Man would need to eat a large amount of nitrogen-containing plants in order to get as much protein as is contained in one egg or in a piece of lean meat the size of an egg. It is because the nitrogen is in such condensed form in meat and eggs that one is likely to take more than the system can handle, if he eats too freely of these two foods, particularly of meat. We will discuss this question more fully under “Proteins.”
Most domestic animals take their food elements from air and water, as well as from the compounds which the plants have formed, while wild animals and some domestic ones, such as hogs and chickens, make use of meat as well.
The greater part of muscle, nerve, and gland is composed of protein.
When the muscles are exercised constantly they use up their protein and must have it resupplied, or the muscle substance will waste. When the muscles are exercised freely, as in the laborer, or the athlete, they need more building material.
The skeleton is composed largely of deposited salts, as calcium. If, therefore, the growing child be not supplied with a sufficient amount of this substance, the bones will be weak and liable to deformity and the teeth will be slow in coming or will be small and malformed. Children need foods rich in lime.
The elements which supply heat and keep up muscular activity are starches, fats, and sugars.
It must be apparent to every thoughtful person, that, since the nerves, muscles, and glands are composed largely of protein and the skeleton largely of calcium salts, in order to furnish the body with the elements necessary for growth and repair, all of these elements, as also the energy-producing substances, must be provided.
Each individual, therefore, should learn how much he requires to replace his daily waste, both for rebuilding tissue and for supplying heat and energy.
The day laborer, though he may do more muscular work than an athlete in training, expends scarcely any nervous energy. Therefore he needs less protein in his diet than one does who expends both nervous and muscular activity, as does the athlete.