The proteins of the digested food, absorbed and distributed in the form of amino acids as described above, soon become available for nutrition; and among other functions they, like the carbohydrates and fats, may be burned * as fuel for muscular work. Pfluger proved that protein may serve as a source of muscular energy by feeding a dog for 7 months exclusively upon meat practically free from fat and carbohydrate, and requiring it throughout the experiment to do considerable amounts of work, the energy for which must in this particular case have been derived largely from the protein consumed.

The experimental facts and theoretical explanations regarding the breaking down of proteins (or of the amino acids arising from them) in the body tissues must now be considered. By experiment it has been found that if a meal extra rich in protein be eaten, an increased elimination of nitrogenous end products can be observed within 2 or 3 hours, and probably much the greater part of the surplus nitrogen will have been excreted within 24 hours of the time it was taken into the stomach. It does not follow, however, that the whole of the protein molecule is broken down and eliminated so quickly, and many experiments have shown that the carbon often does not leave the body so rapidly as does the nitrogen. Evidently, the nitrogenous radicles of the protein may be split off in such a way as to leave a non-nitrogenous residue in the body, and the study of protein metabolism involves a consideration of the fate of both the nitrogenous and the non-nitrogenous derivatives. The fate of the latter may conveniently be considered first on account of its relation to the metabolism of carbohydrates and fats. Of special interest is the problem to what extent the deaminized cleavage products of protein may be actually transformed into carbohydrate or fat in the body.

* It will of course be understood that the protein is not supposed to be burned directly. Protein is split to amino acids, the amino acids deaminized, and the non-nitrogenous residues of the amino acids are burned.