In conventional scientific usage, when the word metabolism is joined with energy, it takes on a somewhat different meaning than when it is joined with protein. The latter – protein metabolism – usually includes consideration of the biochemical pathways of amino acids, the building blocks of protein, whereas energy metabolism is frequently assumed to include only the specific role of energy without consideration, in any detailed way, of the pathways involved in the breakdown and synthesis of the various carbohydrates and lipids that supply food energy.

In this chapter, the major thrust concerned with energy is an emphasis on historical considerations of understanding and meeting human food energy needs. In the case of protein, the role of amino acids in generating energy, protein quality (including digestibility), and human protein requirements will be given emphasis. Finally, there is some discussion of protein–energy relationships, the problems connected with an excess and a deficit of food energy in the diet, and protein–energy malnutrition.

Food Energy

The most pressing problem for humans throughout their history has been the basic one of securing food to satisfy hunger and food-energy needs. But the fact that the human population in different parts of the world (despite subsisting on different diets), seemed before about 1900 to experience approximately the same level of health had led some physiologists to believe that all foods were rather similar (McCollum 1957). In fact, this view was rooted in the ancient “many foods–single aliment” concept of Hippocrates and Galen. By the turn of the twentieth century, however, views were rapidly changing, and although knowledge of the specialized roles of amino acids, vitamins, and minerals was hazy and often contradictory, proteins, fats, and carbohydrates (starches and sugars) could be distinguished and analyzed in foods and diets. Thus, with the advances in food analysis and refinements of nutritional needs the “many foods– single aliment”concept was no longer tenable.