Meat Extractives and the Non-Protein Nitrogen of the Blood *

Liebig14 was the first to try to find some scientific basis for the traditional concept that meat extractives in the form of meat juice and broth are valuable dietary components. He went to the extreme and claimed that the extractives alone are valuable and that the pressed residue is worthless. Later he modified this radical viewpoint but continued, against strongly opposing opinions, to stress the high value of extractives. Dominated by the attitude of Voit and of Rubner19 concerning proteins and their significance for nutrition, investigators concluded that the extractives are negligible as a source of nitrogen and worthless as far as caloric value is concerned. The swing of the pendulum is indicated from conclusions such as the following: The observed heart-stimulating effects of meat extracts were believed to result only from their high content of potassium and one author went so far as to claim that the extract was damaging and tended to shorten life."2 Twenty years later, when he repeated his experiments on a larger scale, Rubner20 reiterated his earlier statement concerning the unimportance of the extract from a caloric standpoint. A small portion of the fed extractive nitrogen was found by Rubner and other authors9 16 to be retained by the body, and from this Rubner concluded that the nitrogen-containing substances present in the extract performed some unknown function. The experiments of Pavlov17 and his school provided an explanation for the beneficial effects of the extract on the appetite and on gastric secretion. The observed effects on the circulation were explained by the activity on the blood vessels of the purines, especially adenylic acid, present in the extracts. Here the matter rested, until attention, accidentally directed to the subject, led to performance of the experiments now reported.