270 alsberg: biochemical analysis of nutrition 



the most important contributions of recent years to the study 

 of nutrition are investigations upon the role of the proteins 

 and of the nitrogenous constituents of the food in the animal 

 organism. You know, no doubt, that in Liebig's time proteins 

 were regarded as that element of the food which supplied the 

 material for growth,- tissue maintenance, and repair, as well 

 as for most of the energy. Though it was soon demonstrated 

 that while proteins did and could furnish energy, under ordi- 

 nary conditions this was supplied in the main by sugar and other 

 carbohydrates and by catabolized fats. The proteins were, how- 

 ever, still regarded as all of about equal value, and their value 

 was estimated as proportional to the amount of nitrogen they 

 supplied to the animal organism. One protein was regarded 

 as of about as much dietary value as another. This was, how- 

 ever, soon found to be an erroneous notion. It was learned that 

 with certain proteins, gelatine for example, as the sole source 

 of nitrogen in the diet, life could not be supported. The proteins 

 then came to be divided into two classes, the true proteins and 

 the albuminoids. Gelatine was classed as an albuminoid. The 

 fact that it and some other proteins were found to be incapable 

 of supporting life was regarded as corroborative evidence that 

 they were not true proteins. Why they are incapable of sup- 

 porting life was not known. It was known that animals could 

 not live without proteins in the diet, but it was believed that rela- 

 tively few of the proteins were incapable of supplying all the 

 nitrogenous needs of the animal organism. 



That is about where matters rested for a long time. Then 

 in 1901 Loewi published a most startling investigation, so startling 

 in fact that it received scant attention. He subjected the pan- 

 creatic gland, which all of you know as sweet-bread, to self- 

 digestion, the process that is technically known as autolysis. 

 As you know, practically all cells and tissues contain many en- 

 zymes or ferments, some of them similar to those secreted in 

 the stomach and intestines for the purpose of digesting pro- 

 teins. Therefore, under suitable conditions tissues can be made 

 to digest themselves. Loewi caused the self-digestion of the 

 pancreas to proceed until the digestion of the proteins in the gland 



