Having surveyed rather rapidly the various substances that function as foods; having shown that the calorific value gives incomplete information; having shown the importance of a judicious distribution of food among the three classes of foodstuffs; having pointed out the importance of mineral salts, of water, of oxygen, and, to a lesser extent, of condiments, flavors and stimulants, it now becomes important to investigate some of these factors a little more carefully.

During the last twenty, and largely during the last ten years, research work in nutrition has revolutionized that science no less than the study of radioactivity has revolutionized our conceptions of matter. This and subsequent chapters will deal with these revolutionary changes - changes made possible very largely by the labors of American men of science.

How the dawn of the modern era arose is an interesting bit of history. It centers itself around a study of the protein food.