. All proteids contain carbon, hydrogen, oxygen and nitrogen; most of them contain sulfur, and a few contain phosphorus, iron, copper, and bromid.

The percentage by weight of the various elements which form proteid matter is about as follows:

The following table gives three groups of proteid substances:

Besides these real proteids there are a few substances known as amido compounds which exist in small quantities in vegetables, and a number of nitrogenous substances which exist in meat and meat extracts, which are not true proteids, as they have little or no nutritive value, but act as stimulants or irritants in the body.

Ptomains are another class of substances which are often found in food products. They are formed by the growth of bacteria, and are in reality the nitrogenous waste-products of bacterial life. Ptomains develop in meats and dairy products held in cold storage, and are sometimes the cause of serious poisoning. Nitrogenous waste-products will be further discussed in Lesson VI, under "Metabolism of Proteids." (See p. 209.)

Albumin is one of the commonest and simplest forms of proteids known. It is found in the white of eggs, in milk, and in blood. It is coagulated by heat, and by certain chemicals, such as acids, alcohol, and strong alkalis. Albumin is soluble in water and in weak solutions of salt, but it is not soluble in very strong salt solutions.

Globulins are much like albumin, but are not soluble in water. They are, however, soluble in dilute salt solutions. Globulins exist in considerable quantities in the yolk of eggs, and in the blood. The globulin in the body could not remain in solution if there were not always present a small quantity of salt in the blood. There are several types of globulins. The fibrinogen of the blood, which coagulates, forming clots, when the blood is exposed to the air, is a globulin. Hemoglobin, which is the chief component of red blood-corpuscles, and which unites with the oxygen in the lungs and carries it to the various tissues of the body, is another form of globulin, and one which contains a considerable amount of iron.

Casein is the most important proteid substance in milk, and is familiar to all as the curd or white substance of clabbered milk. A related form of vegetable casein is found in leguminous seeds, such as beans and peas.

Proteoses and peptones are proteids that are formed by the digestion of other proteids. They exist in the alimentary canal in the partly digested food. Peptones are readily soluble, and for this reason are easily absorbed through the walls of the digestive organs. (See Lesson V, "Digestive Organs"—[The Stomach], p. 137; also "Composition of Gastric Juice," p. 147).

MINERAL SALTS IN FOOD

The subject of salt in food has received considerable attention and discussion by scientific investigators, and many theories have been advanced by those interested in hygiene as to the effect of common salt used in food. The tissues and organs of the body contain certain salts, without which life could not exist, but it does not follow that these salts need to be supplied in mineral form. Common table salt is an inorganic substance, while the mineral salts in green and fresh vegetables are organic, and readily convertible, therefore a valuable aid in the digestion of other foods. A diet of sugar, pure oil, and artificially prepared proteids would be absolutely unwholesome and would fail to nourish the body for any length of time because of the lack of mineral salts. Foods containing mineral salts All natural food products, whether of vegetable or animal origin, contain a limited but ever-present amount of mineral salts. This is especially true of milk, eggs, and the seeds and green portion of plants. The amount of salts in the human body is considerable, especially the calcium phosphates of the bones, but the salts that need to be supplied daily in food is small because the salts are not consumed as rapidly as are other elements of nutrition.

Some grains, especially rice and corn, are somewhat deficient in salts. At the Kansas Experiment Station some pigs were fed exclusively on corn, and others on grain and green forage. At a certain age the pigs were killed, and the bones weighed and tested for strength. The bones of the pigs which had been fed on a corn diet, which is deficient in mineral salts, were about half as heavy and strong as the bones of the pigs fed in a more natural way.

LESSON V

CHEMISTRY OF DIGESTION

DIGESTIVE ORGANS AND DIGESTIVE JUICES

First—THE MOUTH:

The three salivary glands of the mouth secrete the saliva, which is an alkaline substance containing a digestive enzym called ptyalin.

The saliva begins the digestion of starch and moistens food to facilitate swallowing.

Second—THE STOMACH:

The gastric juice secreted by the mucous lining of the stomach is an acid. It contains hydrochloric acid and pepsin, which act on proteids, changing them to proteoses ("intermediate products formed naturally in the process of digestion") and peptone.

The gastric juice also contains rennet, which acts directly on milk, and indirectly on all proteids.

Third—THE LIVER:

The liver secretes a digestive fluid called bile, which is an alkaline substance. Its chief purpose is to emulsify fats and to supply the alimentary tract with the requisite amount of moisture.

Fourth—THE PANCREAS:

The pancreatic juice, secreted by the pancreas, is an alkaline and slightly acidulous substance. It contains three enzyms, the names and action of which are as follows:

Amylopsin completes the digestion of starch.

Trypsin completes the digestion of proteids.

Steapsin converts fats into fatty acids and glycerin.

Fifth—THE SMALL INTESTINES

The intestinal juices secreted by the small intestines are alkaline substances which change sugar and maltose into glucose, and perform the last step in the process of breaking up or subdividing food so fine that it will pass through the intestinal walls into the circulation.

LESSON V

CHEMISTRY OF DIGESTION

The digestive juices of the human body are five in number, namely: Saliva, gastric juice, bile, pancreatic juice, and the several intestinal juices. Beginning with the saliva these juices alternate, first an alkali, then an acid. It is the opinion of the writer that this alternating plan is carried on throughout the entire intestinal tract, as the final dissolution of food matter takes place in the intestinal canal. These five juices are secreted from the blood by special cells or glands. Each of these juices contain one or more enzyms or digestive principles. These enzyms are highly organized chemical compounds which have the property of changing other chemical compounds without being destroyed or used up themselves except in minute quantities.

Malt, which was studied in the last lesson, and which is produced by the sprouting of barley, is a true digestive enzym of the barley. Yeast-cells are minute plants which secrete an enzym that causes the fermentation of bread. It was formerly thought that the fermentation of yeast could not take place except in the presence of a living cell. This has now been disproved, as a German scientist has succeeded in grinding up yeast-cells and filtering off the chemical compound or true enzym which causes the fermentation of sugar.

It is now recognized by scientists that all processes of fermentation and digestion found in plant and animal life are due to definite chemical compounds known as enzyms. The action of digestion is truly a chemical one, and could take place without the body as well as within, if we could manufacture the proper enzym and could produce the exact conditions of temperature, moisture, etc., that are found in the human digestive economy.

The manufacture of predigested foods depends upon various processes of fermentation, or upon the digestion that may be carried on by inorganic chemical agents, such as acids, or by the ferments of bacteria, or other forms of life. The following are illustrations of these processes of predigestion:

1 The manufacture of glucose from starch by the action of sulfuric acid

2 The malting of starch for the production of malt-sugar or of fermented liquors

3 The making of cheese by the action of the enzym rennet which has been extracted from the stomach of a calf

A great amount of discussion, pro and con, has been raised over the subject of predigested food. The foregoing examples will show that the subject of predigested food, taken in its broadest sense, cannot be dismissed summarily with either approbation or disapproval. We must consider the particular chemical process involved in each case and the final chemical products, as well as its mechanical condition. These things must be taken into consideration when we pass an opinion upon the wholesomeness of a so-called predigested food.

With this diversion to illustrate the breadth and the importance of the action of enzyms, I will now return to the consideration of the chemical action of the human digestive organs.

SALIVA

The saliva is the digestive juice of the mouth. It is secreted by three pairs of salivary glands. The secretions from these three glands are slightly different in Starch digestion in the mouth composition, but for our purpose may be considered as one secretion. The saliva is an alkaline fluid, and the principal enzym that it contains is a starch-digesting enzym known as ptyalin, which can act only in an alkaline solution. As the gastric juice is strongly acid, the digestive action of the saliva is stopped soon after the food has entered the stomach, and the enzym is of no further use. The action of the saliva is very weak, and the amount of starch digestion which is accomplished in the mouth is comparatively insignificant.

The chief function of the saliva is to moisten food and to facilitate swallowing. From these statements one might first infer that the emphasis given to thorough mastication is unwarranted. In fact, the mastication of food has a much more important function than the digestion of starch by saliva. This subject will be referred to again when the physical condition of food as a factor in digestion, and the nervous control or co-ordination of the various functions of the digestive system are considered. (See "Composition of Gastric Juice," p. 147.)

GASTRIC JUICE

The importance of the stomach as an organ of digestion has been overestimated in modern times. From the discussions in the average text-book and physiology, one would be led to believe that the stomach is the only organ of digestion, when, as a matter of fact, the chief purpose of the stomach is that of a receptacle for the storage of food for digestion further on. I do not mean by this statement that there is no digestive action in the stomach, but I do mean to say that there are no digestive processes completed in the stomach, and that all foods which are acted on by the gastric juice can also be acted on by the digestive juices in the intestines. This has been proved by the fact that surgeons have successfully removed the entire stomach from both animals and men without seriously interfering with the nutrition of the body. They merely had to eat more often, as the depot or storage receptacle had been removed.

The stomach should be considered as a preliminary organ of digestion. The tables published in the physiologies giving the digestibility of various foods as so many hours, refer entirely to the length of time it takes for the food to pass out of the stomach. According to these tables boiled rice is given as one of the most digestible of foods. As a matter of fact, the chief reason why rice passes out of the stomach more quickly than other grains, is because it contains practically nothing but starch, and as starch is not digested in the stomach, the rice is passed on to the next station where it can be acted on by an alkali.

In this connection it becomes necessary to refer to the interpretation of the experimental results obtained by investigators at the Battle Creek Sanitarium. In these experiments cereal products which had been put through various processes of predigestion were compared with uncooked whole wheat, the contents being removed from the stomach after a given period. The results of this experiment showed a greater amount of starch digestion in the case of the dextrinized or super-cooked foods. These results were published as proof that starchy foods should be put through a process of super-cooking, dextrinization or predigestion. To those who are not familiar with food chemistry, such results would appear very convincing, but to a well-informed food scientist they only illustrate how misinterpretation of scientific facts may indicate conclusions opposed to the truth.

Starchy foods are not intended by Nature to be digested in the stomach, but in the intestines, and the processes of partial digestion of these foods, by artificial means, before entering the stomach, serve only to interfere with Nature's plan, and to deprive both the stomach and the intestines of their natural functions.

COMPOSITION OF THE GASTRIC JUICE

The gastric juice contains three principal enzyms or digestive principles. These are hydrochloric acid, pepsin, and rennet. The hydrochloric acid and the pepsin are secreted by different cells, and could be considered as separate digestive juices, but as the action of one is dependent upon the other, I will consider these actions as one. Pepsin, in the presence of hydrochloric acid, acts on proteids, and changes them into proteoses Peptone and proteoses and peptone. Comparatively little food is completely peptonized in gastric digestion. Proteoses are intermediate products between food proteids and peptone, being the principal product of the action of the gastric juice. Thus it is seen that this stomach-action is only preparatory for the digestive processes of the intestines.

The gastric juice does not act on fat, but in the case of animal food, in which the membranes or connective tissues that enclose the fat-cells are formed of proteid material, the gastric juice sets the fat-globules free by dissolving these enclosing membranes.

The chief action of hydrochloric acid in the stomach is to aid the action of the pepsin. Pepsin alone has no digestive power. There are no other acids produced by the secretive glands of the stomach. If other acids are found in the contents of the stomach, it is because they have been taken in with the food, or produced by abnormal fermentation.

The source of hydrochloric acid is from the sodium chlorid or common salt of the blood. The secreting cells of the stomach-glands are thought to have the power to form hydrochloric acid by uniting the chlorin of the salt with the hydrogen of the water. This is a very unusual chemical process, and has not yet been successfully produced in a laboratory.

One of the chief functions of hydrochloric acid in the stomach is that of an antiseptic. In other words, hydrochloric acid kills bacteria. This is not true of all bacteria, for some germs can live in an acid medium, while others may live best in an alkaline solution. The alternation of the digestive juices from alkali to acid is a provision of Nature which has a dual purpose:

1 To reduce food to the finest possible solution; that is, to subdivide or to digest food elements into a form that will admit of assimilation and use

2 To destroy bacteria and enzyms of plant and animal origin that are taken into the digestive tract with food

(These two facts constitute additional reasons for the thorough mastication of food)

By such plan Nature provides for the digestion of food only by such enzyms and ferments as will produce a finished product wholly suited to the particular requirements of the body. When we attempt by artificial processes to digest our food with other enzyms than those of our own digestive organs, or take into the stomach large quantities of food without proper mastication, which causes fermentation, we may expect that the nutritive material supplied to our tissues will not be perfectly adapted to the needs of human cell-growth, and, as a natural result, consequent derangement of the body-functions will take place.

The rennet of the gastric juice is primarily for the purpose of digestion. Other than this it has no particular function that has yet been discovered.

The problem as to why the stomach does not digest itself has puzzled scientists for many years. Investigations of the twentieth century have at last solved this fascinating question. The walls of the human stomach are composed of proteid material, and should be dissolved by the gastric juice according to all known chemical laws. The explanation formerly given was that the stomach did not digest itself because it was alive. This answer did not satisfy scientists.

There has recently been discovered an enzym, known as antipepsin, which is secreted by the cells in the stomach-walls. This antipepsin destroys the action of the pepsin, thus in turn preventing its action on the stomach-wall itself. Were antipepsin secreted in sufficiently large quantities to mix with the food in the stomach-cavity, no digestion could take place. The presence of this antipepsin in the stomach-walls has been proved in the following manner: The arteries leading to a portion of the stomach-wall of a dog was severed. This portion, receiving no blood supply, did not form the usual amount of antipepsin. The secretion of pepsin went on in the remainder of the animal's stomach, but digested that portion of the stomach-wall which was receiving no blood supply; that is, secreting no antipepsin.

BILE

The bile is a juice secreted by the liver and is alkaline in character. It is collected by the biliary ducts to be conveyed into the duodenum. The most important constituents of bile are bile salts and sodium glycocholate. The chief purposes of bile are to emulsify fats, thus aiding them to pass through the intestinal walls, and to stimulate intestinal peristalsis.

PANCREATIC JUICE

The pancreas is a secretive gland located entirely outside of the intestinal walls, and produces a juice which is poured into the small intestines at the point where the bile enters. Pancreatic juice is acidulous, and also strongly alkaline. As soon as the food, passing from the stomach, comes in contact with the pancreatic juice and the bile, the acid is neutralized, and the mass becomes alkaline.

The pancreatic juice contains three important enzyms:

1 Amylopsin—acts on starch

2 Trypsin—acts on proteids

3 Steapsin—a fat-splitting enzym

Pancreatic juice also has the power of coagulating milk, and is believed to contain some rennet.

Amylopsin, the starch-digesting enzym, appears to be very similar to ptyalin in its power to digest carbohydrates. Amylopsin completes the digestion of starch that was begun by the saliva. It acts on starch with great activity. One part of amylopsin can change forty thousand times its bulk of starch to glucose. This can act only in an alkaline solution, and if any abnormal fermentation takes place in the digestive tract, producing a large quantity of acids, the digestion of starch is stopped. It is interesting to note that this enzym is entirely absent from the pancreatic juice of infants. This explains why infants cannot digest starch.

The second enzym to be considered in the pancreatic juice is trypsin. This is a substance distinct from pepsin, but its action is the same. The chief distinction is that trypsin acts in an alkaline solution, while pepsin acts in an acid solution. Trypsin is much more energetic in its digestive power than the pepsin of the gastric juice. It completes the digestion of proteids that is begun in the stomach, and converts all proteids into soluble forms. A number of forms of proteid that are not acted on at all by the gastric juice are readily digested by the trypsin of the pancreatic juice.

The fat-digesting enzym of the pancreatic juice is steapsin. This is the principal fat-digesting enzym of the body. This substance has power to split fats; that is, to convert them into fatty acids and glycerin of which they were originally composed. This fatty acid then combines with the alkalis of the bile and of the pancreatic juice to form soap. Soap is soluble, and passes through the walls of the small intestines in this form. Having passed through the walls of the intestines, soap is again changed into fat. The probable reason for which Nature adopts such a complex process for the absorption of fat, is because fat is insoluble. If the intestinal walls were so constructed that fat-globules could be taken directly through them, they would also be open for the entrance of germs and other foreign substances.

Fat is not acted on by the gastric juice. This explains why the process of frying is so unwholesome. Frying causes a thin film of melted fat to spread over the surface of the starch and of the proteid atoms, with the result that these atoms cannot then be properly acted on by the saliva and the gastric juice, and therefore cannot undergo the preliminary changes necessary to normal digestion. Fat, taken in its natural form, does not interfere with other digestive processes.

INTESTINAL JUICES

In addition to the digestive juices that are poured into the small intestines from the pancreas and the liver, there is a juice which is secreted from the walls of the intestinal cells. This is called intestinal juice or succus entericus. It is a light yellow fluid with a strong alkaline reaction, due to the presence of sodium carbonate.

One action of the intestinal juice is to change sugar and maltose into glucose, which is then absorbed directly into the blood.

THE SECRETION OF DIGESTIVE JUICES

Within the past few years many remarkable discoveries have been made in regard to the secretion of the various digestive juices. Until some ten or fifteen years ago it was believed that the secretion of the digestive juices depended wholly upon the presence of food in the alimentary canal. The recent discoveries in this branch of physiology are to be accredited chiefly to Professor Palloff, a Russian scientist, and his co-workers. The facts that are now known regarding this part of Nature's work are essentially as follows:

The secretion of the various substances which make up the digestive fluids of the body depend upon two kinds of stimuli:

1 Direct nerve stimulus from the central nervous system

2 The chemical stimulus on the walls of the digestive organs

Depending upon either or both of these sources of stimulation, the digestive juices of the body are regulated in quantity, and what is much more worthy of note, in their actual chemical composition. Thus it will be readily seen how far-reaching in its effect upon scientific dietetic treatment is the knowledge of the influence of various foods, quantities, and combinations.

Professor Palloff's discoveries throw some very important light on the comparative digestibility of foods. The former method of estimating the digestibility of food was first to analyze the food, and then to analyze the intestinal residue, and subtract the undigested remnant of each particular class of food from the amount originally eaten. By such means it was possible to show that certain foods were, say 80 or 90 per cent digestible, as the case might be. By this method no allowance was made for the amount of nutrition or material that was consumed by the body in the digestion of these particular foods. According to these investigations, milk and meat were about equally digestible. It was not known that the digestion of milk requires only a small fraction of the energy that is necessary to digest meat, or proteids from vegetable sources. Thus it is obvious that when it is desirable to get a large amount of available nitrogen into the system, with as little expenditure of energy as possible, milk is a food par excellence. This is very logical inasmuch as the sole purpose of milk is food for animal life.

The amount of acidity in gastric juice that must be secreted for the digestion of meat is much in excess of that required for a given amount of vegetable food. The amount of acidity required is greatest for milk, second for meat, and least for bread. The digestive energy required is greatest for bread, second for meat, and least for milk. From this we learn that starchy foods are unsuitable for those who are afflicted with hyperchlorhydria or supersecretion of hydrochloric acid, as the excess of acid prevents their digestion by neutralizing the alkali of the intestines.