The process of utilizing food in the building of living tissues, and maintaining them in a fully nourished state with the breaking down of tissues incident to living, is spoken of as metabolism. Since heat is liberated in this process we may think of metabolism as running parallel with the heat production in the body always present but varied with activity, the types of food eaten, and the physiological differences to which the individual body is subject.
The burning of wood in a stove may be compared to the chief changes in metabolism. A stove that is closed tight cannot burn its fuel. Heat production is dependent upon the amount of draft or fresh air containing oxygen that passes through the burning mass. The oxygen from the air unites with the wood in the process of burning. Carbon dioxide and water vapor are formed and pass up the chimney as smoke. The ashes remain in the stove.
The burning in the body produces carbon dioxide and water in the tissues which are carried by the blood to the lungs where they are given up. Even at rest some internal activity continues. The heart never ceases beating and breathing continues regularly. The process in its entirety is similar, only the burning in the body takes place at a much lower temperature. For a given substance the amount of heat or energy liberated is constant.
Energy is necessary to maintain the regular functions of the body. As used in the human machine, just as in a piece of machinery, it has a heat equivalent. The unit of heat, which may be thought of as a unit of energy, is called the “calorie.” The calorie may be defined as the amount of heat needed to raise the temperature of one liter (approximately one quart) of water through one degree Centigrade (9/5° F.). This is the large calorie as distinguished from the small calorie used in many scientific computations which is only 1/1000th of this value.
There are three groups of calorie-yielding food substancesthe proteins, carbohydrates, and fats. A balanced diet however, requires that minerals, vitamins, and water be included in the diet, otherwise the processes of metabolism will soon become disordered.
PROTEINS
The proteins, as we have already pointed out, are those foods which contain nitrogen. Absorbed as amino acids after digestion, they are taken up by the blood stream and carried to the liver. Some are passed on into the systemic circulation for repair and growth of cells and tissues; the remainder are deamidized in the liver, i.e., nitrogen in the form of ammonia is broken off and glucose is formed, which if not immediately utilized may further be stored as glycogen or changed to fat. They are not stored for any period of time, but are utilized almost immediately in one of the ways mentioned above. The energy value per gram of protein is four calories. They are not thought of chiefly as calorie-yielding foods.
About two hours after eating a large meal of protein, there is a marked increase in the exchange of gases through the lungs. This continues for from two to four hours. Other foods such as fats and carbohydrates cause only a slight increase. This increase of metabolism accompanying the assimilation of proteins is often referred to as the specific dynamic action of proteins.
CARBOHYDRATES
Carbohydrates of all digestible forms are absorbed as simple sugars, chiefly glucose, which has a caloric equivalent of four calories per gram. These are carried to the liver and thence circulate in the blood stream. What is present above a normal amount is stored in the liver and some other tissues, as glycogen or body starch. When a need for sugar exists, this is changed back to glucose. Under severe stress, adrenalin, a secretion of the adrenal glands, may speedily change some of the glycogen into glucose, available for conversion into energy immediately. The human body has strange powers in changing sugars. Glucose, fructose, and galactose are stored as glycogen, but when glycogen is broken down for use only glucose is formed. Where lactation is present, glucose is again transformed into galactose and combined with glucose so as to form the sugar of milklactose.
In muscular activity lactic acid is formed from glycogen in the muscle. If this accumulates a sense of fatigue results. But when oxygen reaches the muscle about four-fifths of the lactic acid changes back to glycogen, and the. other fifth is oxidized, and finally excreted as carbon dioxide and water. The’ end products of the oxidation of glucose are carbon dioxide and water which are excreted from the body by the lungs, kidneys, and skin.
FATS
Fats are stored sources of energy. The several fats found in human diets are broken into glycerine and fatty acids in the process of digestion. As these pass through the cells covering the villi of the intestine they are resynthesized into the depot or storage fat characteristic of the human organism. The process by which fat is broken down in liberating its energy is more complicated than that present in the oxidation of carbohydrates. The end products, however, are the samecarbon dioxide and water. In some diseases, such as diabetes mellitus when poorly controlled, and during starvation, the appearance of toxic substances in the incomplete oxidation of fats are sometimes observed. These belong to the chemical group of ketones, and when found a state of ketosis is said to be present. Fats supply certain vitamins which become available for body use following digestion and absorption. Because of this, they are essential in the diet, but relatively small amounts are needed. More heat is liberated from fats than from either proteins or carbohydrates, one gram delivering slightly over nine calories.
CALORIC REQUIREMENT
These three groups of food substancesproteins, carbohydrates, and fats, are the sources of energy. The total amount needed in average living conditions includes the energy needed when the body is at rest, plus that needed for body activity, plus a small amount consumed in the utilization of food by the body.
If it were possible for us to measure all of the heat given off by the body during the day we could determine just how much food one should eat. If a man were placed in a room insulated against outside changes of temperature, this could be determined. Rather elaborate equipment must be planned for such observations. When the amount of carbon dioxide given off is known, also the amount of them on. On a mixed diet it is estimated that approximately 10 per cent of the basal requirement should be allowed for this purpose.
The following tables will give a general statement of the total daily caloric needs for children and adults:
TOTAL ENERGY REQUIREMENTS FOR CHILDREN
FOR 24 Hours.
Age Total Calories
Per Day 1% to 2 years ………… 1000-1200 2 to 4 years ………….. 1200-1400 4 to 6 years ………….. 1400-1600 6 to 8 years ………….. 1600-1800 8 to 10 years ………….. 1800-2000 10 to 12 years ………….. 2000-2200 Adolescent youth 2200-5500
TOTAL ENERGY REQUIREMENTS FOR ADULTS
FOR 24 Hours Total Calories per Day Men Women Resting 1200-1500 1000-1200 Very light work 1500-2000 1500-1800 Light work 2000-2500 1800-2200 Mod. heavy work 2500-3000 2200-2500 Heavy work 3000-3500 2500-3000 Hard labor 3500-4000
MINERAL METABOLISM
While minerals are not energy producing foods, they are closely involved in the transference of oxygen in the body and the removal of certain wastes. They are not in isolated form, but exist in simple chemical combinations or in union with organic substances.
7 per cent for each increase or decrease of one degree Fahrenheit above or below the normal level. This is one of the reasons why patients lose so much weight during long-continued fevers.
Glandular and Nervous States. The rate at which oxidation takes place is often influenced by these. One who is afraid, apprehensive, or worried commonly uses more oxygen. Varied rate of oxygen consumption often is determined by glandular states, and where the above factors are constant the abnormality in readings becomes a measure of abnormality in these conditions.
In states of under nutrition, and where insufficient food is eaten, the metabolic rate may be decreased 15 to 20 per cent. The use of excessive amounts of food may be accompanied by some increase in oxidation at the time. Ultimately if weight is added it in turn will modify the readings.
ACTIVITY
Above the basal or bed-rest needs of the body, the food requirements increase in proportion to the physical activities of the individual, modified, of course, by such external factors as climate, clothing, and housing.
It has been estimated that the energy needed for work varies from 20 to 100 per cent above the basal requirement. An estimate of 50 per cent increase for ordinary work, and 75 per cent for heavy work may be suggested. Activity in children’ of school age may be such that two or three times the basal requirements are needed.
INFLUENCE OF FOOD
The digestion and assimilation of food may be thought of as types of work, and some energy is used in carrying oxygen taken in, an indirect estimate of one’s needs can be computed because definite amounts of oxygen are needed to burn one gram of protein, carbohydrate, or fat.
BASAL REQUIREMENT
About twelve to eighteen hours after eating, the body is free from any specific dynamic effect of food, and digestion of the common ingredients is complete. If the rate at which oxygen is then used while at rest can be determined we would know how much energy is needed to keep active the body functions necessary for life. Such a reading would be a minimum or base requirementwe call it the basal metabolic rate. This basal rate will vary with normal individuals, but the variations will follow definite rules, being influenced by surface area, age, sex, body temperature, and glandular and nervous states.
Surface Area. The most important variation is due to differences in the surface areas of our bodies. The heat loss from the body is definitely proportional to the surface area, and is not proportional to the weight as was for a long time supposed. Every one knows that a cup of hot liquid will cool much more quickly when poured out in a thin layer than it will if left in a cup. The fact that all objects cool in proportion to their surface areas has been known to students of physics for many years. That this applied to the animate body as well was not appreciated until 1901 when Voit summarized the basal rate determinations that had been made on man and certain of the lower animals. His comparison of the basal rate determinations made on a man and a mouse illustrate this very well. The man ” weighing 64.3 kilograms, was found to use 32.1 calories per kilogram of body weight and 1042 calories per unit of surface area, while the mouse weighing 0.018 kilogram, used 212 calories per kilogram of body weight and 1188 calories per unit of surface area. Thus, in proportion to weight, the mouse used about seven times as much food as the man, but in proportion to surface area, it used only about 10 per cent more than the man. Such observations indicate why tall individuals need more food than do short persons of the same weight.
The unit of surface area used is the square meter. In the earlier work the surface area of the body was calculated from numerous measurements, but later it was calculated from measurements made by means of an exactly fitting adhesive tape coating which could be cut off and laid flat. From such determinations formulas have been arrived at and charts made whereby from the height and weight of an individual the surface area of the body may now be calculated quickly with but a small percentage of error.
Age. Young people need more food per unit of surface area than old people do. Thus, a child of five years of age requires sixty calories per. square meter of surface area each hour, while a person eighty years of age needs but thirty calories per square meter of surface area per hour.
Sex. Females require less food than males do under the same conditions. Du Bois has shown that between the ages of twenty and thirty the hourly requirements of a female amount to 37.0 calories per square meter of surface area, while those of a male are 39.5 calories per square meter.
Iron. The red coloring matter of the blood, hemoglobin, is an iron containing compound, which forms a loose chemical union with oxygen in the lungs carrying it as the blood circulates to all the tissues. There oxygen is given up, and carbon dioxide taken on as the blood flow returns to the lungs. Insufficiency of iron will result in a lessened content of hemoglobin in the red blood corpuscles, and the supply of oxygen to the tissues will be deficient, which generally results in a depression of oxidation throughout the body.
Iodine. Iodine is intimately connected with the rate of oxygen exchange. Its effect is not direct but is accomplished through the thyroid gland in the production of thyroxin.
Carbonates. Sodium, potassium, and magnesium enter into the composition of a group of carbonates found in the blood stream, which are part of the means by which carbon dioxide is eliminated. The action of all the carbonates may be illustrated by sodium carbonate (commonly known as washing soda). In the plasma or fluid portion of the arterial blood as it is carried to the tissues, a small amount of sodium carbonate is present. As this comes in contact with the carbonic acid which has resulted from oxidation it is changed into sodium bicarbonate, which flows with the venous blood back to the lungs. There the carbonic acid is released to be exhaled as carbon dioxide and water, leaving the original sodium carbonate free to be carried by the blood stream back to the tissues for another load. The proper elimination of carbonic acid from the body in this manner is so vital that should breathing be prevented for even a few minutes, death would result.
A lack of sufficient carbonates probably forms one of the most serious diet errors in athletic training. Muscles are trained for endurance, the heart is trained to pump large amounts of blood, and the lung capacity is in-creased for adequate breathing, but little attention is paid to the actual chemistry of breathing. Training diets should contain ample amounts of fruits and vegetables which contain these compounds. Some of the heretofore unexplained athletic records made by vegetarians may very well be due to this factor.
Phosphates. The phosphates, especially those of sodium and potassium, are used by the kidneys in maintaining the proper degree of alkalinity of the body. The sodium phosphates may be used to illustrate the chemical reaction that takes place. There is an alkaline disodium phosphate which contains two atoms of sodium to one of hydrogen, and an acid sodium phosphate which contains one atom of sodium to two atoms of hydrogen. When diets are too acid the kidneys maintain the alkaline balance of the body by changing large quantities of whatever alkaline disodium phosphate there is avail-able into the acid sodium phosphate and sodium. They excrete the acid sodium phosphate and return the sodium, which is alkaline, to the blood stream.
The alkaline phosphates are present in abundance in fruits, vegetables, and milk. The acid phosphates are found in cereals and meats.
Under normal conditions the sugars, starches, fats, and the major portion of the proteins are burned to carbonic acid. This is a respirable acid in gaseous form, and is automatically eliminated with each respiration as has been explained. It is produced in large quantities and has no effect upon the alkaline balance of the body as long as it can be promptly eliminated. Carbonic acid is not eliminated as it should be in pneumonia, for the functions of respiration are interfered with by the lung condition. In serious heart diseases the venous blood retains the carbonic acid as it is not adequately circulated in the lungs.
Acid-Base Balance. In addition to the elimination of carbonic acid through the lungs, the elimination of non-respirable mineral wastes is accomplished through the kidneys, which contributes to maintaining the normal chemical balance of the body. The principal alkaline mineral elements found in foods are sodium, potassium, calcium, and magnesium, and the acid elements are phosphorus, sulphur, and chlorine. If the correct proportions of these elements are absorbed from the digestive tract, the normal alkalinity of the body is readily maintained.
Acidosis is a word that has been widely used, but not always wisely. It was originally used to describe a condition seen in diabetic patients when acetone was found in the urine. It is now known that this substance may occur in starvation and wasting states, and where one is purposely fed a diet very high in fat constituents. It is due to an incomplete oxidation of fats, acetone bodies (chemically classified as ketones) being a product of this partial process. A better term in view of our present knowledge is “ketosis.” This leaves the word acidosis to be applied when the alkaline elements of the body and tissues are depleted below normal levels, a condition that can be truly diagnosed only with a careful study of the ‘ body chemistry. The chemical reaction of the blood may vary somewhat, but during life it is always alkaline. The reaction of tissues may vary more than that of the blood as the exchanges of metabolism are prompt in some, and in “distant” organs they are slowed to a marked degree. The delicate internal mechanism of the body preserves the alkalinity by means of natural chemical constituents of the blood which serve as buffers, which bind up any excess of weak acid.
In the Research Department of the Cottage Hospital, Santa Barbara, California, a series of rabbits on a diet consisting essentially of barley, and another series of rabbits on a diet consisting essentially of alfalfa hay were studied. There was no significant change in the chemical reaction of the plasma or in the ability of the blood to combine with carbon dioxide in either group for a period of nearly two years. Throughout the experiment the rabbits on the barley diet passed a fairly acid urine, and the rabbits in the alfalfa group passed an alkaline urine. In the rabbits on the barley diets, all of which died during the course of the experiment, fatty changes in the liver and changes in the kidneys were observed. The rabbits maintained on the alfalfa when killed at the end of two years, showed no pathological changes in the post mortem study. Actual decrease in the measurable alkalinity of the blood did not occur until a few days before death. The surroundings of these two groups were the same, and the diets supplied them were identical except that in the first case a portion of barley was used, and in the second a portion of alfalfa hay. The barley was rich in acid forming minerals, while the alfalfa was rich in alkaline or basic elements. Experiments with similar results have been repeated here, and done also by workers elsewhere.
The predominance of acid or alkaline minerals in foods has led to their being classed as acid-ash or alkaline-ash foods. In a few instances there is an exact balance and such a food is classed as having a neutral ash. Lists of foods showing the net ash content were originally pre-pared by Sherman and Gettler, and tested in man by Blatherwick. The taste of food is not a guide to the re-action of its ash. Lemons, grapefruit, and oranges which taste acid, and are acid because they contain citric acid, have an alkaline ash. Their use as food is commonly followed by the appearance of an alkaline urine. Bread, which tastes neither acid nor alkaline, has a high acid-ash.
In general, the fruits, vegetables, milk, and milk products have an alkaline-ash. Meats, fish, fowl, eggs, and cereals have an acid-ash. Concentrated fats and sweets are neutral-ash foods. It seems to us that the alkaline-ash foods should always be used in sufficient quantities to maintain a urine that is close to neutral in reaction. The early death of experimental animals kept for long periods of time on acid-ash dietaries should lead us to consider the mineral balance of our diets more carefully, at least until the subject is more fully understood.
VITAMINS
Vitamins are necessary for life, but their effect on metabolism is probably indirect. Vitamin A is directly concerned with growth and assists in combating infection; vitamin G has to do with the processes of blood building, and so is intimately concerned in oxidation. Vitamin D contributes to establishing calcium in the body, which was mentioned above. Vitamin B has a good tonic effect on the appetite, and so indirectly has much to do with proper food intake.
Water serves as a solvent and vehicle in the processes of the human body. Oxidation of the energy producing foods leads finally to the production of carbon dioxide and water. Evaporation of water from the surface of the skin is important in regulating body temperature, in which variations affect the general rate of metabolism.
ROLE OF ENDOCRINE GLANDS
In any study of metabolism the possible influence of the glands of internal secretion comes up for consideration. These glands, often called the endocrine glands, are not similar to each other so far as anatomical structure goes, but they are similar in having effects upon the functions of the body out of all proportion to their size, and in having interrelated effects on the activity of each other. Their products are secreted directly into the blood stream giving them wide possibilities in modifying the physiology of the body.
The pituitary gland located in a bony pocket beneath the brain is about the size-of a large green pea. It produces a great variety of active physiological agents. , The anterior portion produces a number of hormones, two of which are fairly well understood. The growth hormone regulates the growth of bone and to a degree other body tissue, the individual being large or small as this hormone is produced in quantity or in scant amount. The sex hormone influences sex development, an excess of secretion resulting in early development and a deficiency being accompanied by delayed maturity. With sexual development there are marked changes in the physiology of the human body, but the exact relation that the sex organs may bear to metabolism is not clearly worked out. The posterior portion of the pituitary gland also produces hormones. A decrease in one of these results in the development of the so-called pituitary type of physical buildthe skin becomes smooth, sexual activity is lessened, fat is deposited about the torso, the knees are often close together with the ankles apart. Mentality, how-ever, remains good. A deficient activity of this part of the gland is sometimes accompanied by the production of enormous quantities of urine with marked thirst, as observed in the disease, diabetes insipidus.
In the minds of many, the thyroid gland regulates metabolism. This probably is not altogether true. It is believed now that the pituitary has some degree of regulatory effect on the thyroid.
The thyroid gland is located in the lower front of the neck. Its growth and subsequent activity are dependent upon the amount of iodine available. It has a close relation to the growth and ossification of the bones in child-hood. A tendency to late calcification may be helped by giving suitable thyroid extract.
The rate at which oxidation takes place in the tissues is intimately connected with the proper function of the thyroid. When the gland is underactive oxygen is utilized slowly, and the individual is likely to be over-weight. If a shortage of secretion is marked in infancy the mentality is dull and body growth is retarded. The size of the thyroid is not a true indication of its functioning ability. Often an increase in its size would appear to be the result of an effort to compensate for an inadequate iodine supply.
There are instances in which the thyroid becomes over-active and metabolism is greatly increased. Nervousness, profuse perspiration, increased heart rate, and loss of weight are observed. Medication in which some form of iodine is a part may relieve the symptoms. Or tumors may form in the thyroid influencing its activity. Surgery by which more or less of the gland is removed is perhaps necessary in these instances.
Other glands such as the pineal, thymus, and parathyroids doubtless effect the functioning of the body, but they are not well understood. Adrenalin, a hormone from the medulla or inner part of the adrenal glands, is liberated when stress or strain is experienced; and glycogen or body starch is released from its storage for use. Blood pressure is raised in the presence of an increased output of adrenalin. Insulin, produced in the pancreas and possibly other tissues of the body is essential in carbohydrate metabolism.
The study of the functions of the endocrine glands is fascinating. With the new knowledge which has been gained in recent years concerning them, many former ideas have been changed or abandoned. As more is learned of the close interrelation of function that exists among the various organs of the body we expect to under-stand better the economy and efficiency seen in its metabolic processes.