Protein molecules are made up of smaller molecules called amino acids. These are the "building stones" from which the working tissues of the body are formed. There are on the average about fifteen different kinds of these amino acids in the proteins, which are especially valuable in supplying building material for the tissues of the human body. These amino acids are united in long chains to form the protein molecule, and in this respect can be compared to cars in a train. By the work of digestion, the proteins are broken down into these comparatively simple building stones, which, when absorbed into the circulation, are used by the body in building working tissues as they are needed.
There are a number of classes of proteins; but since the classification is rather complicated, it will not be given here. To group the various foods as to their relative amounts of protein is often of interest. For example, foods very rich in protein, such as the gluten preparations, lean beef, and white of egg, may be regarded as the first class; a second class might be formed of those which are moderately high in protein, such as peas, beans, lentils, and walnuts; a third class having a moderate amount of protein, represented by the cereals and breads; and still a fourth class very low in protein, such as vegetables and fruits.
Protein is the tissue builder of the body; but the actual amount of tissue built new each day is very small, therefore the need for a large supply of protein for this purpose is not apparent. Protein not only supplies tissue-building material, but it can also supply heat and energy in a manner similar to the other classes of food elements, carbohydrate and fat, one ounce of the protein yielding one hundred sixteen calories of energy. The excess taken in may be used in this way, as there is no storage of this material in the body. However, to use this kind of fuel takes more work on the part of the body as a whole, as the nitrogenous wastes must be eliminated by the kidneys.
One can see, then, that a certain amount of protein is needed to keep the tissues in good repair, but that it is better to get most of the heat and energy from the food elements specially designed for that purpose; that is, carbohydrate and fat.
The Chittenden standard of diet gives ten per cent of the total fuel value in the form of protein. On the basis of two thousand five hundred total calories a day, two hundred fifty calories of protein would be required. This is equal to two and one seventh ounces actual dry protein. This amount is thought by some to be low, but experimental evidence seems clearly to prove its adequacy in keeping up nutrition.
Fats
The second group of food elements in our classification are substances having a greasy feeling and taste. They are lighter than water, leave a grease spot upon paper, are insoluble in water, and soluble in such chemicals as gasoline and ether.
Fats have in their molecules the chemical elements carbon, hydrogen, and oxygen. These elements are put together into two groups, or compounds, glycerin and fatty acids, which, when chemically united, form a fat. When fats are exposed to the open air, and thus contaminated with bacteria, they are likely to become rancid; that is, some of the glycerin and fatty acids are set free from each other. If butter is the fat so decomposed, it becomes very disagreeable, on account of the volatile butyric acid that is set free.
Fatty bodies are usually grouped under a general heading called lipins, but the consideration of the other classes is not essential in this study.