THE STATISTICS OF NUTRITION. 489 



has arisen the very natural distinction to which we have already alluded 

 between "tissue proteids " or " morphotic proteids " which are actually built 

 up into the living substance of the tissues and give rise to urea through the 

 metabolism of living substance, and " circulating proteids " or " floating pro- 

 teids " which do not at any period of their career within the body become 

 an integral part of the living substance and by their metabolism set free 

 energy not in the way of vital manifestations, but in the form of heat only. 

 We shall latter on consider what is the exact meaning which we ought to 

 attach to the words " becoming part of the living substance ;" and hence 

 shall defer until then any discussion of the appropriateness of these phrases 

 and of the validity of the distinction which they formulate. 



It was once thought, as we shall presently see, erroneously, that the ex- 

 clusive purpose of proteid food was to supply the proteid tissues, and that 

 all the energy set free in the body in vital manifestations, such as movement 

 and the like as distinguished from heat, had its origin in proteid metabolism, 

 the metabolism of fats and carbohydrates giving rise to heat only. Hence 

 when it first became known that a certain proportion of proteid food appar- 

 ently underwent a metabolism giving rise to heat only, without becoming 

 part of the tissues, this seemed to be a wasteful expenditure of precious 

 material ; and the metabolism of this portion of proteid food was accordingly 

 spoken of as a " luxus-consumption," a wasteful consumption. 



Before leaving this subject we may call attention to a possible analogy 

 between the history of proteids and that of fats and carbohydrates. The 

 uniform composition of the blood, which the body seems ever striving to 

 maintain, probably applies to its proteids as well as to its other constituents. 

 We have seen that a surplus of non-nitrogenous materials in the blood is 

 withdrawn from the circulation and stored up as fat or glycogen, and it is 

 possible that an excess of proteids might similarly be stored up in some tis- 

 sue or tissues, in the hepatic cells for instance, though from the facts pre- 

 viously mentioned it is obvious that the power of storage is far less than in 

 the case of fats and carbohydrates. Such a store of proteid matter would 

 represent a sort of circulating proteid, but nevertheless for its final metab- 

 olism might have to form an integral part of some living tissue unit. 



437. The effects of fatty and of carbohydrate food. Unlike those of 

 proteid food, the effects of fats and carbohydrates cannot be studied alone. 

 When an animal is fed simply on non-nitrogenous food, death soon takes 

 place ; the food rapidly ceases to be digested, and starvation ensues. We 

 can therefore only study the nutritive effects of these substances when they 

 are taken together with proteid material. 



When a small quantity of fat is taken, in company with a fixed moderate 

 quantity of proteid material, the whole of the carbon of the food reappears 

 in the egesta. No fat is stored up ; some even of the previously existing fat 

 of the body may be consumed. As the fat of the meal is increased, a point 

 is soon reached at which carbon is retained in the body as fat. So also with 

 starch or sugar; when the quantity of this is small, there is no retention of 

 carbon ; as soon, however, as it is increased beyond a certain limit, carbon is 

 stored up in the form of fat or, to a smaller extent, as glycogen. Fats and 

 carbohydrates, therefore, differ markedly from proteid food in that they are 

 not so distinctly provocative of metabolism. This is exceedingly well shown 

 in the results obtained on the pig previously mentioned. It was found that 

 472 units of fat were laid on for every 100 units of fat taken as such in 

 the food (which consisting of barley-meal, etc. contained a very small amount 

 of actual fat), while for every 100 units of the total dry non-nitrogenous food 

 including fat, starch, cellulose, etc., no less than 21 units were retained in 

 the body in the form of fat. No clearer proof than this could be afforded 



