It is impossible to decide intelligently how the money available for food shall be distributed among different food materials without understanding something of the composition of these food materials, and of the relation of food to the needs of the body. Experience has taught us many things, but the accumulation of experience needs interpretation by definite scientific knowledge. Until lately this knowledge was in the hands of only a few, and even then in so indefinite a form that it was not available for the housekeeper, no matter how well trained, and hardly for an educated physician.
Much progress has been made, but even to-day the housekeeper is often a little slow in availing herself of the knowledge she needs. This is partly because of the common feeling that what our fathers and mothers knew is enough for us, and partly because so much of the information is still locked up in more or less technical books, and the ordinary housekeeper, even though she be well educated, has not the key. It is to furnish the key to some of this knowledge that this series of lessons is written.
We all know in a general way that food nourishes us and makes us strong. But when we try to interpret this general idea into specific terms we find that we do not realize its meaning. Nothing is in the strict sense a food unless it performs at least one of three functions, (1) that of building the body, (2) furnishing heat, and (3) giving power to work.
The first function of food, that of building the body, is exercised not only in the growing child, where the material that can be transformed into bones and muscles, blood and nerve tissue, must be furnished by food, but in the adult, since even after growth has ceased, the constant waste of the body tissue must be repaired by food. So far as this function is concerned, the composition of the body must determine to a great extent the kind of material that may be used as food. It is easy to see that the body can be built only by foods containing the same elements, and that the proportion of these elements must bear some relation to their proportion in the body. It is reasonable to expect that the elements are combined in food in a way similar to that in which they are combined in the body.
The body of a man of average weight has been estimated to contain the following amounts of the various combinations known as the proximate principles:
It will be judged from this that so far as the organic food principles proteid, carbohydrate and fat are concerned, proteid holds the chief place as a tissue former.
Not only must the body have its actual material furnished by the food, but from this also must be derived its energy.
ATWATER'S RESPIRATION CALORIMETER.
A Man Lives in the "Box" for Days and the Actual Heat and Energy Obtained from the Food Consumed is Determined. (See page52).
The two forms of energy with which we are especially concerned in our study of the body are heat and power to work.