The important foods derived from the animal kingdom are - Meat - Poultry and Game - Fish - Shell Fish - Eggs - Milk and Milk Products - Sweetbreads - Gelatin - Beef Preparations - Meat Broth and Jellies.

Composition

Milk is the only animal food in which all the nutrients are represented in forms and proportions suitable to supply all the needs of the body. It is properly called a " complete food." Eggs, also often called " complete food," contain both body-building and fuel food-stuffs, but are incomplete in the sense that the amount of carbohydrate is so small as to be almost negligible.

Meats are valuable primarily as sources of protein. "When fat is present they are also useful for fuel.

Meat extracts are chiefly valuable for their flavor and stimulating properties.

The Proteins of animal foods are of many forms, conspicuous among which are albumin of egg-white; myosin of muscle; casein of milk; fibrin of blood; nucleo-protein of liver and sweetbreads; collagen of connective tissue.

The Fats are chiefly mixtures of stearin, palmitin and olein in varying proportions; small amounts of esters of volatile fatty acids; and compounds of fats with phosphorus in the form of lecithin and related substances.

Extractives

Nitrogenous extractives are found chiefly in meat, to which they help to give the characteristic flavor and the stimulating properties. They consist chiefly of purins (adenine, guanine, xanthine, hypoxanthine and uric acid); and creatin and creatinin.

Carbohydrates

The carbohydrates are represented mainly by lactose in milk, and glycogen in shellfish. As a rule, carbohydrates are negligible in animal foods.

Water

"Water is present in all animal foods. Some examples, showing the varying proportions in different kinds, are given in the following table:

Oyster solids ........................ 88%

Milk ................................ 87%

Eggs ................................ 73%

Lean beef ........................... 75%

Salt cod ............................. 50%

Mineral Matter

Milk and eggs are more valuable for their ash constituents than any other animal foods. Calcium, iron and phosphorus especially, are here found in most available forms.

Meats are not particularly useful in this respect. They are deficient in calcium, and while they contain considerable iron, it is not in as useful a form as that in eggs. Most of the ash constituents found in the body are, however, represented in flesh foods.

Digestibility

As a class, animal foods are easy of digestion. They contain little indigestible residue, and the nutrients are very perfectly absorbed. Prepared for the table, they are more concentrated than most vegetable foods. Hence animal foods are frequently chosen for invalids and convalescents, regardless of their high protein content.

Disadvantage Of A Purely Animal Diet

The chief disadvantages of a purely animal diet are that it does not afford sufficient bulk to maintain the proper functioning of the alimentary tract, owing to the lack of indigestible residue; and that it loads the body with an excess of nitrogenous material, which is not only unnecessary for repair, but may cause harm if not speedily eliminated. Furthermore, protein raises the metabolism so that more food is actually required than if carbohydrates and fats are used as fuel. For further discussion of the relative merits of animal and vegetable diets, see page 233.