348 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



enous equilibrium. Apparently this is not the case, as is seen from experiments 

 of the following character : When an animal is allowed to starve, the nitrogen in 

 the urine, after the first few days, becomes practically constant, and represents 

 the amount of oxidation of proteid tissue taking place in the body. If, now, the 

 animal is given an amount of proteid just equal to that being destroyed in the 

 body, nitrogenous equilibrium is not established; some of the body-proteid con- 

 tinues to be lost, and to get the animal into equilibrium a comparatively large 

 exee— of proteid must be given in the food. The same result holds if carbo- 

 hydrate- and fats are given along with the proteid, with the exception that upon 

 this diet nitrogen equilibrium is more readily established — that is, less proteid 

 is required in the food. Upon the theory of circulating proteids and tissue- 

 proteids, this fact may lie accounted for by saying that of the proteid given as 

 food, a part always undergoes destruction as circulating proteid without going 

 to form tissue, so that to cover tissue-waste a larger amount of proteid must be 

 taken a- food than would be necessary if it could all be used exclusively for the 

 repair of tissue. Carbohydrates and fats diminish the amount of proteid 

 destroyed as circulating proteid, and thereby enable us to keep in nitrogen 

 equilibrium on a smaller proteid diet. With albuminoid food (gelatin) the 

 facts seem to be different. If albuminoids be given in the food together with 

 proteids or with proteids and a non-nitrogenous food-stuff (fats or carbo- 

 hydrates), nitrogen equilibrium may be established upon a much smaller 

 amount of proteid thau in the case of a diet consisting of proteid alone or of 

 proteid together with fats and carbohydrates. It seems probable that albu- 

 minoids can take the place entirely of circulating proteids, so that only enough 

 proteid need be given to cover actual tissue-waste. This point will be referred 

 to again in speaking of the value of the albuminoids. 



Z/UXU8 < bnmmption. — The fact that normally more proteid is eaten, even 

 in a mixed diet, than is necessary to cover the actual tissue-waste led some of 

 the older physiologists to speak of the excess as unnecessary, a luxus, and the 

 rapid destruction of the excess in the body was described as a " luxus con- 

 sumption." There can be no doubt about the fact that proteid may be, and 

 normally is, eaten in excess of what is necessary to repair tissue-waste, or in 

 excess of what is requisite to maintain nitrogenous equilibrium at a low 

 level. But it is altogether improbable that the excess is really a "luxus." 

 It has been stated, in speaking of nitrogenous equilibrium, that an animal 

 may be kept in this condition upon a certain minimal amount of pro- 

 teid, or upon various larger amounts up to the limit of the power of the 

 alimentary canal to digest and absorb; but it has also been shown (Munk 1 ) 

 that if an animal i< i^(\ upon a diet containing quantities of proteid barely 

 sufficient to maintain N equilibrium, it will after a time show signs of mal- 

 nutrition. It seems to be necessary, as Pfliiger pointed out, that the tissues 

 should have a certain excess of proteid t<> destroy in order that their nutri- 

 tional or metabolic powers may be kept in a condition of normal activity. 

 Hence we find that well-nourished individuals habitually consume more proteid 

 than would theoretically suffice for N equilibrium. For example, the average 

 1 I>n Bois-Reymond's Arehivjur Physiologie, 1891, S. 338. 



