350 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Practically, however, the use of gelatin in diets is restricted by its unpalata- 

 bleness when used in large quantities. Whatever may be the physiological cause 

 of this peculiarity, there seems to be no doubt that when employed largely in the 

 diet both animals and men soon develop such an aversion to it that it is necessary 

 to discontinue its use. Munk ' has attempted to determine how tar the proteids 

 offood maybe replaced by gelatin. In these experiments a dog was brought into 

 a condition of nitrogenous equilibrium upon a diet of flesh, meal, rice, and lard, 

 containing 9.73 grams of nitrogen. During the period this diet was continued the 

 animal, whose weight was 16.5 kilograms, was oxidizing in its body 3.7 grams 

 ofproteid daily for each kilogram of weight. In a second period lasting four 

 davs the quantities of rice and lard were the same as before, but the proteid in 

 its diet was reduced to 8.2 grams, which contained 1.3 grams of nitrogen ; the 

 balance of the necessary nitrogen was supplied in the form of gelatin, so that in 

 round numbers only one-seventh of the required daily amount of nitrogen was 

 given as proteid. r Fhc result was that the animal maintained its nitrogen equi- 

 librium for the short period stated. It was found that the experiments could 

 not be continued longer than four days, owing to the growing dislike of the 

 animal for the gelatin food. During the second period the animal was receiving 

 in its food and burning in its body only 0.5 gram of proteid daily for each 

 kilogram of weight, as against 3.7 grams upon a normal diet. It is usually 

 stated that it is not possible to substitute fats or carbohydrates for the proteids 

 of our diet to the same extent, but the experiments of Siven quoted on the 

 preceding page indicate that this common belief may be incorrect. 



Nutritive Value of Fats. — The fats of food are absorbed into the lacteals 

 as neutral fats. They eventually reach the blood in this condition, and are 

 afterward in some way consumed by the tissues. The final products of their 

 oxidation must be the same as when burnt outside the body — namely, 0O 2 

 and H 2 — and a corresponding amount of energy must be liberated. Speak- 

 ing generallv, then, the essential nutritive value of the fats is that they furnish 

 energv to the body, and, from a chemical standpoint, they must contain more 

 available energy, weight for weight, than the proteids or the carbohydrates 

 (see p. 365). In a well-nourished animal a large amount of fat is found 

 normally in the adipose tissues, particularly in the so-called "panniculus 

 adiposus" beneath the skin. Physiologically, this body-fat is to be regarded 

 a- a reserve supply of nourishment. When food is eaten and absorbed in 

 excess of the actual metabolic processes of the body, the excess is stored in 

 the adipose tissue as fat, to be drawn upon in case of need — as, for instance, 

 during partial or complete starvation. A starving animal, after its small 

 supply of glycogen is exhausted, lives entirely upon body-proteids and fats; 

 the larger the supply of fat, the more effectively will the proteid tissues be 

 protected from desi met ion. In accordance with this fact, it has been shown 

 that when subjected to complete starvation a fat animal will survive longer 

 than a lean one. Our supply of fat is called upon not only during complete 

 abstention from food, but also whenever the diet is insufficient to cover the 

 oxidations of the body, as in deficient food, sickness, etc. 



1 Pjlugcr's Arrhii fur 'He gesammte Phyniologie, 1894, Bde. lviii. S. 309. 



