A prolonged deposition of protein in the normal adult, when fat is given with it, is demonstrably difficult. .

The question arises, Does the ingestion of large quantities of fat also cause an increase in the metabolism until fat combustion is balanced by its ingestion?

Rubner1 has shown that this is not the case. He cites the record of the following long respiration experiment on a dog which was given 80 grams of meat and 30 grams of fat daily:

Absence Of The "Secondary Dynamic Rise" In Fat Metabolism On A Meat-Fat Diet

The diet was 58.7 per cent, above the starvation requirement. It contained 354 calories, of which 21.5 per cent, were in protein. The mean heat production during the period of ingestion of food was 256.0 calories, and in the following starvation days 223.2 calories, showing an increase in metabolism of 11.2 per cent, caused by an excess in food of 58.7 per cent. During the later days the animal was in nitrogenous equilibrium. Notwithstanding an excess of fat in the diet, and a continued deposit of fat in the body, there was no increase in the metabolism during the time of experimentation. The secondary dynamic action noted by Rubner as regards protein does not, therefore, take place as regards fat. The storage of fat in the body is consequently a matter of comparative ease.

Rubner2 has compared the metabolism of a boy who was obese with that of his brother, who was a year older, but thin. They were the children of parents of small means and would not naturally be overfed. The interesting point of the experiment was whether obesity was due to a reduced metabolism with the consequent deposition of fat. Each boy was given a maintenance diet, or one which balanced his metabolism, without adding to or subtracting from his body substance. The general results are as follows:

1 Rubner: "Energiegesetze," 1902, p. 251.

2 Rubner: "Beitrage zur Ernahrung im Knabenalter," 1902.

The comparison shows that the fat brother had a larger total metabolism than the thin one, but the fat boy also had the larger surface. Per square meter of surface the metabolism was the same (see p. 129). The gradual increase in the area of the body caused by filling out the fat cells may therefore increase combustion, but this is not due to the specific action of the fat on metabolism as in the case of the secondary dynamic rise after protein ingestion, but rather to the increase in the size of the body. Carbohydrates, which in excess are converted into fat, must behave in the same way. It will be noticed that in the experiment where 80 grams of meat and 30 grams of fat were daily ingested, although the protein metabolism gradually fell, the fat metabolism gradually rose, and in isodynamic relation to the fall in protein. Allowing for the difference in specific dynamic action, protein and fat replace each other in metabolism in isodynamic quantities.

The preceding chapters have dealt entirely with the subject of the metabolism of protein and fat. The metabolism of carbohydrate has been touched upon incidentally in describing the intermediary metabolism of protein, but the fuller details remain to be considered. Generally speaking, two-thirds of the energy produced by the human organism is derived from the oxidation of carbohydrate. Not without warrant is bread considered the staff of life.