Most interesting is the study of the food as it passes through the processes of digestion, absorption, assimilation, and oxidation—all definite and important parts of the great cycle through which everything we eat passes on its way from the table to the tissues. Elimination is the last step in nutrition, and is the process by which the body rids itself of the broken down cells and other poisonous and useless wastes. These various phases of bodily nutrition may be expressed in a single term—metabolism.

What we eat and how much we eat must be carefully planned, for our body temple is really made of what we eat. If you were erecting a beautiful mansion you would not think of allowing cheap, trashy, and inferior building materials to enter into the construction of your home. Neither should you permit unfit and inferior materials to become a part of the daily dietary of your little boy or girl, thus to become a part of their bodily structure.

ASSIMILATION OF FOOD

Following the process of digestion in the stomach and intestine, the nutritive food elements are absorbed through the wall of the bowel by the wonderfully adapted little villus, and distributed by various routes to the uttermost parts of the body. The sugars (all starches are changed into sugar) are carried in the portal blood stream to the liver, where they are actually stored away in the form of glycogen which, in a most intelligent manner, is dealt out to the body from hour to hour as it is needed for fuel. If all the sugar, after a hearty meal, were poured into the circulation at once, the blood stream would be overwhelmed and the kidneys would be forced to excrete it in the urine. This unnecessary waste is avoided by the liver's storing sugar after each meal and dealing it out to the body as required.

Likewise, the proteins also pass through the liver on their way to the body. Just what action the liver exerts upon proteins is not wholly known at the present writing. The digested fats are absorbed at once by the lacteals, the beginning of the intestinal lymphatic system, by which they are carried to the large veins at the root of the neck and there emptied into the blood stream. We have now traced our various food elements through the processes of digestion and absorption in the alimentary tract, some going through the liver, and others through the lymphatic system, until they circulate in the blood stream itself.

It is from these food substances, circulating in the blood stream, that the various cells of the body must assimilate into themselves such portions as they require for purposes of heat and energy and for the repair of their cell substance. This specialized work of cell assimilation converts the dissolved watery food in the blood into solid tissues, exactly reversing the process of digestion.

With a most profound intelligence, each of these body cells and tissues, bone and nerve fiber, muscle and organ, selects from the blood stream just its supply or portion of the food elements requisite to its upbuilding and maintenance. The mysteries of assimilation are effected by means of chemical substances called "enzymes," similar to those found in the digestive organs, but acting in an entirely different manner, in that they build up solids out of liquids instead of converting solids into liquids.

ELIMINATION OF BODY WASTES

Metabolism consists of a twofold rÔle—an upbuilding and a tearing down process. After the food is all digested, absorbed, and assimilated, having become a part of the bodily organ, bone, muscle, and nerve fiber, then begins the work of tearing it down—of liberating its heat and energy—to be followed by its elimination from the body through the sweat glands, uriniferous tubules of the kidneys, etc. The carbohydrates (starches and sugars), together with the fats, are completely burned up in the body and are then eliminated in the form of water (thrown off through the sweat) and carbonic acid gas given up by the lungs.

The proteins, or nitrogenous foods, are not so completely burned up in the body. The ashes which result from their combustion are not simple substances like the water and CO₂ of the carbohydrates. This protein ash is represented by a number of complicated substances, some of which are solid (protein clinkers), which accumulate in the body and help to bring about many diseases, such as gout, headache, fatigue, biliousness, etc.

These protein ashes and clinkers are further acted upon—split up and sifted—by the liver, and are finally eliminated by the kidneys in the form of urea, uric acid, etc. The body being unable to store up protein, is often greatly embarrassed when one eats more of this substance than is daily required to replenish the waste of the body, for it must all be immediately split up in the system, and the over-abundant and irritating ashes must be carried off by the eliminating organs. Now, the overeating of sugars, starches, or fats, is not such a serious matter, as they may be stored in the liver and subsequently used; and even if they are eaten in excess of what the liver can care for they accumulate as fat or add extra fuel to the fires of the body, their ashes being carried off in the form of such harmless substances as water and carbon dioxide (CO₂); but the overeating of protein substances is always a strain on the body and should be avoided.

ELEMENTS OF NUTRITION

There are seven distinct elements entering into the composition of human foods—protein, starch, sugar, fat, salts, cellulose, and water, not to mention enzymes, vitamines, and other little-known chemical principles. These elements are all variously concerned in the nourishment, energizing, and warming of the body.

PROTEINS

The proteins are the structure builders of the body. While starches, fats, and sugars may be compared to the coal that feeds the locomotive, the proteins represent the iron and steel that are used from time to time to repair the engine and replace its worn parts. The essential chemical difference between starch and protein is that the latter contains nitrogen and a small amount of sulphur and phosphorus. The most common forms in which protein is used for food are the glutens of the grains, the legumes, nuts, cheese, the white of egg, and lean meat.

STARCHES

The starches are by far the most abundant of all elements in human food. They enter largely into the composition of nearly all plants and seeds. Under the influence of the sunlight, the green-colored plants gather up the CO₂ of the air and, with the water absorbed from the ground, build up starch. The plant takes all the carbon from which starch is made from the air, but while the atmosphere contains almost eighty per cent of nitrogen, the plant is unable to use it; it must secure its nitrogen from the decaying refuse of the soil. Thus the plant utilizes the waste products found in air and earth in the building of its food substances.

Starch exists in the form of small granules. Since each little starch granule is surrounded by a woody envelope of cellulose, it becomes necessary to cook all starches thoroughly in order to burst this cellulose envelope and thus enable the saliva to begin, and other secretions to continue, the work of digestion.

FRUIT SUGARS

The sugar of fruits represents a form of food requiring practically no digestion; while the sugar found in beets, the cane plant, and the maple tree, must be acted upon by the digestive juices of the intestine before their absorption can take place. During the winter, the maple tree stores its carbohydrates in its roots in the form of starch. With the advent of spring Mother Nature begins the digestion of this starch—actually turns it into sugar—and in the form of the sweet sap it finds its way up into the tree trunk to be deposited in the leaves and bark in the form of cellulose, a process very similar to that performed by digestion in the human body, where starch by digestion is first turned into sugar, and afterwards deposited in another form in the liver and muscles.

Dextrine is a form of sugar resulting from thoroughly cooking or partially digesting starch. There are about twenty-five stages or forms of dextrine between raw starch and digested starch or fruit sugar. Dextrine is found in the brown-colored portions of well-toasted bread.

FATS

Fat is a combination of glycerine and certain fatty acids. As a food, it is derived from both the animal and the vegetable kingdom. Animal fat consists of lard, suet, fat meat, etc., while fat of animal origin is represented by cream, butter, and the yolks of eggs. The vegetable fats are found in nuts, especially the pecan, cocoanut, Brazil, and pine nuts; also in the grains, particularly oats and corn. The peanut also contains a considerable amount of fat. Of the fruits, the banana and strawberry contain a trace of fat, while the olive is the only fruit rich in fat.

As a food, fat is used in three forms. The emulsified form is represented by cream, olive oil, and nuts. When the tiny globules of fat, which are each surrounded by a little film of casein, are crushed—united into a solid mass—we have a free fat. This form is represented by butter and other animal fats. Another form is fried fat—fat which has been chemically changed by heat with the development of certain irritating acids.

MINERAL SALTS

The mineral elements comprise but a small part of human food as regards weight, but they are extremely important to the health of the child as well as the adult. As found in the food, they are not in the form of mineral salts, like common table salt. The salts of food are living salts, organic or organized salts, such as are found in the growing plant. These salts are of great value to the various fluids of the body, and also as stimulants to nerve action, but more particularly in the work of building up the bones.

Salts are found largely in the cereals. A small amount is also found in vegetables, particularly the potato, as well as in most fruits.

CELLULOSE

Cellulose represents the great bulk of all vegetables and fruits. It is digested by most animals, but in man it is digested only to the extent of about thirty per cent. The presence of a large amount of cellulose in the food enables us often to satisfy the appetite without injury from overeating. It serves to give bulk to the food, and thereby possibly acts as a preventive to constipation.

WATER

Water fills an important place in the nutrition of the body. The food changes in connection with digestion, assimilation, and elimination, can take place only in the presence of water. Water constitutes from fifteen to ninety-five per cent of the various foods. The watery juices of vegetables and fruits consist largely of pure, distilled water, in which fruit sugar is dissolved, with added flavoring substances. Water is absolutely essential to the performance of every vital function connected with human metabolism.

ANIMAL HEAT

The source of heat in the animal body was the subject of much superstitious speculation on the part of ancient scientists. It is now known that animal heat is derived from the food we eat by means of a peculiar process of vital oxidation—effected in the presence of oxygen—by the action of water and enzymes upon the food elements absorbed by the living cell. This process of oxidation liberates the heat and energy stored by the sun in the food, and thus the body is kept warm by this constant combustion of the digested foodstuffs. The starches and sugars, together with the fats, represent food elements which serve as the body's fuel. By this means we are able to maintain a constant body temperature of almost one hundred degrees.

The average human body produces enough heat every hour to raise two and one-half pounds of water from the freezing point to the boiling point. This is equivalent to boiling about seven gallons of ice-water every twenty-four hours. Differently expressed, the body gives off each hour the same amount of heat as a foot and a half of two-inch steam coil. This is the same amount of heat which would be produced by burning about two-thirds of a pound of coal.

FUEL VALUE OF FOODS

Expressed in terms of English weight, the fuel value of the three different food elements would be:

It will be observed that fat contains more than twice as much heat as the carbohydrates. This is due to the fact that fat contains more carbon than either starch or sugar. Next to fats, starches and sugars are the most important fuel elements. Protein is a very extravagant form of food for fuel purposes. Proteins are the most expensive elements of human food; they are incompletely burned in the body, and inasmuch as they leave behind distressing and disease-producing ashes, it is clearly evident that only sufficient amount of proteins should be eaten each day to supply the demand of the body for repairs. We should depend more largely upon the carbohydrates and fats for heat and energy.

A large part of our food is required to furnish heat to take the place of that lost by radiation from the skin, and this is why children require more food than adults—they have a larger skin surface in proportion to their weight, and therefore lose more heat by radiation, and it is for this reason that the food for the growing child must be wisely and carefully selected.

DIET FOR CHILD TWO TO THREE YEARS OLD

Breakfast, 7-8 a. m.: Fruit; cooked or toasted cereal served with thin cream; a soft boiled or coddled egg; bread (two or more days old) and butter; plenty of milk.

Dinner, 12-1: Soups; creamed vegetables—tomato, corn, peas, and celery; any two of potatoes—creamed, mashed, or baked—carrots, beets, spinach, peas, cornlet, squash, cauliflower, asparagus tips, string beans; protein dish—the puree of dried beans, peas, or lentils; macaroni or carefully selected meats; dessert—apples, baked or sauce—or other fruits, junket, custard, milk.

Supper, 5-6 p. m.: Fruit, bread (bran bread if constipated); milk; porridge, with rich milk or milk toast; sweetened graham crackers.

FOODS ALLOWED CHILDREN OF FOUR YEARS AND OLDER

Protein Dishes: Purees of dried peas; lentils; beans; macaroni; eggs—soft boiled, poached, scrambled, or omelette; meats—steak, chops, chicken, turkey, broiled fish.

Cereals: All the toasted-flake foods; toasted and not too fresh bread, including both graham and bran; hominy; corn meal; oatmeal; farina; rice; barley; tapioca; sago, etc.

Soups: Creamed vegetable soups of all kinds and broths.

Vegetables: Potatoes; all the small green vegetables; lettuce; stewed celery; beets; squash; cauliflower, etc.

Fruits: All, if stewed or baked. Raw fruits—pears, peaches, ripe apples, berries, oranges, persimmons, grape-pulp without seeds, etc.

Desserts: Custard; jellos; junkets; home-made ice cream; sponge cake; baked fruits with whipped cream, etc.

FOODS TO BE AVOIDED BY YOUNG CHILDREN

1. Doughy breads, griddle cakes, insides of muffins, hot biscuits, etc.

2. Fried meats, such as sausage, oysters, pork, ham, veal, salt fish, corned beef, dried beef, etc.

3. Foods that are hot when they are cold—such as catsup, horse radish, mustard, highly spiced pickles, sauces, etc.

4. Rich pastries, puddings, unripe fruit, salted peanuts, and highly concocted dishes.

5. Certain salads, containing coarse but easily swallowed foods, with highly seasoned sauces.

6. Tea, coffee, and all alcoholic beverages.

7. Soft candies, chocolate creams, bon-bons, patties, etc.

Average normal children crave sweets, and since their normal food is about seven per cent sugar it is not to be wondered at. There are many forms of pure, hard candies which may be taken by the three-year-old child. They are stick candy, fruit tablets, sunshine candies, and other varieties which may be sucked.

All soft candies, such as chocolate creams, bon-bons, patties, etc., are to be avoided. Hard candies, taken along with the desserts at meal time, in no wise injure the normal stomach of the healthy child.

The other members of the family should set a correct example by sucking the hard candies rather than chewing them; for if the hard candies are allowed to dissolve slowly in the mouth they produce a weak solution of sugar, which does not interfere with digestion as do the strong and concentrated sugar solutions which result from chewing chocolate creams, bon-bons, etc.

Candy, cookies, sandwiches, or bits of cake should never be allowed between the meals.

EATING BETWEEN MEALS

Children who do not eat well at the breakfast table, if given a "piece" at 10 a. m., will not be ready for the 12 o'clock meal; and then another "piece" at 2 p. m. interferes with the normal appetite at 6 p. m. Digestion is disturbed, the nervous system irritated, and a "puny child" is often the result.

Bring the three-or-four-year-old to a well-selected breakfast some time between 7 and 8 a. m. Then nothing—absolutely nothing—but water must pass the lips between that breakfast hour and the 12 o'clock meal, which should be a good one. Then the interval until 5 or 6 p. m. is passed in the same manner. At the evening meal the appetite is again whetted: and a good appetite always means good gastric juice to digest the meal. And so, good mother, guard carefully the interval between meals if you would have good digestion and good health for the little folks.

DAILY FOOD REQUIREMENT

The following table, taken from The Science of Living,^[4] shows the minimum of calories or food units required by boys from five to fourteen years of age and girls from five to twelve:

⁴ Sadler, William S., The Science of Living; or, The Art of Keeping Well. A. C. McClurg & Co.