Food principles or elements are commonly grouped into the following classes:
- Proteins
- Fats
- Carbohydrates
- Inorganic salts
- Vitamins
- Water
- PROTEINS
The first class of food substances mentioned above are of very great of importance to the body. The term “protein” really means “of first importance”. These compounds are represented by such foods as the white of egg, lean meat, gluten of wheat and casein of milk. Chemically, proteins are very complex, more so than any other class of food materials. They have in their structure the chemical elements carbon, hydrogen, oxygen, nitrogen, often sulfur and phosphorus, and less commonly iron. The nitrogenous element seems to be the most important, since the others mentioned can be obtained from other classes of food; but as these classes of food cannot take the place of protein, it seems clear that the nitrogen is the important constituent.
Most proteins coagulate on heating. All illustration of this property is the coagulation of the white of an egg when the egg is cooked. The proteins all undergo decomposition quite easily. This is evidenced by the ease with which eggs and meat spoil.
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 materials 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 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 tissues 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 may be used 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.
