THE PRODUCTION OF FLESH Animals fed normally grow from birth to maturity increasing in weight and developing their bones and muscles. What constituents of feeding stuffs are essential to this growth? We know that the muscles, blood, tendons, hair, skin, nerves, brain and all the tisues of the body contain nitrogen. The only con stituent of feeding stuffs that contains nitrogen is the protein. Protein is, therefore, absolutely essential to grow ing animals. Is protein also necessary for mature animals that have made their full growth? When the urine of animals is ana lyzed nitrogen is always found in it, This is true even when the foods fed contain no nitrogen whatever. Even when animals receive no food at all and are starving, nitrogen is found in the urine. In these cases the only possible source of the nitrogen found in the urine is the protein previously stored in the body. The nitrogen in the urine repre sents the protein that was used in the body in carrying on its vital functions.
In the living animal of whatever age, a certain amount of protein or flesh is constantly destroyed in the life processes and is removed from the body through the urine, while new tissue is constantly built up out of the protein obtained from the digested food to replace that broken down and destroyed. Protein is used constantly in the body of all ani mals. This is true whether the animals are young or old and whether they are at work or at rest. If the protein in the foods supplied is more than sufficient to maintain the vital activities of the ani mal, the excess, within limits, may be stored as flesh.
Forms of protein the basis of careful experiments, it is believed that protein exists in the body in two forms. In one form it is unstable and rapidly destroyed. In the other form it is com paratively stable and only slowly de stroyed. It Las been suggested that in the first instance the protein destroyed may be that which has been carried to the cells of the body, but which has not yet become organized, while in the latter case the protein has become organ ized into the tissues of the different organs.
Organized protein or flesh constitutes by far the greater part of the protein of the body, and in mature animals re mains nearly constant, the amount de stroyed each day being practically less than one per cent. The quantity of pro tein not organized into flesh, or "circu latory protein" as it has been called, on the other hand may vary greatly. In starving animals the circulatory protein may be practically nothing, while in an imals well fed on foods containing a large amount of digestible protein it may equal as much as 5 per cent of the organized protein. But while less than 1 per cent of the organized protein is destroyed each day in the vital processes, 70 to SO per cent of the circulatory pro tein may be destroyed.
It is thus seen that the bodily func tions are maintained largely at the ex pense of the circulatory protein which varies in amount from day to day with the amount of digestible protein in the food.
A certain amount of protein in the food is essential to life. If the protein supply in the food is increased above this amount, a temporary gain in flesh occurs and a rapid increase in the cir culatory protein. With every increase, however, in the amount of protein sup plied in the food there is a correspond ing proportionate increase in the amount of protein consumed in the body and excreted in the urine. No matter, there fore, how great the amount of protein supplied in the food any excess beyond the normal amount required to keep the animal in good condition will not be formed into flesh, but will be consumed in the body and excreted in the urine.
On this point Armsby states that the "Animal body puts itself very promptly into equilibrium with its nitrogen sup ply, and no considerable or long contin ued gain of proteid tissue can be pro duced in the mature animal by even the most liberal supply of proteid food." This means that in fattening mature animals there is practically no increase in lean meat. The gain is largely due to deposits of fat which contains no ni trogen.
Other constituents Thus far we have spoken only of the protein constituent of the ration in flesh formation. Pro tein, however, is seldom fed alone. It is usually combined in feeding stuffs with either fats or carbohydrates and usually with both. Especially is this true with all vegetable feeding stuffs like the hays and grains. What effect do carbohy drates and fats have on flesh formation when they are fed with protein? Less Protein Required Experiments have shown that when either carbohy drates or fats are fed in the ration with protein a much smaller quantity of pro tein is required for the vital functions, and that the gain in flesh may extend over a longer period than when protein is fed alone.
Armsby illustrates this in the case of a dog that, when fed on lean meat, re quired 1500 grams to maintain himself in good condition. When fed only 500 grams the dog lost flesh and was starv ing. When, however, 200 grams of fat was fed with the 500 grams of lean meat the dog not only kept in good condition, but actually gained in weight. The an imal was actually better nourished on 500 grams of protein and 200 grams of fat than on 1500 grams of protein. The use of the 200 grams of fat saved 1000 grams of protein. It is probable that had the dog been able to have eaten the 200 grams of fat in addition to the 1500 grams of protein he would not have The best proportions of protein, fat and carbohydrates to feed different ani mals at different stages of growth, for the different purposes of growth, fatten ing, work, milk, egg and wool produc tion have been studied very extensively and have been formulated into feeding standards which will be taken up and discussed farther along in this acccount.
In summing up this phase of the ques tion it may be said that animals can live and form flesh on protein alone. But by the addition of carbohydrates or fat to the ration they can live and form flesh for a longer period on a much small er quantity of protein. Lean meat is formed entirely from the protein sup plied in the food. In the next section we will see how fat is formed.