The greater the amount of protein that is ingested, the greater will be the amount of urea excreted, and so we are forced to the conclusion that the body can break up the protein molecule and discharge its nitrogenous moiety in order to obtain the use of the carbonaceous portion for the production of energy.

The organism cannot freely store up protein, and this is an explanation of nitrogenous equilibrium being able to be maintained on varying quantities of protein food. As Folin says, "The ordinary food of the average man contains more nitrogen than the organism can use, and increasing the nitrogen still further will therefore necessarily only lead to an immediate increase in the elimination of urea, and does not increase the protein katabolism involved in the creatinin formation any more than does an increased supply of fats and carbohydrates."

The normal body can store fats and carbohydrates, and their katabolism consists chiefly of oxidations setting free heat to keep up the temperature or to appear as mechanical work. The removal of nitrogen from protein by deamidisation sets free very little heat or energy, but yields a non-nitrogenous portion of great value, capable of oxidation if required. When not used in this manner it is probably converted in part into carbohydrates or stored up as fat. It is notable that large eaters of meat are usually corpulent, and after a certain age are apt to become glycosuric.

Thus we have true tissue or endogenous katabolism with creatinin as its index, and exogenous katabolism - a splitting-up of the excess of food protein with the elimination of unnecessary nitrogen by deamidisation and the formation of a carbonaceous moiety later on - with urea as its index.

It is rather difficult to assess the relative proportions of exogenous and endogenous katabolism, but with an intake of 100 grams protein quite three-quarters will be excreted as urea and not more than one-quarter will represent true tissue katabolism.

It is also difficult to say which of the amino-acids are utilised for tissue formation and which of them are broken down into urea. Injection of glycine, leucin, and arginin directly into the blood stream is followed by an increased formation of urea, whereas in a similar experiment with tyrosin and phenyl-alanine a negative result ensues. It is, therefore, conjectured that these two last mentioned amino-acids are examples of building stones which actually take their place in the growth and renewal of the bodily tissues.

As we have just stated, an extremely difficult point to decide is what quantity of tissue protein undergoes decomposition and requires renewal each day. Muscular exercise has very little influence on the excretion of urea, while it enormously increases the expiration of carbon dioxide, iudieating that muscular energy is dependent on the combustion of non-nitrogenous material, and it is believed more particularly of carbohydrate. When an insufficiency of carbo-hydrate and fat is supplied to the muscles, or when an excess of work is done, they are then compelled to fall back on their protein contents for the production of energy.

The principal end-products of the katabolism of proteins in the body besides urea and creatinin are carbonic acid, water, and sulphuric acid (in combination as sulphates).