It is a valuable piece of information to know that one may diet an obese patient on a food containing little protein and two-thirds the body's energy requirement without danger of protein loss. The other third of the necessary energy will be furnished by the body's own store of fat. It is not remarkable that the body is capable of great physical effort on such a diet, for a fasting man is also competent in this direction (see p. 71); In Chapter on p. 156 mention was made of the sparing action of gelatin on protein metabolism, and its ingestion was found to prevent about 23 to 37.5 per cent, of the protein loss during starvation. Murlin1 in an extensive series of experiments has shown that the sparing power of gelatin is greater than this when it is ingested with a mixed diet. He finds that if the quantity of nitrogen eliminated in fasting be taken as one, then nitrogen equilibrium may be maintained in dogs and in man on ingestion of a diet rich in carbohydrates, whether the nitrogen of the diet be protein nitrogen equal to one or whether it contain one-third protein plus two-thirds gelatin nitrogen. This is shown in the following experiment on a man, the results being expressed in averages per day:

1 Chittenden: "Physiological Economy in Nutrition," 1904, pp. 14, 40.

2 Rubner: "Zeitschrift fur Biologie," 1879, xv, 130.

Effect Of Administering Gelatin In A Mixed Dift In Man

N elimination on a third day of fasting = 13.23 gm.

Murlin2 also showed that the sparing power of gelatin was due to its immediate chemical nature, and not to the 60 per cent, of glucose which can arise from it in metabolism (see p. 174). For example, a fasting dog was given 12 grams of glucose daily for four days after thirteen days of fasting; then 20 grams of gelatin were substituted during a period of four days. The glucose scarcely exerted any sparing power over the protein metabolism, whereas the ingestion of gelatin showed the usual sparing of 31 per cent.

1 Murlin: "American Journal of Physiology," 1907, xix, 285. 1 Murlin: Ibid., 1907, xx, 234.

The same fact was demonstrated on a man who was brought into nitrogen equilibrium on an adequate mixed diet containing 10 grams of nitrogen and carbohydrates enough to supply 50 per cent, of the energy. The state of nitrogen equilibrium was not quite maintained when gelatin was used as the source of two-thirds of the nitrogen in the diet. Murlin explained this as being due to a dislike for sweets on the part of the individual so that he could not take carbohydrates in large excess. However, when the nitrogen of the diet was reduced so as to contain only protein nitrogen equal to one-third that eliminated in fasting, together with the 60 per cent, of glucose which could have originated from the gelatin previously ingested, the waste of body nitrogen rose far above that observed when gelatin and other protein were given. The experiment may thus be presented:

Influence Of Gelatin In Metabolism

Figures are for the last day of each period.

* Two-thirds meat N + one-third vegetable N in wheat, oats, and rice.

Here the rise in the metabolism of body protein corresponds to the withdrawal of gelatin from the diet even in the presence of a considerable intake of carbohydrate. Hence Landergren's1 interpretation that the rise in nitrogen elimination, which takes place on changing from a pure carbohydrate to a pure fat diet, is due to the body's absolute requirement for carbohydrate and that it obtains this by increasing its protein metabolism is scarcely tenable, although even now this point is emphasized by many writers.

1 Landergren: Inaugural Dissertation, 1902: "Maly's Jahresbericht," 1902. p. 685.

It is evident that the "wear-and-tear" quota of protein metabolism must be covered by the ingestion of an equal "repair" quota, while the additional "dynamic" quota may be supplied by protein or by gelatin. Murlin found that the "repair" quota was best administered in the form of beef heart, and that the proteins of biscuit meal were very inefficient as sparers of body protein.

In the course of his experiments Murlin found that the longer the animal had fasted, that is, the lower its protein condition, the more readily did gelatin reduce the waste of body protein.

Murlin also showed that three-quarters of the starvation nitrogen ingested as gelatin and one-quarter as protein were not able to maintain nitrogen equilibrium in the dog. Two-thirds the starvation nitrogen requirement ingested as gelatin and one-third as protein maintain nitrogenous equilibrium. Carbohydrates ingested alone reduce protein metabolism to one-third that found in starvation. One-third the starvation quantity seems to be the lower limit of protein metabolism compatible with life.

It may also be noted that in a fasting diabetic dog the protein metabolism may rise to fivefold that noted in simple fasting (see p. 463), or fifteenfold the irreducible minimum of the "wear-and-tear" quota. Under these circumstances the writer has found that pure gelatin given alone is more effective as a protein sparer than it is in simple fasting. Thus after giving 30 grams of gelatin to a fasting phlorhizinized dog the following results were obtained on analyzing the urine every twelve hours:

If the fecal nitrogen, which is very small after gelatin ingestion, be neglected, it may be calculated that body protein was spared to the extent of 63.7 per cent, after the administration of gelatin instead of 30 per cent, as in ordinary fasting. One may, therefore, conclude that the great waste of body protein which takes place in diabetes belongs in Rubner's category of "dynamic" protein metabolism, for which gelatin may be largely used as a substitute.

McCollum1 gave to a pig a diet of starch and salts containing 90 calories per kilogram of body weight for twenty-four days and then during eight days added gelatin, the nitrogen content of which equalled the urinary nitrogen excretion at the end of the starch period. The results showed a sparing of the minimal endogenous protein metabolism (the "wear-and-tear" quota) equal to 40 per cent., as appears below:

The creatinin nitrogen, which remained at the same daily level throughout the experiment, was at the start 18.3 per cent, of the total urinary nitrogen. It is interesting to note that the reduction in the amount of endogenous protein metabolism brought about by the ingestion of gelatin is exactly the same quantity which may be withdrawn from the endogenous metabolism in the form of glycocoll following the ingestion of sodium benzoate (see p. 188). The creatinin excretion is not affected in either case. It is interesting to speculate whether the exogenous amino-acids of gelatin replace in metabolism that part of the "wear-and-tear" quota which involves the endogenous production of glycocoll.

Curiously enough, the endogenous protein metabolism may be greatly reduced when ammonium acetate or citrate are added to a rich carbohydrate diet. This subject was first studied by Grafe,1 who announced that nitrogen equilibrium could be maintained with carbohydrate and ammonium acetate in the diet, and who saw in this a synthetic formation of protein within the organism. Even the ingestion of ammonium chlorid reduced the amount of protein metabolism. A paper by Abderhalden2 followed quickly, which showed that though ammonium acetate when given with starch, sugar, fat, and bone-ash greatly reduced the endogenous metabolism, yet nitrogen equilibrium could not be attained under these circumstances. Abderhalden believes it possible that the animal cell may synthesize alanin, serin, or even cystein under these conditions, although he thinks that the heterocyclic and aromatic amino-acids are much less likely to be formed. He suggests that the mass action of ingested ammonia may prevent the deamination of some of the amino-acids, which may therefore be used once again for the repair of the tissue. Abderhalden's explanation seems the more rational of the two. A considerable sparing of endogenous protein metabolism was observed by Grafe3 to take place after the administration of ammonium citrate with carbohydrate, and this has been confirmed by Underhill,4 who, however, could find no influence exerted by ammonium chlorid.

1 McCollum: "American Journal of Physiology," 1911-12, xxix, 215.

1 Grafe and Schlapfer: "Zeitschrift fur physiologische Chemie," 1912, lxxvii, 1.

2 Abderhalden: Ibid., 1912, lxxviii, 1. A vast literature, experimental and polemical, has arisen from these two papers.

3 Grafe: "Zeitschrift fur physiologische Chemie," 1912, lxxxii, 347.

4 Underhill and Goldschmidt: "Journal of Biological Chemistry," 1913, xv, 341.