In order not to interrupt the narrative of the Yale experiments, we have foregone defining certain of the technical terms which it was necessary to use. It will be well, before going further, to give a simple description of the manner in which the food we eat is transformed in the body into tissue building material and energy: a process the many parts of which are grouped by physiologists under the name of Metabolism.

When you take a mouthful of food it enters on a journey through the body in which it traverses more than thirty feet of the intestinal tube before that part of it which the body cannot use is ejected; the process of metabolism begins the moment the lips touch it. The six salivary glands which are located in the mouth manufacture saliva, which flows out through numerous openings, and mixes with food as it is chewed. The saliva not only moistens the food, thus allowing it to be more easily swallowed, but it also has a most important chemical office, converting all starchy food matter into sugar, and thus performing the first and one of the most essential steps in the process of digestion.

After the food has been masticated and saturated with saliva, it passes down the throat through the gullet, which performs a peculiar muscular contraction, thrusting downward the particles of food. The conversion of the starch in food into sugar, or glucose, which is begun by the saliva in the mouth, is continued as the food passes into and down the gullet, but stops almost completely when the food once reaches the stomach.

THE WORK OF THE STOMACH

It is in the stomach, on the other hand, that most of the work of digesting the albuminoids, or proteids, of food is performed by the gastric juice. The stomach is a pear-shaped bag, that holds about three pints of material, or three-quarters of an ounce for every inch of the individual’s height. Food enters it through the gullet on the upper left hand side, just below the heart. Myriads of glands in the walls of the stomach are active in the formation of either pepsin, or an acid fluid which, when combined with pepsin, forms the gastric juice.

At the back of the stomach, partly overlapping it, lies the liver, which discharges a liquid called the bile into the alimentary canal just below the stomach. Behind the stomach, lies a large gland called the pancreas, which discharges a remarkable fluid, named pancreatic juice, into the intestine through the same opening which the liver uses for its bile. Connected with the stomach is the small intestine, which is the narrow portion of the alimentary canal, and the largest and most important of all the digestive organs. It is some twenty-five feet in length, and its walls are everywhere covered with glands which secrete and exude mucous and other fluids.

At the lower end of the intestine is the colon or large intestine which is not a digestive organ in itself, but is a reservoir in which the food is stored up for a short time, to allow opportunity for complete absorption of the digested portions.

THE ELEMENTS OF FOOD

Although there may be thousands of different dishes, and combinations of foodstuffs, fundamentally they are all closely akin, and can be all resolved into a few quite simple elements: Proteid, Carbohydrate, Fat or Mineral Salts, or combinations of these; the Proteid class having many subdivisions, and the Carbohydrates being made up of the various sugars and starches.

We also know definitely just what use the body makes of these various substances. The Proteid is the up-builder of tissue, the essential foodstuff without which life cannot exist. If we compare the human body to an engine, as nearly all physiologists seem bound to do, we may say that Proteid is like the brass, or other metal, of which the structure is composed. The various Carbohydrates and the Fats are the fuels from which are derived the energy which animates and operates the mechanism. The Mineral Salts are used to supply various important bodily needs, such as elements required by the bones, or the delicate tissue in the eyes, the enamel for the teeth, and so forth.

THE WORK OF THE DIGESTIVE JUICES

As there are five main food elements, namely, proteid, starch, sugars, fats, and salts, so also there are five main digestive fluids, the saliva, the gastric juice, the bile, the pancreatic juice, and the intestinal juice.

The saliva is an alkaline fluid that digests starch. Its work is checked by the presence of acid substances; which explains why the digestive action of saliva ceases soon after it enters the stomach. Hence the importance of giving the saliva ample opportunity to perform its function, by complete mastication, is obvious.

The gastric juice, of which about seventy ounces is formed by the stomach daily, contains in addition to hydrochloric acid, a quantity of pepsin, which with the acid dissolves all sorts of proteids or albuminous substances, like meat and eggs; and it also contains rennet, which coagulates milk. The gastric juice digests proteids by converting them into pepsin, an exceedingly soluble substance which passes readily into the blood.

The bile manufactured by the liver has the function of digesting fats. Fats are not changed chemically, as are starches and proteids. They are only broken up into particles so small that the cells of the mucous membrane can take them up and effect their removal into the blood stream.

The pancreatic juice is able to perform the work of all the three digestive fluids which we have already named. In fact, it is even more powerful than saliva in the digestion of starch, since it is able to digest raw as well as cooked starch, which the saliva cannot do. It is also able to convert proteid into peptone, as does the gastric juice; and it emulsifies fats, as does the bile.

The intestinal juice digests cane sugar, and is supposed to have a digestive influence upon all the other food elements.

The mineral salts which are taken into the body are dissolved by all the digestive fluids which we have named, some by the saliva and the juices of the intestinal tube, and others, which require acids for their solution, by the gastric juice.

Nearly all these digestive fluids are also powerful antiseptics and are able to destroy germs when the health of the body as a whole is good. The gastric juice, for instance, acts as an antiseptic, preserving the stomach contents from putrefaction during the digestive process. It is a remarkable fact that the gastric juice, although it is so essential to life, is a deadly poison, which, when introduced into the blood produces insensibility and death.

These digestive juices and organs are able completely to dispose of all the food elements which are introduced into the body, save proteid alone. The sugars and starches are either completely absorbed and oxidized, or stored up in the form of surplus fat. The oxidation or burning up of proteid, however, is never complete. There is always a certain amount of unburnable substance left behind from the processes of metabolism, which the liver and kidneys of the body have to dispose of. If only as much proteid as is needed by the body for the upbuilding of its tissue, and the repair of waste, is taken, the body can very readily handle it; but an excess of proteid is highly disadvantageous. Professor Chittenden, in his great work, “The Nutrition of Man,” has set forth in elaborate detail the process of the assimilation of proteid. It appears that there are many kinds of proteid; the proteid of eggs is different from that of meat, and that again from the proteid of beans, and so on; and human proteid is different from all. Consequently, the body is obliged to transform every kind of proteid which is brought to it. This proteid is then absorbed by the blood, and carried to the tissues, which are kept perpetually bathed in a supply of nutritive material. The taking of more proteid than is needed would not be so dangerous if it were simply passed on without being digested; nor even if it were digested and transformed, and then promptly eliminated. But what actually happens is that the new proteid taken in is passed through all the stages of assimilation, and drives out in front of it, so to speak, the proteid which has already been prepared, but has not yet been used. And the result is, of course, to throw a double strain upon the liver and the kidneys, the organs of elimination.

Professor Chittenden also points out the common blunder which is made in assuming that persons who are doing hard work need an additional amount of proteid substance. One commonly hears the phrase that laborers and athletes can eat meat in large quantities, and “work it off.” As we have seen, one can “work off” sugars and starches and fats completely; but one cannot “work off” proteid completely. Professor Chittenden is now recognized as the leading authority of the world upon this particular question; and he sets forth clearly in his book the fact that the quantity of proteid needed is not increased by muscular activity. One may work as hard as he pleases, but his body will use no more nitrogen, save only in the case where a sufficiency of other food elements is not supplied. Only as a last resort will the system undertake the labor of burning up proteid to make energy.

HOW MUCH SHOULD WE EAT

When foodstuffs are taken into the body, digested, assimilated, and used up, they produce the same amount of heat and other forms of energy as if burned outside of the body; and hence it follows that the number of calories, or units of heat, represented in a given foodstuff, is taken by scientists as a common measure of its food value.

A calory is a heat unit, which has been adopted as a means of estimating the nutritive value of foodstuffs. It represents the amount of energy required to raise the temperature of four pounds of water one degree Fahrenheit. The number of calories contained in food is obtained by burning the food and measuring the heat produced by means of a calorimeter.

It has been calculated that the normal, average person needs from one hundred and sixty to two hundred and forty calories of proteids every day, in order to build blood and tissues. He needs daily from five hundred to nine hundred calories of fats, which supply heat.

He needs of carbohydrates, which are the starches and sugars, and which the body uses to produce energy for work and heat, from one thousand to one thousand four hundred calories daily. It is declared by Chittenden and Kellogg, whose work has overset the old notions, that the total number of calories, or food units, should rarely exceed two thousand.

Two thousand calories are furnished respectively by twenty-eight ounces of bread, or ninety-six ounces of milk, or sixty-two ounces of potatoes, or nine ounces of butter. One quarter of each of these, or any other fractions which together equal unity, will make up a ration containing two thousand calories.

It is quite impossible, however, to make a hard and fast rule in this matter. Every individual differs from others in his requirements. Moreover, the weather, the season of the year, the amount and kind of work done, are all factors in the situation. Hard physical work and exposure to cold demands the largest food supply. A person who naturally perspires freely needs more food than a person who does not, because of the large amount of heat carried off from his body by the evaporation of sweat from the skin. Adults require food chiefly to repair waste and losses. Growing children require in addition to food to repair waste and losses, material for tissue building. According to the best authorities upon the diet of children, a growing infant utilizes fully one-third of its total intake of food in tissue building. When an adult becomes emaciated he requires more tissue building material than the normal adult, his need for it being practically the same as that of a growing child.

We give below a table showing the average number of food units or calories required daily by people of various heights and weights. This table is one drawn up by Dr. J. H. Kellogg, Superintendent of the Battle Creek Sanitarium. In calculating the number of calories required in a given case, the estimate should be based not upon the actual weight of the individual, but upon the weight of the average person of his height.

“Persons who are in good health,” says Dr. Kellogg, “and find their weight somewhat greater than the figures given in the table, should not necessarily consider themselves obese. While above the average in weight, their condition is probably natural, and no attempt should be made to reduce the weight to any considerable amount, as injury may result. The average for adults applies especially to healthy adults between twenty and thirty years of age. Most people who are above forty years of age have a natural tendency to increase of flesh, which requires no attention unless it becomes excessive. Any reduction in foods made by an obese person should be in carbohydrates rather than in proteids or fats, unless these latter have been taken in excess.”

Table No. 1

Showing for different ages the average height, weight, and the number of food units or calories required daily.

Boys

Girls

Men

Women

PRACTICAL APPLICATION OF DIETARY RULES

While dieticians have ascertained the number of food units daily required by the average person, yet on no point do they reach more thorough agreement than in saying that the average person should not establish any hard and fast rules as to the quantity and kinds of food he consumes. It is really only an invalid, one who is in a physician’s care, who needs to have his food regulated in this precise fashion. The average person should be careful to practice thorough mastication, and should see to it that the proteid part of his meals is not excessive, but he should avoid worrying about his food habits. Any person who fusses and fumes about the kind of foodstuffs and the number of calories they contain, will be apt to cause himself harm; for science has proved by laboratory experiments, which we shall describe later on, that worry, in fact any of the unpleasant emotions, exercises a prohibitive effect upon the flow of digestive juices.

The really important thing to do is to follow a simple dietary, which at the same time is well balanced in its food elements, well cooked, and tastefully served. The housewife will see to it that the foodstuffs she chooses represent more of carbohydrates and fats than of proteids; her guiding rule in this matter being that the proportion of proteids to the other food elements be ten per cent. The United States Department of Agriculture has prepared a list of foodstuffs, comprising all those in common use, which shows the proportion of their constituents, and their total energy value, in calories, per pound of material.

This is “Bulletin No. 28, Revised Edition,” the work of two of the leading physiological chemists of America, W. O. Atwater and A. P. Bryant; and may be had on sending five cents to the Department. We have inserted in the Appendix a selected list of foodstuffs taken from this publication; and we give here a rough classification of foods, from which one can see at a glance their leading elements:

Foodstuffs which are Rich in Proteids

• Eggs
• White of Egg
• Skimmed Milk
• Buttermilk
• Yogurt
• Cottage Cheese
• Nut Products

Foodstuffs which are Rich in Fats

• Butter
• Nut Oils
• Olive Oil
• Cream
• Olives
• Nuts (except chestnuts)
• Egg Yolks

Foodstuffs which are Rich in Carbohydrates

• Potato
• Rice
• Breads
• Cereal Preparations

Pure Carbohydrates

• Fruits (raw and cooked)
• Fruit Juices
• Fruit Jellies
• Honey
• Malt Honey
• Marmalades

Foodstuffs which are Rich in Proteids and Fats

• Nuts
• Nut Butters
• Eggs
• Cheese
• Nut Products

Foodstuffs which are Rich in Proteids and Carbohydrates

• Peas
• Beans
• Lentils
• Chestnuts
• Skimmed Milk
• Gluten Preparations.

Foodstuffs which contain all the Food Elements in Fairly Good Proportion

• Crackers
• Batter Breads
• Pastry
• Malted Nuts
• Custards
• Puddings
• Salads
• Sandwiches
• Soups (other than meat or fish soups).