Animal foods contain practically the same classes of nutrients as are found in vegetable foods. The protein in meat differs from that in vegetables in three ways. It is more abundant, more easily digestible, and very different in appearance. This compound in meat is considered under the three heads of albuminoids, gelatinoids, and extractives. The albuminoids are the chief flesh formers of meat. They are capable of forming muscle, tendon and fat, and of yielding energy in the form of heat and muscular strength. So far as scientists have yet learned, gelatinoids cannot be said to change to the composition of albuminoids, but they do protect the albuminoids from being consumed. The extractives give flavor to the meat and presumably tone to the system. That these cannot build tissue nor furnish heat to the body is believed to be an established fact. Scientists and medical men agree that they do serve a purpose, but just what that is they cannot explain. The extractives seem to exert some action on the nervous system which aids the body in making use of the food consumed. Scientists are of the opinion that "flavoring materials and an agreeable appearance do not directly increase thoroughness of digestion, but serve to stimulate the digestive organs to greater activity as regards the actual amount digested. This stimulation is probably not of so great moment as is commonly supposed. Meat that has been extracted with water so as to be entirely tasteless has been found in actual experiment to be as quickly and completely digested as an equal weight of meat roasted in the usual way."* There seems to be inherent in man a desire for flavor. Dr. Livingston tells us that the savages in the wilds of Africa throw the meat over the coals long enough to sear it a little, then swallow it, hot, but it has a different flavor, though nearly raw.
The value of meat for food depends in part upon the proportion of protein to fat. While the protein can take the place of fats and carbohydrates, nothing else can do the work of the proteids. It is not wise to allow the pro-teids to take the place of the other elements of food, because the changing necessary to fit them for use entails too much work on the organs, and it is substituting the more expensive for the much cheaper food substances. Neither is it wise to attempt to wholly substitute vegetable protein for animal protein. The nutrients in vegetables are enclosed in woody fibre which resists the action of the digestive juices, and probably oft-times hastens the food along the digestive tract too rapidly, owing to its irritating action on the lining membrane of the intestines, to insure complete digestibility. The fat in meat performs the same office as starch, sugar, and digested cellulose in vegetable foods. It forms fatty tissue, or serves as fuel, and yields energy in the form of heat and muscular strength. There is no perceptible difference in the digestibility of animal and vegetable fats. Animal foods contain much more fat than vegetable foods.
*U. S. Department of Agriculture, Farmers' Bulletin No. 34 - p. 17.
The following charts are from "Meats for Farm Use." - Andrew Boss, Associate Professor of Agriculture, University of Minnesota.