Chapter 2: The Constant Need of Food


The light and heat of the sun playing on the green leaf of the plant cause carbon dioxide and water to unite to form sugar. Heat is absorbed in the process and oxygen is given off to the atmos­phere. If one gram of sugar be placed in a very strong, closed steel receptacle into which oxygen, under a pressure of 450 pounds to the square inch, is conducted, and then if the sugar be kindled by an electric spark it will be completely burned to carbon dioxide and water and exactly the same quantity of heat will be liberated as was obtained from the sun in the original manufacture of the substance. If the steel receptacle be placed in a liter of water in which a thermometer has been put it may be noticed that the temperature of the water rises nearly 3.75°. After making certain corrections, it may be proved that 1 gram of glucose, when oxidized, yields heat sufficient to raise one liter of water 3.755° C. Since the measure for heat is a calorie or that quantity of heat required to raise 1 liter of water 1° C, it fol­lows that 1 gram of glucose yields 3.755 calories of heat. This apparatus measures the heat of combus­tion of organic substances like sugar, starch, meat, fat, etc., and is called a bomb calorimeter (a measurer of calories). The calorie or heat unit is as much an exact value for measurements of heat as are a quart or a pound for measurements of volume or weight.

When protein is burned in the bomb, the nitrogen of it is converted into nitric acid, but when protein is burned in the body, its nitrogen is not oxidized but is eliminated in the form of urea, so the heat produced from protein in the body is always less than that measured by the bomb. Sugar, however, yields the same products in the bomb and in the body and, therefore, the amount of heat produced is identically the same, no matter where the oxidation takes place. The same is true of fat.

One gram of the ordinary food stuffs when oxidized in the body yields the following number of calories:

Glucose ................................. 3.755
Cane sugar .............................. 4.0
Starch .................................. 4.1
Fat ..................................... 9.3
Protein .................................. 4.1

It has been said by some that they never will be converted to the belief that a knowledge of calories in nutrition is valuable. These persons must be reasoned with and persuaded to listen and they cannot then but be convinced.
The law of the conservation of energy holds that power cannot arise from nothing. Power must be derived from some store of energy, from energy which is potential. The store of power in the food stuffs is liberated when they are oxidized in the body. This power becomes the source of the mo­tions of life, and in the resting organism is finally liberated as heat. If this be true, then if one can measure the quantity of protein, fat and carbo­hydrate (sugar) oxidized in an animal in twenty-four hours one can calculate the quantity of heat which will arise from this process. If, at the same time, the animal can be placed in a calorimeter, which measures the heat actually given off during the period, the two computations should exactly agree.

To Rubner belongs the glory of being the first to have demonstrated this truth.

Calorimeters have since been constructed to measure the heat production in man. The labors of Atwater, Rosa and Benedict have confirmed the application of the law of the conservation of energy to man. The oxygen intake and carbonic acid outgo give a measure of the oxidation of the food stuffs, and the heat given off by the body is found to be equal to the quantity of heat which would have arisen from the oxidation of just that quantity of protein, fat and carbohydrate estimated to have been destroyed.

In a calorimeter built for the Russell Sage Insti­tute of Pathology in Bellevue Hospital, it has been found that if a definite amount of alcohol be burned in a lamp within the apparatus, the heat measured during a four-hour period is exactly that amount which the theory would call for.
Theory Found
212.57 211.88

Doctors Du Bois and Warren Coleman have dis­covered that a typhoid patient, during a period of five hours of rest in this calorimeter, produced the same number of calories as were calculated he should produce from the materials which were oxidized in his body.

Theory Found
422.59 419.78

Contemplation of such a result as this drives home the fact that if this typhoid patient is to be kept from losing his own body muscle and fat, he must be given the equivalent of 422 calories in food substances during a five-hour period.
If one measures the hourly heat production of a normal resting man, one must be convinced of its constancy. The human furnace requires a certain quantity of fuel to support the activities of life. Measurement of the total heat production, there­fore, becomes a measurement of the intensity of the life processes.

The quantity of heat produced by mammalia of the same size is fixed and definite and can be closely predicted in advance. It is not dependent on the weight of the animal nor upon the relative size of the individual cells. Thus, the size of the cells which make up the substance matter of a mouse is not very different from the size of the cells of the horse, yet a mouse produces 452 calories per kilo­gram of body weight in 24 hours and the horse 14.5 calories. The mouse requires thirty times more food per unit of body weight than the horse. How­ever, Rubner has shown that all well-nourished mammals produce the same number of calories per square meter of surface.

A normal man, well nourished, who is resting quietly in his bed in the morning, having been with­out food for fifteen hours, will manifest a minimum level of heat production. This level may be called the basal heat production. The following table has been prepared to show the constancy of energy pro­duction under these circumstances:

B. ... G. L. D. B. R. ...
C. ... J. R. . H. ... G. ... T. C.
Calories per hour
Weight per
in per square meter
kilos kilo surface
83 1.01 35.7
78 1.03 36.5
74 1.01 35.3
74 0.95 32.5
68 0.96 31.7
66 1.00 32.7
62 1.15 37.0
56 1.07 34.0
49 1.13 37.7

In the light of this exposition no educated man can say that he "does not believe in calories," when the energy in the food stuffs constitutes the basis of his being, and calories eliminated from his body are a measure of the sum total of his physical activities.

Food is the fuel of the human furnace, and must be furnished to that furnace in accordance with its needs.
The basal heat production of an average man weighing 156 pounds (70 kg.) will be 70 calories per hour or 1,680 calories in twenty-four hours. If food be taken extra heat is produced in the body. This extra amount does not exceed 10 per cent of the basal heat production or 7 calories per hour and 168 per day, so that the maintenance requirement of this man, resting quietly in bed, would be 1,848 calories in the daily diet.

Beyond this, the amount of fuel needed depends upon the quantity of mechanical work done. It becomes purely a matter of supplying fuel for the machinery.

It has been shown by Atwater and by Benedict that if a person sits in absolute quiet in a chair the heat production is 8 per cent greater than when he is lying on a bed. If, however, those ordinary move­ments are made which are associated with daily life when sitting in a chair, the heat production may rise 29 per cent, or from a basal level of 70 to a level of 90 calories per hour, an increase of 20 calories.

Since the influence of food is to increase the metabo­lism 7 calories per hour during twenty-four hours, and the influence of a sedentary life adds 20 more calories per hour during the 16 hours when a man is up in his chair, the total energy requirement would be:

Night (70 + 7)X8= 616
Day (70 + 7 + 20) X 16= 1,552

A hospital patient must be liberally fed when he receives this amount during convalescence. Addi­tional fuel in the food may be considered as a charitable contribution, a welfare fund for future use.

No normal man leading a life involving sedentary occupation, should live without exercise. Only this will keep his body in proper condition. One may attempt to calculate the additional energy require­ment needed for this purpose. To walk one hour on a level road at the rate of 2.7 miles requires energy to the amount of 160 additional calories. Therefore, if the man of sedentary occupation walks two hours daily to and from his business, 320 calories must be added to the 2,168 required to sup­port him without exercise, a total of 2,488. This figure is not far from Rubner's average allowance of 2,445 calories for such men as writers, draughts­men, tailors, physicians, etc.

It follows, therefore, that about 2,500 calories are required in the daily food of a man whose occupa­tion is of sedentary character. As a matter of fact, statistics show that the inhabitants of cities take this amount of fuel daily. The latest statistical proof that the food supply of a great city is regulated by the needs of its inhabitants may be found in the report of Gautier which shows that in Paris an average of 2,500 calories of energy are daily sup­plied to each inhabitant.
If the exercise taken be vigorous and include hill climbing, the quantity of energy needed will be greater than when a level road is traversed. To climb on a path at the rate of 2.7 miles an hour so that the summit of a hill 1,650 feet high is attained during the hour, requires 407 extra calories.

An expert bicycle rider at hard work has indi­cated an increase in oxidation corresponding to 529 calories per hour (Atwater and Benedict).

The fuel requirement, therefore, depends upon the quantity of work accomplished.
The Kaiser Wilhelm Institut has recently granted funds to Rubner in Berlin in order to establish a special laboratory in which to determine the specific fuel needs of individuals engaged in various occu­pations and trades.
Computations of the diets of farmers show the following interesting similarity in the fuel values of their food:

Farmers in Connecticut .................. 3,410
Farmers in Vermont .................... 3,635
Farmers in New York ................... 3,785
Farmers in Mexico .................r___ 3,435
Farmers in Italy ........................ 3,565
Farmers in Finland ...................... 3,474
Average ............................. 3,551

These figures, representing the food-fuel con­tained in the dietaries of individuals in widely dif­fering communities but engaged in the same occu­pation, show a plus or minus variation of only 6 per cent from the mean average.
From the present available data one may estimate the daily fuel requirement of well-nourished adults after the following fashion:

Occupation Calories
In bed 24 hours ................................. 1,680
In bed 8 hours, work involving sitting in a chair 16
hours ...................................... 2,170
Bed 8 hours, in a chair 14 hours, moderate exercise
2 hours .................................... 2,500
Farmers ........................................ 3,500
Rider in a six-day bicycle race................... 10,000

It is apparent that the great numbers of men em­ployed as clerks or those employed in watching machinery require about 2,500 calories in their daily food.
A boy of twelve requires about 1,500 calories daily. A baby when first born requires 100 calories per kilogram of body weight per day and later about 70. Many cases of reported chronic malnutrition of infants are in reality due to persistent undernutri-tion carried out in ignorance of the proper amount of food required by the child.
In fever, the production of heat may be 50 per cent above the normal. In cases of hyperthyroidism (Graves disease) even greater increases have been observed, whereas in hypothyroidism (myxoedema) the heat production falls below the normal. It fol­lows, therefore, that increased nourishment is indi­cated in fever and in Graves disease whenever this is possible.
The great practical importance of food fuel in sufficient quantity for the human machine in health and disease warrants its consideration in greater measure than has heretofore been given it.

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