Although food constitutes the chief item in the expense of living among the wage-earning class, and is more intimately concerned with the promotion of health and strength than are clothing, shelter and climate, it is only very recently that any serious thought has been given to the nature of food, and to the processes which it undergoes in being utilized for the bodily upkeep. Man has adjusted himself to such food supply as he found available, and accordingly latitude, climate and soil and proximity to large bodies of water have been the determining factors in establishing his dietary habits. Common observation led to the knowledge that people in different regions lived on diets of widely different character, yet without any markedly different success in physical development. Indifference to the nature of the food supply, except as to palatability, was therefore natural, since there was no obvious evidence that the character of the diet had anything to do with well-being, provided a sufficient amount of food could be had.

The paucity of our knowledge concerning nutrition that existed toward the middle of the nineteenth century is well illustrated by the views of Dr. William Beaumont, a surgeon in the United States Army, expressed in his book, "Physiology and Experiments," published about 1832. He had the good fortune to study the processes of digestion with the hunter, Alexis St. Martin, who had a fistulous opening into the stomach as the result of a gunshot wound. Beaumont collected gastric juice from his subject's stomach and studied its effect on various foods. He also introduced foods into the stomach and observed through the opening, the behavior of this organ during digestion. He stated very definitely that he believed that the views of others to the effect that there are various kinds of nutrient substances, were in error. In his opinion there was but one kind of food or "aliment" as he termed it. This was, he believed, present in all foods, and was simply dissolved out by the action of the gastric secretion. Dr. Beaumont was one of the most progressive investigators of his time (1).

The chemistry of both inorganic and organic substances advanced with great rapidity during the nineteenth century, and among the facts which were established was the widespread occurrence of proteins, carbohydrates and fats as components of foods, thus disproving the view that there was but "one kind of aliment." Of these nutrient principles only the first named contains the element nitrogen. It is the oxidation of these food substances that yields the energy which keep the body warm, and enables it to do mechanical work. The laws governing energy metabolism were next developed.