The most prominent component of the body tissues, the skeleton excepted, is protein. This substance is the basis of the muscles and glandular tissues, and overshadows all other organic compounds in relative abundance. Liebig believed that a muscle does mechanical work at the expense of energy stored in it in the form of protein. The energy was supposed to come from the change of the complex protein molecule into simpler products, analogous to the change of complex molecules of wood into carbon dioxid and water during combustion, the energy set free being capable of appearing in part in the form of work.

If this view were correct, the amount of protein which is broken down daily should be profoundly influenced by the amount of work performed. Modern investigations have, however, established that work is ordinarily performed at the expense of the energy of carbohydrates such as starch and sugars taken in the food, or from fat. This was first shown by Pettenkofer and Voit (1) in 1866. These investigators observed that even a fasting man showed no increased protein destruction as measured by the amount of nitrogen eliminated in the form of waste end-products in the urine. Instead of prodigally wasting his muscle substance (protein) he burned fat as a source of energy. Recent investigations have proven that following severe work there is an increased elimination of nitrogen (1).

For many years physiologists assumed that the tissues of the human or animal organism were not capable of synthetic transformations so as to give rise to the formation of complex organic compounds from simple ones. The building up of organic substances from inorganic compounds was believed to occur only in plants during growth. The logical inference was that the wear quota of the protein-rich tissues must be replaced by new proteins from the food. Early observations on fasting men and animals established the fact that even during periods of abstenance from food, tissue waste still goes on, and in the absence of protein in the diet the muscles and other tissues are used up. During a subsequent period of feeding, the lost muscle substance may be completely restored. One of the problems which early occurred to the inquiring mind of Carl Voit of Munich (2) was that of determining the amount of protein in the form of meat, which would suffice to prevent a nitrogen (protein) deficit in dogs. He did not actually determine the minimum amount of protein necessary to accomplish this, but instead, showed that with moderately large intakes of meat protein, the nitrogen which appeared in the urine as waste products of metabolism, plus that eliminated in the feces, was equal to that ingested in the food. In other words the body was in nitrogen (or protein) equilibrium.