Having now followed the foods in all their physical and chemical changes, until we find them in circulation in the blood, we come to a consideration of those processes which are really the fundamentally important ones. They are the end. All these other processes are only means leading toward the end. Foods are taken into the body for the energy which they contain. The essential and most important food change is the one in which energy is set free, and we are now ready to consider this change.

The chemical changes within the body are collectively called metabolism. These chemical changes are subject to classification in two general groups:

First, those changes in which the complex food materials or tissue constituents are oxidized and broken down into simpler materials, which are finally excreted. Incident to this oxidation, the energy of the complex material is given up in the form of heat, motion, etc. This general process is called catabolism. In the animal body most of the chemical changes belong to this group - that is, for the most part the chemical changes in the animal body are catabolic in character.

Second, there are certain chemical processes in the animal body in which simpler substances are built up into more complex ones. This process of building up complex substances out of simpler ones is called anabolism. An example of anabolism is found in the first change which the fats undergo after absorption, when they are absorbed as soaps. This change, as described above, consists in the combination of fatty acids with glycerin to form molecules of neutral fat. The neutral fat molecule contains three molecules of fatty acid with one of glycerin, and is therefore much more complex than the molecules of which it is built up. This is a typical anabolic process. Another example is the formation within the epithelium of the intestine of serum albumin and serum globulin out of the peptone absorbed from the intestine. The steps of this process are without doubt a reversing of the steps of digestion. Digestive steps in proteolysis are hydrolytic cleavages. A hydrolytic cleavage is a splitting accompanied by the addition of water. A reversing of such a process would naturally be a combination with the release of water, and this it is which takes place in the epithelium of the small intestine, and results in the formation there of proteins out of peptones. These proteins are serum albumin and serum globulin.