THE SIGNIFICANCE OF THE FOOD-STUFFS 645 



substance, urea, which is the almost exclusive nitrogenous end-product of 

 the energy metabolism of protein. 



THE FOOD-VALUE OF CERTAIN SUBSTANCES ALLIED 

 TO PROTEINS 



PROTEOSES AND PEPTONES. In the digestion of the naturally oc- 

 curring proteins the first products of hydration consist of a mixture of 

 substances known as proteoses and peptones. In the further processes of 

 digestion, under the influence of the ferments of the pancreas and small 

 intestine, these substances are converted into the amino-acids which we have 

 learnt to regard as the proximate constituents of the protein molecule. 

 Many experiments have been performed in order to determine the nutritive 

 value of these digestive products. In nearly all cases it has been found 

 that the meat in the diet of an animal can be replaced by a corresponding 

 quantity of the products of digestion of the same meat without interfering 

 with the nitrogenous equilibrium of the animal, and Loewi and others have 

 shown that the same result may be attained by feeding an animal on the 

 products of pancreatic digestion of protein, i.e. a mixture consisting almost 

 entirely of amino-acids. Since the proteins of the body differ in their 

 composition from the majority of the proteins of the food, it is evident that 

 each food-protein molecule has to be taken to pieces and reconstructed before 

 it can take its place in the body fabric, and it is therefore only natural that, 

 so far as metabolism is concerned, the results should be identical whether 

 we feed the animal with the ordinary food- protein or with the products 

 of its metabolism. This mode of feeding cannot, however, be regarded as 

 presenting any advantages. Under normal circumstances the food molecules 

 are broken down by degrees. Their products of hydrolysis are set free in 

 small quantities at a time and can be therefore absorbed and disposed of in 

 proportion as they are set free. On the other hand, a sudden flooding of the 

 alimentary canal with a ferge quantity of the products of digestion intro- 

 duces an abnormal factor which must tend to produce wastage of nitrogen 

 in the body and to disturb the normal chain of processes involved in the 

 regular course of digestion. 



COLLAGEN AND GELATIN. Among the various sclero-proteins or albu- 

 minoids which occur as normal constituents of our foods these two are the 

 only substances which Undergo digestion and solution in the alimentary 

 canal to any appreciable extent, other substances, such as elastin and keratin, 

 reappearing for the greater part in the faeces. As we have seen, gelatin, the 

 first product of hydration of collagen, represents, so to speak, an imperfect 

 protein. When it is hydrolysed by acids its disintegration products include 

 the ordinary amino-acids of the fatty series, including a considerable amount 

 of glycine and also a certain amount of phenyl alanine and proline. The oxy- 

 phenyl group which occurs in all the food-proteins in the form of tyrosine, as 

 well as the indol-containing group tryptophane, are absent. On this account 

 purified gelatin does not give either Millon's test or the Hopkins- Adam- 

 kiewicz test with glyoxylic acid. As might be expected from its composition, 



