Protein and amino acids

While the biochemists were exploring the vitamins, the physiologists were busy investigating the needs of the whole animal. Nearly two centuries ago the first measurements of the energy expenditure of animals were made, by enclosing them in an insulated chamber and observing the amount of ice melted by heat from their bodies. Lavoisier also measured the gaseous output of animals and men, and showed that more oxygen was used up after meals and exercise. Atwater and Benedict in 1899 were the first people to build a Calorimeter for measuring the direct heat output of humans. Their work was so precise and well documented that it is only in the 1970’s that scientists have thought it necessary to repeat and expand upon their original work. About one hundred years ago scientists also began to investigate man’s requirements of protein. In 1881 Carl Voit suggested that man needed at least 145 grams of protein daily to maintain himself in good health. His contemporary, Rubner, was so confirmed in this view that he advised the German government to direct their agricultural policy towards livestock production rather than cereal growing during the First World War. This may have contributed to famine and the eventual downfall of the German troops. Chittenden in 1909 showed that adult men survived in good health on only 40 grams of protein daily for periods exceeding a year. In 1920 Sherman concluded that the average of all published recommendations was 44 grams of protein per day for a man weighing 70 kilograms.

As early as 1915 Mendel suggested that there were two types of protein, one that supported growth and one that did not. This lead to the concept of primary and secondary proteins which has persisted to this day, although it is now outdated and misleading. Today we talk of animal or vegetable proteins and either can be growthpromoting, depending on the amino acid composition of the protein. Amino acids are the subunits of proteins. Some can be synthesized by the body and the ones which cannot are termed essential amino acids, and must be present in the diet in order for proper body functioning. The value of the protein to the body is called protein quality and is determined by its amino acid composition. The work on essential amino acids was pioneered by Rose who discovered the last of these in 1935. Nevertheless the protein problem is by no means solved. The 1965 FAO/WHO report on protein requirements received much criticism and the issue is still very hotly debated by the experts. We still do not know whether there is an optimum level of protein intake for good health.