In Lesson 6 we discussed various types of engines, and it was learned that engines do not create energy, but instead merely take energy in a form available for doing work, and convert a part of this into useful work. All of this energy is finally degraded into the unavailable form as waste heat. In the present lesson we wish to focus attention on a very remarkable engine that has not been previously discussed, namely, the human body.

Calories. A steam engine, as we saw, takes in coal and oxygen, and gives out, as products of combustion, water vapor, carbon dioxide, and cinders. Besides this it produces heat and work in driving the steam engine. In an analogous manner, the human body takes in food and oxygen, and gives out carbon dioxide, water vapor, and waste products. Besides this, heat is produced inside the body, and the body is enabled to do work. Human food is just as much a fuel as is coal or gasoline, or wood. The same kind of tests have been made to determine the heat value of food as were described in Lesson 5, to determine the heat value of coal, gasoline, etc. The apparatus that is used to determine the heat value of fuels is called a calorimeter.

The ‘calories’ contained in various kinds of food have become a household expression, but few people realize what is meant; what is actually meant is that food of a certain kind has been burned in a calorimeter and the heat produced by one gram of food has been carefully measured and stated in terms of kilocalories produced by one gram of food. Hence, the ‘calorie’ that one commonly hears spoken of in regard to food is a kilogram-calorie.

Heat Value of Foods. There are three fundamental kinds of food substances: proteins, carbohydrates, and fats. Chemically, a protein consists of carbon, hydrogen, oxygen, and nitrogen plus a small amount of sulfur and mineral matter. Both carbohydrates and fats are composed of carbon, hydrogen, and oxygen.

  TABLE 5    
Proteins ……………………………. 52.00%                7.00% 23.00% 16.00%
Carbohydrates …………………… 44.40%                6.20% 49.40%  
Fats …………………………………. 76.60%               11.90% 11.50%  

(Percentages in the above table are by weight.)

Examples of proteins: White of eggs, curd of milk, and lean meat.

Examples of carbohydrates: Sugar and starch.

Examples of fats: Fat of meats, butter, lard, and olive oil.

Most foods are a mixture of proteins, carbohydrates, and fats.




FOOD                                                                                                                         PER GRAM

Protein ………………………………………………………………………………………        4.1

Carbohydrates ……………………………………………………………………………        4.1

Fats …………………………………………………………………………………………..        9.3

On the average, in temperate climates, out of each 100 grams of food eaten, approximately 16 grams are proteins, 75 grams are carbohydrates, and 9 grams are fat. This food is taken into the body, oxygen in the air is taken in by breathing, and combines chemically inside the body with the food. Energy in the form of heat and work is released.

Food + oxygen → carbon dioxide + water + waste products + energy (heat and work).

The heat produced by 100 grams of this average diet would be about 457 kilogram calories, provided all of this were digested.  This provides us with a scientific way of rating human beings; we can rate them by the amount of energy they consume or degrade per day. Men, on the average, consume about 2,800 kilogram-calories per day and women about 2,000.  The average energy consumed per capita per day by all the people in the United States, young and old alike, is about 2,300 kilogram calories.

The significant thing about all this for our purpose is that it is possible to determine exactly how much energy is contained in various kinds of foods, and then after they are eaten to determine how much heat and work they can produce. This latter is accomplished by placing a man in a large heat-tight calorimeter and measuring very accurately over a given time period the amount of heat given off by his body. At the same time, the amount of oxygen he breathes, and the amount of carbon dioxide that he gives off, are also accurately measured. If the person is lying quietly and doing no work, it has been found that the heat given off in a given time is exactly equal to that contained in the food ‘burned’ or oxidized in that time.

By this manner it is also possible to determine how much work a given amount of food can be made to produce, or the efficiency of the human engine. This is accomplished by having the man turn a crank or pedal a bicycle attached to an instrument called an ergometer. The ergometer measures how much work has been done by the man; the calorimeter at the same time measures the heat given off. In this case it has been found that the energy represented by the heat given off and the work done by the man are exactly equal to the energy contained in the food ‘burned’ during that time.

Efficiency of the Human Engine. Remembering that the efficiency of any engine is determined by the ratio of the work done by that engine to the total energy degraded in a given time period, it is now possible to determine the efficiency of the human engine. The maximum efficiency of the human engine has been found to be only about 25 percent. Due to the fact that the human engine, while still alive, never completely shuts down, and therefore never ceases to degrade energy, the efficiency is zero when no outside work is being done; that is to say, when the body is at rest. This basic rate of consuming energy while at rest amounts on the average to 1700 kilogram calories per adult person per day.

When physical work is done the rate of energy consumption very rapidly increases. A strong man doing heavy physical labor can perform approximately 2,000,000 foot-pounds of work in a ten-hour day, or one-tenth of 1 horsepower for a 10-hour day. In order to do this he will require approximately 5,000 kilogram calories per 24 hours.

By way of contrast, work involving little physical activity, such as writing, or various kinds of desk work, involve very little energy expenditure. It has been found that the additional energy required for intense mental work amounts only to about 4 kilogram-calories per hour. In other words, the most difficult thinking requires additional energy per hour equal approximately to 1 gram of sugar or to one-half a peanut. Indeed, so small is the amount of energy required to ‘think’ that a housemaid engaged in sweeping and dusting the study of a college professor would expend as much energy in 3 minutes as the professor would expend in an hour of intensive study.

One frequently hears careless talk about ‘nervous’ energy, ‘mental’ energy, ‘creative’ energy and other such expressions, which imply not only that there are numerous unrelated kinds of energy, but that energy associated with the human body is different from energy as manifested in calorimeters and steam engines. It is also implied that human beings are somehow or other spontaneous sources of work or energy. From what has been shown in this lesson it becomes evident that all such expressions have no basis in fact and are therefore sheer nonsense. There is only one fundamental energy which, as we defined above, is the capacity to perform physical work.

Engines of any kind are not creators of energy; they are, instead, converters of energy from one form to another in exact accordance with the first and second laws of thermodynamics. The laws of thermodynamics are no respecters of persons, and they hold as fast and rigorously in the case of the human body as they do in man-made engines.

A human body takes the chemical energy from food and converts it into heat and work on a 24-hour basis. Rarely is as much as 10 percent of this energy converted into work. Consequently, in spite of anything we can do, man is a dissipater of energy and it is not possible for him by any amount of work he may do ever to repay the amount of energy that he required in doing that work.


  • The Chemistry of Food and Nutrition, Sherman.
  • The Exchange of Energy Between Man and His Environment, Newburgh and Johnston.
  • Living Machinery, Hill (Out of print).
  • The Science of Nutrition, Lusk.